An insulin syringe measures volume, not medicine. The numbers printed on the barrel are units of liquid, and they mean the same thing no matter what is in the vial. That one fact resolves most of the confusion people have the first time they hold one, so it is worth stating before anything else.
This page explains what the markings are and how to convert between them. It does not tell you what to take — that is between you and whoever prescribes for you.
The label describes the scale, not the contents. A U-100 syringe divides one millilitre into a hundred equal parts, and each part is called a unit.
Two consequences follow, and both are worth sitting with.
Units are volume. A unit is a hundredth of a millilitre — a measure of how much liquid, carrying no information about how much drug. Two syringes drawn to the same mark from two differently concentrated vials hold different amounts of substance. The syringe cannot tell the difference, and neither can you by looking.
The conversion never changes. Multiply millilitres by 100 for units; divide units by 100 for millilitres. Nothing about the liquid alters this, because the scale is a property of the barrel.
Insulin syringes come in three common sizes. All three share the U-100 scale — what changes is how finely the barrel is marked, because a smaller barrel has more room to print lines.
| Barrel | Holds | Numbered at |
|---|---|---|
| 0.3 mL | 30 units | half-unit or 1-unit intervals |
| 0.5 mL | 50 units | 1-unit intervals |
| 1.0 mL | 100 units | 2-unit intervals |
The intervals above follow Healthline’s syringe-size reference, which states that 0.3 mL syringes are “numbered at 1/2-unit or 1-unit intervals”, 0.5 mL syringes “at 1-unit intervals”, and 1.0 mL syringes “at 2 units per interval”.
The practical consequence. A volume you can read precisely on a 0.3 mL barrel may fall between printed lines on a 1 mL barrel. The liquid is identical; the resolution is not. If you find yourself squinting for a mark that is not there, the barrel is too large for the measurement rather than the other way round.
Check your own syringe rather than trusting the table. Manufacturers vary, and some barrels are marked more finely than the numbering suggests — numbered every 2 units can still mean a printed line every 1 unit.
Work in units and the arithmetic disappears, because the scale is already in units. The question “where is 0.18 mL?” is better asked as “where is 18 units?”
On a 0.5 mL barrel marked every unit, that is the eighteenth line. On a 1 mL barrel numbered every two units, it is the ninth numbered mark.
Read at the plunger’s forward edge — the flat face nearest the needle, the one the liquid touches. Hold the syringe at eye level; reading at an angle shifts the apparent position of the plunger against the scale.
Deliberately generic — substitute your own figures. The method is the point, not the numbers.
Suppose a vial contains 5 mg of powder and you add 2 mL of liquid to it.
Step 1 — concentration. Divide the powder by the liquid:
Step 2 — volume. Divide the amount you intend to draw by the concentration. For 1 mg:
Step 3 — units. Multiply by 100:
Forty units fits a 0.5 mL barrel and lands on a printed line on all three sizes. It is the position drawn in the diagram above.
Change any input and everything downstream moves. Add 3 mL of liquid instead of 2 and the concentration becomes about 1.67 mg/mL, so the same 1 mg is 0.6 mL, or 60 units — which no longer fits a 0.5 mL barrel at all. The volume of liquid you add matters as much as the powder in the vial.
It often will. Arithmetic does not respect graduations.
A syringe can only be read to the lines printed on it. Where a volume falls between two marks, the honest options are to draw to the nearest line and accept that you have measured slightly more or slightly less than you calculated, or to change the volume of liquid you reconstitute with so the arithmetic lands on a mark.
Vialkeeper always rounds down to the nearest line, never up, and tells you the amount you would actually be measuring rather than hiding the difference. The reasoning is written out on the methodology page.
Worth being explicit, because the scale’s simplicity invites over-trust.
Every one of those is your record to keep, which is the problem Vialkeeper exists to solve.
Vialkeeper works this out for you and keeps the record — what the vial held, what you added, what you drew, and how much is left.
Free, and no account needed.