Why you need the label
A milligram is a weight and a milliliter is a volume, so there’s no single conversion between them. The link is the liquid’s concentration, printed on the label as an amount of drug in an amount of liquid, such as 125 mg per 5 mL or 25 mg/mL.
The formula
mL to give = dose ordered (mg) ÷ label mg × label mL
If the dose and the label use different units, convert them to the same one first. 1 g is 1,000 mg, and 1 mg is 1,000 mcg. The calculator does this for you.
mL to mg
To find how much drug is in a volume, read the label the other way: mg = volume (mL) × label mg ÷ label mL. For example, 10 mL of a liquid labeled 125 mg per 5 mL holds 250 mg.
Worked examples you can try
| Dose ordered | Label | mL to give |
|---|---|---|
| 12.5 mg | 25 mg per 1 mL | 0.5 mL |
| 250 mg | 125 mg per 5 mL | 10 mL |
| 160 mg | 80 mg per 1 mL | 2 mL |
| 0.25 mg | 50 mcg per 1 mL | 5 mL |
| 6.966 mg | 2.5 mg per 1 mL | 2.8 mL (2.7864) |
These come from Open RN’s Nursing Skills and WisTech Open’s Nursing Dosage Calculations, and the calculator’s tests check every one of them.
Rounding
Open RN rounds liquid doses of 1 mL or more to the tenth and doses under 1 mL to the hundredth, and says to follow your agency’s policy. The calculator shows the rounded answer and the unrounded working. For orders written per kilogram of body weight, use the dosage calculator.
Sources
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.9 Liquid concentrations. Working out mL from a label concentration (12.5 mg from 25 mg/mL is 0.5 mL). Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.4 Equivalencies. 1 g is 1,000 mg and 1 mg is 1,000 mcg. Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.5 Rounding. Rounding liquid doses to the tenth or hundredth. Checked October 16, 2026.
- WisTech Open, CC BY 4.0, Nursing Dosage Calculations, oral medication worked example and answer key. The liquid examples the calculator is tested against. Checked October 16, 2026.