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Syringes · Sizes and units

Syringe sizes, needles, and units versus mL

If you have never held a syringe, the numbers on the box and the barrel can look like a code. This page explains them: what the barrel sizes and unit marks mean, what the needle's gauge and length tell you, why a U-40 syringe is dangerous to mix up, and how millilitres turn into units, with worked examples you can open in our calculator.

Last updated October 28, 2026 · Reviewed by Grey Peptides Editorial Board · ✓ Every claim sourced · No products sold, no seller named

📋 Key takeaways

The syringes most people use to inject under the skin at home are U-100 insulin syringes. They come in three barrel sizes: 0.3 mL (30 units), 0.5 mL (50 units) and 1 mL (100 units). On one maker's syringes the 0.3 mL barrel is marked every unit, with half-unit marks on some versions, the 0.5 mL every unit and the 1 mL every two units.12

A unit on a U-100 syringe is a volume, one hundredth of a millilitre. It says nothing about how much drug is in that volume; that depends on the concentration. Our units explainer covers the arithmetic, and the reconstitution calculator does it for you.

A U-40 syringe puts 40 units in a millilitre, so its units are two and a half times bigger. Mixing the two syringe types up gives two and a half times too much or too little, and FDA has recalled U-100 packs that held U-40 syringes.78

Higher gauge means a thinner needle. Clinical recommendations say the shortest needles, the 4 mm pen needle and the 6 mm syringe needle, are safe, effective and less painful for injections under the skin.113

Pick the smallest barrel that holds the whole volume you will draw: the marks are farther apart and easier to read, and FDA has linked overdoses to people drawing small volumes with syringes much larger than needed.615

On this page

  1. The parts of a syringe
  2. U-100 insulin syringes: 0.3, 0.5 and 1 mL
  3. U-40 syringes, and why a mix-up is dangerous
  4. Tuberculin syringes: marked in mL, not units
  5. Needle gauge: a higher number is a thinner needle
  6. Needle length, and why short, fine needles suit injection under the skin
  7. Fixed versus detachable needles, and dead space
  8. How mL relates to units
  9. Worked examples, in the calculator
  10. How to read the barrel
  11. Which size to pick for which volume
  12. Where to go next
  13. Frequently asked questions
  14. Related on Grey Peptides
  15. Sources

The parts of a syringe

A syringe has four parts you need to know by name:

  • The barrel: the clear tube that holds the liquid. It is printed with lines and numbers, the scale, that measure how much is inside.6
  • The plunger: the rod that slides inside the barrel to pull liquid in and push it out. At its lower end is a rubber seal, and the seal is what you line up with the marks.6
  • The needle: on insulin syringes it is short, thin and coated with a fine layer of silicone, and a cap covers it until use.6 On most insulin syringes it is permanently attached to the barrel.12
  • The packaging: the box and the barrel say which syringe it is. Look for the strength it is made for (U-100, or U-40), the barrel size in mL, and the needle's gauge and length, such as 31G and 6 mm.1

Everything else on this page explains those four labels. The buying page covers where and how to get syringes, state by state.

U-100 insulin syringes: 0.3, 0.5 and 1 mL

"U-100" means made for insulin at a strength of 100 units per millilitre, so the barrel is marked with 100 unit lines to the millilitre: one unit is 0.01 mL.9 One maker's FDA filing for its U-100 insulin syringes lists the three barrel sizes and how each is marked:1

BarrelHoldsSmallest markNeedles listed in the filing
0.3 mL (also written 3/10 mL or 0.3 cc)30 units1 unit; half-unit marks on some versions1229G to 31G; 6, 8 or 12.7 mm1
0.5 mL (1/2 mL)50 units1 unit128G to 31G; 6, 8 or 12.7 mm1
1 mL (1 cc)100 units2 units127G to 31G; 6, 8, 12.7 or 16 mm1

The same filing says the syringes are for subcutaneous (under the skin) injection of U-100 insulins, are single use, and have needles permanently attached to the barrel.1 A patient-education page lists the same sizes, plus a 0.25 mL (25-unit) barrel, which is less common.6 Other makers' syringes may be marked differently; the barrel itself is the authority, so read it before you rely on any table (our reasoning).

The 1 mL barrel's two-unit spacing matters: an odd number of units, such as 75, sits halfway between two lines on it. The worked example below shows one.1

Half-unit marks

Some 0.3 mL syringes add a mark at every half unit, 0.005 mL apart; the maker's filing lists "1 unit increments and ½ unit increments (0.3mL)" as a scale option.2 Half-unit marks exist for people who need very small insulin amounts, such as young children (our reasoning from the 30-unit barrel being the one recommended for children6). They make the barrel busier to read, so check whether yours has them: if every line is a half unit, counting lines as whole units would double the volume.2

U-40 syringes, and why a mix-up is dangerous

A U-40 syringe is marked for insulin at 40 units per millilitre, so each of its units is 0.025 mL, two and a half times the volume of a U-100 unit. U-40 insulin is used mainly for dogs and cats: one approved veterinary insulin's maker says to use it "with U-40 insulin syringes only to avoid dosing errors", and that drawing it with a U-100 syringe would give the animal two and a half times less insulin than intended.8

The danger runs both ways. Drawing to a mark on the wrong scale delivers either 2.5 times or 0.4 times the intended volume. The arithmetic: "10 units" on a U-40 syringe is 10 × 0.025 = 0.25 mL, which is 25 units on a U-100 syringe; "10 units" on a U-100 syringe is 0.1 mL, which is 4 units on a U-40 syringe.8

This is not hypothetical. In 2008 FDA listed as a Class 1 recall a lot of 1 mL syringes whose package was labelled for U-100 insulin but held syringes for U-40 insulin, a risk of insulin overdose; 471,000 syringes had been distributed nationwide.7 Japan's medicines regulator, warning about syringe mix-ups, reports a case in which a syringe for 50 units was taken in place of the usual 30-unit syringe.9

What to check: the barrel and box of a U-100 syringe say U-100. If anything says U-40, every conversion on this site and in our calculator is wrong for that syringe, because they all assume U-100 (our reasoning). Our units explainer has more on U-40 against U-100.

Tuberculin syringes: marked in mL, not units

A tuberculin syringe is a 1 mL syringe marked in millilitres, named for the tuberculosis skin test, which injects 0.1 mL into the skin. WHO's guidance for that test calls for "a 1 mL graduated syringe or tuberculin syringe that can dispense 0.1 mL solution accurately", with a short (8 to 13 mm) 27-gauge needle.10 Insulin syringes are printed "UNITS"; tuberculin and other general-purpose syringes are not.9

That difference has caused real errors. Japan's medicines regulator collected cases where staff confused units with millilitres: in one, 4 units was read as 0.4 mL and 40 units were given with a tuberculin syringe; in another, a general-purpose syringe was used to prepare 7 mL (700 units) instead of 7 units. Its advice is to use insulin syringes for insulin, always.9

If a source gives a volume in mL and you have a U-100 insulin syringe, multiply by 100 to get units: 0.1 mL is 10 units, 0.25 mL is 25 units.9 If it gives units and you have a tuberculin syringe, divide by 100 to get mL. Either way, write down which scale you are reading before you draw (our reasoning). FDA's 2024 alert on compounded semaglutide traced overdoses partly to "confusion between different units of measurement (e.g., milliliters, milligrams and "units")".15

Needle gauge: a higher number is a thinner needle

Gauge, written G, is the needle's width. The scale runs backwards: the higher the gauge, the smaller the needle's hole, so a higher number means a thinner needle.11 Insulin syringes in one maker's filing carry needles from 27G to 31G.1 For comparison, CDC's job aid for vaccines given under the skin uses a 23- to 25-gauge needle, wider than an insulin needle.12

On the box you will see gauge and length together, for example "31G × 6 mm" or "30G × 5/16 inch".11 Lengths in inches convert as 5/16 inch ≈ 8 mm and 1/2 inch ≈ 12.7 mm; these are the 8 and 12.7 mm needles in the filing.1

Needle length, and why short, fine needles suit injection under the skin

A subcutaneous injection goes into the fat layer under the skin, not into muscle. How deep that is decides which needle length is enough.

  • The skin is thin. An ultrasound study of 388 US adults with diabetes measured the skin at injection sites at about 1.9 to 2.4 mm thick on average, varying little with body size, while the fat layer below varied much more (about 10 to 15 mm on average by site, thicker in women). It concluded that 4 to 5 mm needles inserted straight in reach the fat with minimal risk of hitting muscle in virtually all adults, while needles of 8 mm or longer inserted straight in may often reach muscle in the limbs of men and of people with a BMI under 25.4
  • The recommendations follow. The FITTER recommendations, written by 183 diabetes experts from 54 countries in 2015, say the shortest needles, then the 4 mm pen needle and the 6 mm syringe needle, are safe, effective and less painful and should be the first choice for all patients, and that intramuscular injection should be avoided.3
  • Longer needles need a pinch. A syringe maker's technique sheet says 4 and 5 mm needles go straight in at 90° without pinching the skin (children aged 2 to 6 and very lean adults may still need a pinch), while 6, 8 and 12.7 mm needles go in at 90° into a pinched-up fold of skin.5

Insulin syringe needles come in 6, 8 and 12.7 mm, and some 1 mL syringes in 16 mm; 4 and 5 mm needles are pen needles.1317 So the range you will meet runs from about 4 to 13 mm, with 16 mm on a few 1 mL syringes. A 6 mm syringe needle is the shortest a syringe offers; it is also the one the FITTER panel named.13 Where on the body to inject, and what to expect, is a separate question; that page in this section is coming next.

Fixed versus detachable needles, and dead space

Insulin syringes have the needle permanently fixed to the barrel.1 Many other syringes, including many tuberculin and general-purpose ones, take a separate needle that screws or pushes onto the tip (a "Luer" fitting), which lets you change needles (our reasoning: this is what "detachable" means).

Dead space is the liquid left in the syringe's tip, the needle's hub and the needle itself when the plunger is pushed all the way in. It is drawn up but never injected. A study of delivery devices measured it: syringes with fixed ("staked") needles had relatively low dead space and wasted less, so more 0.1 mL doses could be drawn from a vial than with a detachable needle and syringe.13 A review of low dead space syringes notes that most of those available are 1 mL or smaller with fixed needles.14

What it means in practice (our reasoning): with a fixed-needle insulin syringe, the volume you read on the barrel is close to what goes in, and little is wasted. With a detachable needle the barrel marks still measure what the barrel delivers, but the hub and needle fill as well, so each draw uses up more of the vial than the marks show, and any air in the hub has to be cleared first.

How mL relates to units

On a U-100 syringe the rule is fixed: 1 mL is 100 units, so 1 unit is 0.01 mL.9 Converting needs nothing else:

  • mL to units: multiply by 100. 0.05 mL is 5 units; 0.4 mL is 40 units; 0.75 mL is 75 units.
  • units to mL: divide by 100. 12 units is 0.12 mL; 30 units, a full 0.3 mL barrel, is 0.3 mL.

What the units do not tell you is how much drug is in them. That depends on how much powder is in the vial and how much water went in; the units explainer works through the arithmetic and shows why "10 units" alone is not a dose. Any amount you plan to draw comes from a source: the dosing box on each encyclopedia entry names one, and a clinician should check it with you first.

Worked examples, in the calculator

These are arithmetic examples, not doses. The vial sizes and amounts were chosen to show the arithmetic and the syringe sizes, not taken from any peptide. Each link opens our reconstitution calculator on exactly these inputs, and every figure here is the one the calculator gives. The calculator's arithmetic: concentration = vial ÷ water; volume = amount ÷ concentration; units = volume × 100 on a U-100 syringe.

  1. A 0.3 mL syringe, a third full. 5 mg in a vial with 2 mL of water is 2.5 mg/mL. An amount of 0.25 mg is 0.25 ÷ 2.5 = 0.100 mL, which is 10.0 units: the tenth line on a 30-unit barrel. The vial holds 20 such amounts. Open in the calculator.
  2. A half-unit mark. 2 mg with 1 mL of water is 2 mg/mL. An amount of 0.13 mg is 0.065 mL, which is 6.5 units. That needs a 0.3 mL syringe with half-unit marks; on a whole-unit scale it falls between the 6 and 7 lines. Open in the calculator.
  3. A 0.5 mL syringe. 10 mg with 2 mL of water is 5 mg/mL. An amount of 2 mg is 0.400 mL, which is 40.0 units: too much for a 30-unit barrel, so a 50-unit (0.5 mL) syringe, 80% full. The vial holds 5 such amounts. Open in the calculator.
  4. A 1 mL syringe, between the lines. 6 mg with 3 mL of water is 2 mg/mL. An amount of 1.5 mg is 0.750 mL, which is 75.0 units. That fits only the 100-unit barrel, whose lines are two units apart, so 75 sits halfway between the 74 and 76 lines. The vial holds 4 such amounts. Open in the calculator.
  5. Too big for the syringe. 5 mg with 2 mL of water, as in the first example, but an amount of 1 mg: that is 0.400 mL, 40.0 units, more than a 30-unit syringe holds, and the calculator warns "Exceeds capacity". Choose a larger syringe, or the amount has to be drawn in two parts. Open in the calculator.

The same amount can sit on very different marks: 0.25 mg is 10 units at 2.5 mg/mL in the first example, but would be 5 units at 5 mg/mL (more concentrated) and 12.5 units at 2 mg/mL. That is why a number of units means nothing without the concentration, and why a change of water between vials changes every mark.

How to read the barrel

The plunger's rubber seal is what you read. Line the seal's top ring, the edge facing the needle, up with the mark you want: one patient-education source says the rubber seal "is matched with the line on the barrel to measure the correct amount", and a hospital system's insulin instructions say to pull the plunger back "until the tip of the plunger is at the line" for the number of units.616 The seal has more than one ring; reading the bottom ring, or the middle of the seal, over-reads by the seal's thickness (our reasoning).

  1. Find the scale first. Read the largest numbers on the barrel (30, 50 or 100) to know the size, then count the small lines between two numbers to learn what one line is worth: 1 unit, a half unit, or 2 units.12
  2. Hold it at eye level, with the needle pointing up when you check, so the line and the seal's top ring are level with your eye (our reasoning).
  3. Clear the bubbles. Tap the syringe so air bubbles rise to the top, push them out, and check the reading again; air takes up room that the liquid should fill.18
  4. Read the scale you have, not the one you expect. On a 1 mL syringe each line is 2 units; on a half-unit syringe each line is a half unit.12

Which size to pick for which volume

The rule from patient-education sources is to use the smallest syringe that holds the whole amount you need: the lines on small barrels are farther apart and easier to see.6 A maker's patient guide gives the ranges:17

Volume you will drawIn units (U-100)Syringe
Up to 0.3 mLUp to 30 units0.3 mL (30 units)17
0.3 to 0.5 mL30 to 50 units0.5 mL (50 units)17
More than 0.5 mL, up to 1 mL50 to 100 units1 mL (100 units)17
An amount that needs half unitse.g. 6.5 units0.3 mL with half-unit marks2

Why it matters: FDA's 2024 alert describes patients told to draw a small volume with a 1 mL syringe, 5 units (0.05 mL), who drew 50 units, ten times the amount, and recommends giving patients "the appropriate syringe size for the intended dose".15 A 5-unit amount fills a sixth of a 30-unit barrel but only a twentieth of a 100-unit one (our arithmetic).

A volume close to the top of a barrel is fine as long as it fits; if it is close, the next size up leaves room to clear air (our reasoning). For needle length, the shortest syringe needle, 6 mm, is the clinical first choice for injection under the skin.3

What to check on the box

  • U-100, not U-40.7
  • Barrel size in mL or units, matched to your volume.6
  • Scale: whole units, half units, or two units per line.12
  • Needle: gauge and length, such as 31G × 6 mm.13
  • Sterile, single-use and in date; the maker's filing describes the syringes as sterile and single use.1

Where to go next

This section's first-time path, in order:

  1. Buying syringes: the law in every state. Whether a pharmacy can sell you syringes without a prescription in each state and DC, the limits, and how buying works.
  2. Sizes, needles, and units versus mL. U-100 insulin syringes, barrel sizes, needle gauge and length, and how 1 mL relates to 100 units.
  3. Injecting under the skin. Where subcutaneous injections go, whether a needle in belly fat can hit a vein, blood and bruises, and when to get care.
  4. Reuse, sterility and disposal. Why needles are single-use, preparing the skin and the vial, never sharing, sharps containers and travel.
  5. Insulin syringe units explained. How the unit marks on a syringe relate to millilitres.
  6. Reconstitution calculator. Enter the vial, the water and the dose; it gives the mark on the syringe.
  7. Bacteriostatic water guide. The water used to mix a vial, from the approved products' labels.
  8. How to reconstitute peptides. Mixing a vial step by step.
  9. The grey-market buyer's guide. Twelve steps for a first-time buyer; step 7 covers water and syringes.
  10. Dosing guides and dosing boxes. Every encyclopedia entry has a dosing box that names its source; the dosage hub collects the guides.

Frequently asked questions

How many units are in 1 mL?

On a U-100 insulin syringe, 100 units: each unit is 0.01 mL. On a U-40 syringe, 40 units, each 0.025 mL.98

Is a 0.3 mL syringe the same as a 30-unit syringe?

Yes, for U-100 syringes: 0.3 mL holds 30 units, 0.5 mL holds 50 and 1 mL holds 100.16

What does 31G x 6 mm mean?

31 gauge, a very thin needle (a higher gauge is thinner), 6 millimetres long. The 6 mm syringe needle is the shortest syringe needle and the first choice in clinical recommendations for injection under the skin.113

Can I use a U-40 syringe with a U-100 calculation?

No. A U-40 unit is two and a half times bigger, so every mark means a different volume. Use a U-100 syringe with any U-100 conversion, including our calculator.8

Can I use a tuberculin syringe instead of an insulin syringe?

A tuberculin syringe is marked in mL, not units, so every conversion has to be done by hand, and regulators have recorded overdoses from units-for-mL mix-ups with them. If the source gives units, use a U-100 insulin syringe.9

Why do the numbers on a 1 mL syringe skip?

Its lines are 2 units apart, so an odd number of units falls halfway between two lines.1

Syringes hub Buying syringes: the law in every state Injecting under the skin Reuse, sterility and disposal Insulin syringe units explained Reconstitution calculator Bacteriostatic water guide How to reconstitute peptides The grey-market buyer's guide Dosing guides and dosing boxes Syringe glossary

Sources

All sources read October 28, 2026. A syringe maker's own FDA filings are cited for its products under our rule on manufacturers as sources; no retailer or peptide seller is named or linked.

  1. Becton, Dickinson and Company (the maker). 510(k) summary K190054, BD Insulin Syringe: sizes 0.3, 0.5 and 1 mL; scale marks; needles 27G to 31G, 6 to 16 mm, permanently attached; for subcutaneous injection of U-100 insulins. FDA clearance October 21, 2019. Read October 28, 2026. Source
  2. Becton, Dickinson and Company (the maker). 510(k) summary K212499, BD Insulin Syringe (0.3 mL): 1-unit and half-unit increments; 29G to 31G, 6, 8 and 12.7 mm. FDA clearance March 9, 2022. Read October 28, 2026. Source
  3. Frid AH, Kreugel G, Grassi G, et al. New Insulin Delivery Recommendations (FITTER). Mayo Clin Proc. 2016;91(9):1231-55. PMID 27594187. Read October 28, 2026. Source
  4. Gibney MA, Arce CH, Byron KJ, Hirsch LJ. Skin and subcutaneous adipose layer thickness in adults with diabetes at sites used for insulin injections: implications for needle length recommendations. Curr Med Res Opin. 2010;26(6):1519-30. PMID 20429833. Read October 28, 2026. Source
  5. BD (the maker). Injection technique for different needle lengths, patient sheet 190173624ITPK, © 2018. Read October 28, 2026. Source
  6. Columbia University Irving Medical Center health library (WebMD Ignite staff, clinician-reviewed). Insulin Syringes. Current as of May 1, 2026; read October 28, 2026. Source
  7. U.S. Food and Drug Administration, medical device recall database. Class 1 recall Z-0284-2009: 1 cc insulin syringes, package labelled for U-100 insulin containing syringes for U-40 insulin; initiated October 9, 2008. Read October 28, 2026. Source
  8. Merck Animal Health (maker of an FDA-approved U-40 veterinary insulin). Syringe administration (canine): use U-40 syringes only. Read October 28, 2026. Source
  9. Pharmaceuticals and Medical Devices Agency (PMDA, Japan). PMDA Medical Safety Information No. 23: Precautions in handling of insulin vial preparations (ensuring the use of insulin syringes). April 2011, revised November 2020. Read October 28, 2026. Source
  10. World Health Organization, TB Knowledge Sharing Platform. Annex 2: Tuberculin skin testing: administration, reading and interpretation (Module 5, children and adolescents). Read October 28, 2026. Source
  11. Minnesota Department of Health, STD/HIV/TB Section. Needle gauge and length (from syringe services programs in Minnesota). November 7, 2019; read October 28, 2026. Source
  12. Centers for Disease Control and Prevention. Vaccine administration job aid: subcutaneous (Subcut) injection. Dated September 25, 2025; read October 28, 2026. Source
  13. Jarrahian C, Rein-Weston A, Saxon G, et al. Vial usage, device dead space, vaccine wastage, and dose accuracy of intradermal delivery devices for inactivated poliovirus vaccine (IPV). Vaccine. 2017;35(14):1789-96. PMID 28189403. Read October 28, 2026. Source
  14. Zule WA. Low dead-space syringes for preventing HIV among people who inject drugs: promise and barriers. Curr Opin HIV AIDS. 2012;7(4):369-75. PMID 22627710. Read October 28, 2026. Source
  15. U.S. Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 26, 2024; read October 28, 2026. Source
  16. Virginia Mason Franciscan Health. How to self-administer insulin (vial and syringe steps). Undated page; read October 28, 2026. Source
  17. BD (the maker). Getting started with insulin injections, patient guide ID1105E, © 2005 (over two years old; used only for its size-by-volume ranges, which match the 510(k) barrel sizes). Read October 28, 2026. Source
  18. MedlinePlus (U.S. National Library of Medicine). Drawing medicine out of a vial. Review date January 14, 2026; read October 28, 2026. Source

This section explains syringes in plain words; it is not medical or legal advice. Grey Peptides sells nothing, links to no seller and rates none. If you have injected something and feel unwell, contact a clinician or Poison Help (1-800-222-1222 in the US); in an emergency call 911.