How Many Units Are in 1 mL? Insulin Syringe Guide + Units-to-mg Converter

Key takeaways

• On a U-100 insulin syringe, 100 units = 1 mL, so 1 unit = 0.01 mL, 10 units = 0.1 mL and 50 units = 0.5 mL. The 0.3 mL and 0.5 mL syringes use the same scale and just stop at 30 and 50 units.
• "Units" on the syringe measure volume. How much peptide is in 10 units depends on the concentration of the vial, which depends on how it was reconstituted.
• To convert: mcg = units ÷ 100 × concentration (mcg per mL). To go the other way: units = mcg ÷ concentration × 100.
• Two syringes will get you the wrong dose: U-500 syringes (for concentrated insulin) and U-40 veterinary syringes. A tuberculin syringe is safe only if you work in mL.
• Use a new syringe for every injection, and the shortest needle that reaches subcutaneous fat; 6 mm is the shortest insulin-syringe needle.

The short answer

A standard insulin syringe in the United States is a U-100 syringe. The scale is built for insulin that contains 100 units in every millilitre, so the barrel is marked so that 100 units = 1 mL. From that one fact:

Units to mcg and mg: the converter

For peptides, the units on the syringe are only a volume. The amount of drug in that volume is set by the concentration of your vial, which you (or the pharmacy) set at reconstitution. Enter the concentration from your label and the converter does the rest.

The arithmetic behind it:

• mcg in a dose = units ÷ 100 × concentration (mcg per mL)
• units for a dose = dose mcg ÷ concentration × 100
• 1 mg = 1,000 mcg

Quick reference: what 10 units contains

The same 10 units can hold 100 mcg or 500 mcg. That is why "I take 10 units" means nothing without the concentration, and why a pharmacy that changes the reconstitution volume between refills must tell you.

If you still need to work out the concentration from the vial size and the water you added, the reconstitution calculator does that step first.

The three U-100 syringe sizes

BD, the largest manufacturer, sells insulin syringes in 1 mL, 0.5 mL and 0.3 mL capacities with 6 mm, 8 mm and 12.7 mm needles. All three use the U-100 scale; they differ in how far the scale goes and how finely it is marked.

Choose the smallest syringe that holds your dose. The marks are spread further apart, so reading 12 units on a 0.3 mL syringe is easier and more accurate than on a 1 mL syringe, where 12 falls between marks.

The "cc" on the packaging is the same as mL: 1 cubic centimetre is 1 millilitre.

Needle length and gauge

Needle gauge (G) is thickness; a higher number is thinner. Insulin syringes come in 28 to 31 G; 31 G is the thinnest and most comfortable for watery solutions, which includes every peptide.

Length is about reaching subcutaneous fat without entering muscle. The 2016 international insulin delivery recommendations (the FITTER consensus, published in Mayo Clinic Proceedings) concluded that 4 mm pen needles are suitable for all adults regardless of body size and that 6 mm is the shortest syringe needle available, and recommended against the 12.7 mm length for most people because of the risk of intramuscular injection, especially in lean patients and in the thigh. For insulin syringes, 6 mm or 8 mm at a 90-degree angle is the usual choice; a lifted skin fold is advised with 8 mm needles in slim people.

The syringes that will get you the wrong dose

U-500 syringes

Humulin R U-500 is a concentrated insulin with 500 units per mL, five times U-100. Measuring it in a U-100 syringe was a well-documented source of five-fold errors in either direction, and in 2016 FDA approved a dedicated U-500 syringe whose scale reads the actual U-500 dose. The reverse mistake also happens: if a U-500 syringe is used with U-100 insulin or with a peptide, every mark delivers one-fifth of the volume you think it does. Do not keep U-500 syringes in the house unless someone uses U-500 insulin, and never use one for a peptide.

U-40 veterinary syringes

Pet insulin is often U-40 (40 units per mL), and U-40 syringes, usually with red or orange markings, are scaled so that 40 units = 1 mL. "10 units" on a U-40 syringe is 0.25 mL, two and a half times the volume of 10 units on a U-100 syringe. They are sold online next to human syringes. Check the box: human insulin syringes are labeled U-100.

Tuberculin syringes

A tuberculin syringe is a 1 mL syringe graduated in hundredths of a millilitre (0.01 mL) with no unit scale. It is accurate, and in hospitals it is the recommended alternative for measuring U-500 insulin by volume. The hazard at home is reading "0.15" and thinking "15 units", which happens to be correct only because 0.15 mL is 15 U-100 units; with any other scale in your head, the conversion fails. If you use one, write doses in mL.

Reading the syringe correctly

1. Draw the dose with the needle in the solution and the vial upside down, then hold the syringe at eye level.
2. Read the line at the top edge of the black rubber tip of the plunger, not the slanted part.
3. Tap bubbles up and push them out before reading. Air in the barrel does not hurt a subcutaneous injection, but it takes the place of drug.
4. If the dose lands between marks on a 1 mL syringe (which is marked in twos), switch to a 0.3 mL or 0.5 mL syringe rather than estimating.
5. Use a new sterile syringe and needle for each injection. A reused needle is dull, painful and a route for bacteria into the vial.

Bottom line

100 units is 1 mL on any U-100 insulin syringe, and every unit is 0.01 mL. The drug in those units depends only on the concentration of the vial, so always convert from mcg through the concentration, never from "units" alone. Keep U-500 and U-40 syringes out of the picture, pick the smallest syringe that holds the dose, and use a fresh one every time.

Frequently asked questions

+−How many units are in 1 mL?

On a standard U-100 insulin syringe, 100 units equal 1 mL. Every unit mark is 0.01 mL. That is true for the 1 mL, 0.5 mL and 0.3 mL sizes; the smaller syringes simply stop at 50 and 30 units.

+−How many units is 0.5 mL?

50 units on a U-100 insulin syringe. 0.25 mL is 25 units, 0.1 mL is 10 units and 0.05 mL is 5 units.

+−How many mg is 10 units?

It depends entirely on the concentration of the solution. 10 units is 0.1 mL. At 1,000 mcg per mL that is 100 mcg (0.1 mg); at 2,000 mcg per mL it is 200 mcg; at 5,000 mcg per mL it is 500 mcg (0.5 mg). Units measure volume, not drug.

+−Is 100 units equal to 1 mL on every syringe?

No. It is true for U-100 insulin syringes, which are the standard in the United States. A U-500 syringe for concentrated insulin and a U-40 veterinary syringe use different scales, and a tuberculin syringe is marked in mL with no units at all. Using the wrong one produces a 2.5-fold to 5-fold dosing error.

+−Why does my pharmacy write the dose in units instead of mL?

Because insulin syringes are marked in units, and 15 units is easier to read and remember than 0.15 mL. The two are the same measurement. A good label gives the dose in mcg or mg as well, so you can check the arithmetic if the concentration ever changes.

+−What is a half-unit syringe?

A 0.3 mL or 0.5 mL insulin syringe with marks every half unit (0.005 mL). It makes small or odd doses, such as 7.5 units, easier to measure accurately. The 1 mL syringe is usually marked in 2-unit steps.

+−Can I use a tuberculin syringe for peptides?

You can, if you convert the dose to mL: a tuberculin syringe is a 1 mL syringe marked in 0.01 mL steps with no unit scale. The risk is reading an mL mark as a unit mark. Most clinicians prescribe insulin syringes for subcutaneous peptides because the dose, the label and the syringe all speak the same language.

Sources

1. SourceBD. BD insulin syringes with BD Ultra-Fine needle: 1 mL, 0.5 mL and 0.3 mL capacities; 6 mm, 8 mm and 12.7 mm needle lengths (product family page).
2. RegulatoryFDA. FDA approves dedicated syringe to be used with Humulin R U-500 insulin. Drug Safety and Availability, 2016. (2016)
3. LabelEli Lilly. Humulin R U-500 (insulin human) injection, 500 units per mL. Prescribing information (DailyMed).
4. SourceISMP Consumer Med Safety. U-500 insulin errors: why using a U-100 syringe with U-500 insulin causes 5-fold dosing errors.
5. GuidelineFrid AH et al. New insulin delivery recommendations. Mayo Clin Proc 2016;91:1231-55. (2016)
6. GuidelineCDC. Injection safety: clinical safety guidance for healthcare providers. Reviewed March 2024. (2024)