mg to units — it depends on the concentration

There is no fixed number of units in a milligram. A milligram is an amount of drug. A unit is a volume of liquid — one hundredth of a millilitre. How many units carry a milligram depends on how much drug is dissolved in each millilitre, which is the concentration on your vial.

units = mg ÷ (mg/mL) × 100. So 1 mg at 10 mg/mL is 10 units. The same 1 mg at 2.5 mg/mL is 40 units. Same drug, four times the liquid.

mg → mL → unitsU-100 syringe

Dose mg

Concentration mg/mL

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Dose and concentration. The concentration is on the vial label, or it's vial mg ÷ water mL.

The formula, whichever way round you need it

You have	You want	Sum	Example
dose in mg, concentration	mL	mg ÷ (mg/mL)	2.5 ÷ 10 = 0.25 mL
mL	units	mL × 100	0.25 × 100 = 25 units
units, concentration	mg	units ÷ 100 × (mg/mL)	25 ÷ 100 × 10 = 2.5 mg
vial mg, water mL	concentration	vial ÷ water	20 ÷ 2 = 10 mg/mL

The ×100 is the only place the syringe comes in, and it's the definition of a U-100 syringe: 100 units to the millilitre. If your syringe says U-40, it's ×40 — see units to mL.

mg to units at the common concentrations

Units on a U-100 syringe. Red: more than one syringe holds. Find your vial's mg/mL along the top.
Dose	1 mg/mL	2.5 mg/mL	5 mg/mL	10 mg/mL	20 mg/mL	40 mg/mL
0.25 mg	25	10	5	2.5	1.25	0.63
0.5 mg	50	20	10	5	2.5	1.25
1 mg	100	40	20	10	5	2.5
2 mg	200 ✗	80	40	20	10	5
2.5 mg	250 ✗	100	50	25	12.5	6.25
5 mg	500 ✗	200 ✗	100	50	25	12.5
7.5 mg	750 ✗	300 ✗	150 ✗	75	37.5	18.75
10 mg	1000 ✗	400 ✗	200 ✗	100	50	25
12.5 mg	1250 ✗	500 ✗	250 ✗	125 ✗	62.5	31.25
15 mg	1500 ✗	600 ✗	300 ✗	150 ✗	75	37.5
20 mg	2000 ✗	800 ✗	400 ✗	200 ✗	100	50
25 mg	2500 ✗	1000 ✗	500 ✗	250 ✗	125 ✗	62.5

Red on the right means it doesn't fit in a syringe. Red on the left means it's under 4 units — a draw so small that one mark is a large percentage of the dose. Both are fixed by a different concentration, not by better aim.

Units to mg

The other direction: you drew a number of units and want to know the milligrams. Same concentration, opposite sum.

Units → mgU-100 syringe

Units drawn

Concentration mg/mL

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Units and the vial's concentration.

For the two drugs people actually ask this about, with the common numbers written out: tirzepatide units to mg · semaglutide units to mg.

Insulin is the exception

If the drug is insulin, you don't convert. Insulin is prescribed in units, the U-100 syringe was built for it, and the number your prescriber gave you is the mark you draw to. "1 mg of insulin" isn't a dose anyone is given. The conversion on this page is for drugs that come in milligrams — compounded GLP-1s, mostly — and get drawn with an insulin syringe because it's the right size.

mg to mL, when there's no syringe involved

Drop the ×100. mL = mg ÷ (mg/mL). A 250 mg dose from a 50 mg/mL vial is 5 mL. It's the same first line of the table above; the units step is only there because insulin syringes are marked in units rather than millilitres.

If you're starting from a vial and the water you added rather than a printed concentration, the main calculator does the whole chain in one go — and the reconstitution page covers the mixing.