Why the mark is not obvious
Your prescription says a dose in milligrams or units. Your syringe is marked 0 to 100. Those are two different kinds of number. One is an amount of medication; the other is a volume of liquid. What connects them is the concentration printed on your vial, and if the vial is a powder, the concentration is whatever you make it by choosing how much liquid to add.
Nearly every wrong draw is a case of treating one kind of number as the other: milligrams typed where micrograms were meant, a U-40 syringe read as if it were U-100, a syringe "unit" taken to be a unit of medication, or a single-dose vial drawn as if it held several doses. The calculator does the division, draws the answer on the syringe you actually hold, and prints every intermediate number so you can check it with a pencil. It does not know what dose is right for you, which is why the dose field is always left for you to fill in, and why nothing here suggests, stores or judges one. The about page explains the rest of those limits.
By medication
Choose a medication and the vials on its FDA label appear as choices, with the concentration already filled in from the label. The dose is always typed by you.
Check the answer yourself
Three lines of arithmetic. If your pencil and the calculator disagree, stop and find out why before you draw anything.
A liquid vial skips the first line, because the label already prints the concentration. A U-40 syringe changes the last line to × 40. The rounding rules and the worked examples the site is checked against are on how it works.
How much bacteriostatic water do I add to a powder vial?
The amount the label or your pharmacy told you to add. It is not a free choice, and it is not a detail: the liquid you add sets the concentration, and the concentration decides where your dose sits on the syringe. A 5 mg vial with 1 mL added is 5 mg/mL; the same vial with 2 mL added is 2.5 mg/mL, and the same dose becomes twice as many marks. More liquid makes a small dose easier to read; less liquid keeps a large dose inside the syringe. The calculator flags a draw that is smaller than one mark or larger than the syringe, and shows the volume that would fix it, but the volume you actually use should be the one in your instructions. Somatropin and HCG labels give a range; the somatropin and HCG pages show what those ranges do to the mark. Storage and handling of the water itself are on the bacteriostatic water labels on DailyMed.
mg, mcg, units and IU: four words, three meanings
Milligrams and micrograms are both mass, and a microgram is a thousandth of a milligram. Typing 250 mcg into a field that expects mg is a thousand-fold error, which is why the unit is a separate choice with no default and your dose is read back in both. Units and IU measure a medication's activity, not its mass; there is no conversion between units and milligrams that this calculator will do, because there is no general one. And the "units" printed on an insulin syringe are a third thing again: a volume scale, 100 to the millilitre on a U-100 syringe and 40 on a U-40. When someone says "50 units" you cannot tell which of the three they mean until you know what is in the vial.
Which syringe?
The one your pharmacy gave you. What matters is the U number on the box and the barrel, and then the size.
U-100 and U-40 syringes look the same and read differently; the U-40 scale exists for veterinary insulins such as Vetsulin. U-500 insulin has its own syringe and its label says not to convert, so it is not offered here. More on the insulin page.
Mixing tools
Worked answers
Common questions
Who is this for?
Anyone drawing a prescribed medication out of a vial with an insulin syringe: people on Zepbound vials, people with diabetes and pet owners sorting out U-100 from U-40, fertility and growth-hormone patients whose diluent sets the concentration, and caregivers doing it for someone else. Nursing and pharmacy students use it as a worked example. If your vial is compounded, the calculator still works; you type the concentration from your own pharmacy label instead of picking a preset.
Is a syringe unit the same as an IU?
No, and this is the mix-up that produces ten-fold errors with HCG. A syringe unit is a hundredth of a millilitre on a U-100 syringe. An IU measures how active the medication is. 500 units of HCG at 1,000 units per mL is 0.5 mL, the 50 mark, not the 500 mark, which does not exist.
My vial says 5 mg but no mg/mL. What do I enter?
That depends on whether you add the liquid. If you do, choose powder and type 5 mg and the millilitres you add; the calculator divides. If it came as a liquid, the label should say the volume somewhere, for example 5 mg in 1 mL, which is 5 mg/mL. If it gives an amount and nothing else, the pharmacy that filled it is the only place the volume can come from.
What mistakes does it catch?
Milligrams typed as micrograms: the dose is read back in both, and the unit has no default unless a label preset sets it. A U-40 syringe used with a U-100 preset or the other way round. A dose larger than the vial. A draw larger than the syringe, with the syringes that would fit listed. A draw below the smallest mark, shown hatched on the drawing. What it cannot catch is a wrong number that is internally consistent, which is why the read-back line is there.