Is 20 units the same as 20 mg?

No. Units measure the volume drawn into the syringe, not the amount of drug. 20 units on a U-100 syringe is always 0.2 mL — but the mg in that 0.2 mL depends entirely on the concentration of your vial.

How do I convert units to mg?

mg = units ÷ 100 × concentration (mg/mL), for a U-100 syringe. Concentration = vial mg ÷ bacteriostatic water mL. The calculator does both steps for you.

Education · not medical advice

Units vs mg: why they're not the same thing

The single most common (and most dangerous) mix-up in reconstituted medication: treating syringe "units" as if they were a dose.

"Units" is a volume, not a drug amount

An insulin-style syringe is marked in units. On a U-100 syringe — the standard — 100 units = 1 mL. So units are just a fine-grained way to measure volume. They say nothing about how much drug is in that volume until you know the concentration.

Concentration is what you set when you reconstitute

When you add bacteriostatic water to a vial of dry powder, you choose the concentration:

concentration (mg/mL) = vial strength (mg) ÷ bacteriostatic water (mL)

Put 2 mL of water into a 5 mg vial and you get 2.5 mg/mL. Put 1 mL into the same vial and you get 5 mg/mL — twice as strong. Same vial, same "20 units" drawn, double the dose.

A worked example

5 mg vial + 2 mL water → 2.5 mg/mL. You want 0.25 mg:

volume = 0.25 mg ÷ 2.5 mg/mL = 0.1 mL
units = 0.1 mL × 100 = 10 units

Reconstitute that same vial with 1 mL instead, and 0.25 mg is only 5 units. This is exactly why "just draw 20 units" advice from a forum is meaningless without the mix behind it — and why this calculator always asks for your vial mg and water first.

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Sources

This tool converts the numbers you enter — it never recommends a dose. Its constants are cited below; the warnings are arithmetic checks, not clinical limits.

Syringe "units" are a volume marking, not a drug amount: a U-100 syringe reads 100 units = 1 mL (U-40 = 40 units/mL). The calculator converts mg ↔ mL ↔ units on this standard — it never assumes units equal mg.Standard insulin concentration, U-100 = 100 units/mL (PMC5505430) ↗
The volume to draw is (drug ordered ÷ drug available) × the vial's total mL — i.e. dose ÷ concentration, where concentration (mg/mL) = vial mg ÷ bacteriostatic water mL. You enter every number; the tool only does this arithmetic.Dose-volume formula — StatPearls "Dose Calculation" (Bookshelf NBK430836) ↗
An opened (punctured) multi-dose vial is dated and discarded within 28 days unless the manufacturer states otherwise.CDC Injection Safety (per USP General Chapter <797>) ↗

Educational only — not medical advice and not a recommended dose. Always confirm your product, concentration, and dose with your prescriber or pharmacist.

Built by the makers of Glipath — the private GLP-1 companion for iPhone.