Insulin syringe units, explained

The one fact that matters

U-100 means 100 units per 1 mL. So 1 unit = 0.01 mL on every U-100 syringe, no matter the barrel size. A 30U syringe is 0.3 mL total, a 50U is 0.5 mL, a 100U is 1 mL. The number printed last on the barrel is the capacity.

Converting units to mL and back

Divide units by 100 for mL: 12 units = 0.12 mL. Multiply mL by 100 for units: 0.25 mL = 25 units. If you only have a 3 mL syringe, convert the calculator's result to mL and draw that.

Reading the barrel

Read at the top edge of the black plunger tip, at eye level. On a 100U syringe each printed line is 2 units, so 15 units sits halfway between the 14 and 16 marks. Pull slightly past the line, then push back to it to clear air.

Needle gauge and length

Insulin syringes are sold as one piece with the needle attached. Common choices are 29G, 30G and 31G with a 5/16 inch (8 mm) or 1/2 inch (12.7 mm) needle. Higher gauge is thinner. The gauge has no effect on the arithmetic.

Why small draws are a problem

A half-unit read error is 10% of a 5-unit draw and 2% of a 25-unit draw. That is why the calculator warns under 4 units and suggests more water: the goal is a draw between roughly 10 units and the syringe's capacity.

U-40 and U-50 syringes

These exist for veterinary insulin and use a different scale. The calculator assumes U-100. If your barrel says U-40, the unit lines are not 0.01 mL and the numbers will be wrong.

Common questions

How many mL is 10 units on an insulin syringe?

0.10 mL. On any U-100 syringe, units divided by 100 is mL.

What is the difference between a 30U and a 100U syringe?

Only the capacity and line spacing. Both are U-100, so a unit is 0.01 mL on each. The 30U holds 0.3 mL and is marked every unit; the 100U holds 1 mL and is marked every 2 units.

Can I use a 3 mL syringe for peptides?

For adding water, yes. For measuring a draw, convert units to mL first (units divided by 100) since 3 mL syringes are marked in 0.1 mL steps.

What needle size is used for subcutaneous injections?

Insulin syringes come with 29G to 31G needles, 5/16 or 1/2 inch long. Thinner (higher gauge) and shorter needles are the common choice for subcutaneous use.