Dose calculation guide
This guide explains the math behind the calculator and helps you avoid the most common mistakes. It does not replace advice from your doctor or pharmacist.
How it works
The calculation has three steps:
- Concentration (mg/mL). For powder (lyophilized), divide the vial’s mg by the mL of water added. For a ready-made liquid, use the label value.
- Volume (mL). Divide the dose in mg by the concentration.
- Units. On a U-100 insulin syringe, 1 mL = 100 units. Multiply the volume by 100.
Example: 10 mg vial + 2 mL of bacteriostatic water = 5 mg/mL. A 2.5 mg dose is 2.5 ÷ 5 = 0.5 mL, which is 50 units.
How to read a U-100 syringe
U-100 insulin syringes are marked in units, not mL. The most common sizes are:
- 0.3 mL (30 units): a mark every 1 unit (some every 0.5). Good for small doses.
- 0.5 mL (50 units): a mark every 1 unit.
- 1 mL (100 units): a mark every 2 units. Holds more, but is less precise.
Read at the front edge of the plunger (the needle side), not the back. Remove air bubbles before injecting: they take up space and reduce the dose.
How much water to add to the vial
The amount of water only changes the concentration, not the amount of medication. More water makes the solution more dilute: each dose takes more units on the syringe.
- With little water, small doses become very few units and are hard to measure (below 2 units the relative error is large).
- With a lot of water, larger doses may not fit in the syringe.
Many people use 1 to 3 mL, but follow the instructions of whoever prescribed it. Write down how much you added: without that number you cannot calculate later.
Common mistakes
- Mixing up mg, mL and units. They are different units; the calculator converts them.
- Using a syringe that is not U-100. U-40 or other scales read differently.
- Forgetting how much water was added. Without it, the concentration is unknown.
- Reading from the back of the plunger. The correct reading is at the needle-side edge.
- A dose larger than the syringe. Use a larger syringe or split it into two injections, as instructed.
Frequently asked questions
What are units (IU) on the syringe?
They are the marks on a U-100 insulin syringe. Every 100 units equal 1 mL. The calculator converts your dose in mg into those marks.
How much bacteriostatic water should I add?
It depends on the vial and your doctor’s instructions. More water makes the solution more dilute, which increases the units per dose and makes small doses easier to measure.
Is my data stored?
Calculations stay in your browser. The share link carries the data inside the address itself and never goes through our servers.
Can I use it for any medication?
The math works for any injectable measured in mg with a U-100 syringe. The dose itself must always be prescribed by a professional.
Does the calculator tell me which dose to take?
No. It only converts the dose you enter. See the Medical disclaimer.