# How do I work out a dosage by weight?

Does the arithmetic of a weight-based dosage: a prescribed dose in mg/kg times body weight, split per dose, and the liquid volume from the concentration on the label. It suggests no doses.

- Page: https://www.acalculator.org/health/dosage-calculator
- JSON spec: https://www.acalculator.org/health/dosage-calculator.json
- Version: fc645c10a4b2

## Default answer

Example with the default inputs (The prescribed dose is mg/kg per dose, Prescribed dose (mg/kg) 15, Weight 44.0924524369755 lb, Doses per day 4, Liquid strength: mg 160, Liquid strength: per mL 5): Each dose is 300 mg, 1,200 mg per day.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| basis | The prescribed dose is | Whether the prescribed mg/kg is for each dose, or for the whole day split into doses. |
| mgkg | Prescribed dose (mg/kg) | The dose in milligrams per kilogram of body weight, as written in the prescription. |
| weight | Weight | Body weight. |
| n | Doses per day | How many doses are given in 24 hours. |
| cmg | Liquid strength: mg | Optional: the milligrams of drug in the stated volume on the label, for example 160 in "160 mg per 5 mL". |
| cml | Liquid strength: per mL | Optional: the volume in mL that holds those milligrams, for example 5 in "160 mg per 5 mL". |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| dose | Each dose | The amount per dose in mg, to 2 decimals. |
| daily | Total per day | The amount per day in mg, to 2 decimals. |
| volume | Volume per dose (mL) | The liquid volume that holds one dose, in mL, to 2 decimals. Shown when the liquid strength is given. |

## Method

Per dose: mg/kg × weight (kg), or mg/kg per day × weight ÷ doses per day. Volume (mL) = dose (mg) ÷ (label mg ÷ label mL).

## Assumptions

- The dose in mg/kg comes from the prescription or the drug label. The calculator does not suggest, check, or cap doses.
- Pounds are converted to kilograms exactly (1 lb = 0.45359237 kg).
- Each amount and volume is rounded to 2 decimals, halves up; each is worked out from unrounded values.

## Worked examples

1. basis = dose, mgkg = 15, weight = 20, n = 4, cmg = 160, cml = 5 gives dose = 300, daily = 1,200, volume = 9.38. Source: Gage and Preuss 2023, StatPearls, Dose Calculation (https://www.ncbi.nlm.nih.gov/books/NBK430836/), desired ÷ have × quantity: 15 × 20 = 300 mg.
2. basis = day, mgkg = 40, weight = 19.958064, n = 2, cmg = 250, cml = 5 gives daily = 798.32, dose = 399.16, volume = 7.98.
3. basis = dose, mgkg = 0.1, weight = 70, n = 1, cmg = 10, cml = 1 gives dose = 7, daily = 7, volume = 0.7.
4. basis = dose, mgkg = 5, weight = 12, n = 3 gives dose = 60, daily = 180.

## FAQ

### How do you calculate a dose by weight?

Multiply the prescribed dose in mg/kg by the body weight in kilograms. For 15 mg/kg and 20 kg: 15 × 20 = 300 mg. If the weight is in pounds, divide by 2.2046 first (1 lb = 0.45359237 kg).

### How do I turn mg into mL of a liquid?

Use the strength on the label. Volume = dose ÷ strength. For a 300 mg dose of a liquid with 160 mg in 5 mL: 300 ÷ 160 × 5 = 9.375 mL. Nurses call this "desired over have times quantity".

### What does mg/kg per day mean?

A daily dose is the total for 24 hours. Split it by the number of doses. 40 mg/kg per day for a 20 kg child in 2 doses is 800 mg a day, or 400 mg per dose.

### Does this calculator tell me what dose to take?

No. It has no drug list and no suggested doses. The dose comes from your prescriber, pharmacist, or the drug label. Use the measuring device that comes with a liquid medicine, and ask your pharmacist if a result does not match your instructions.

## Sources

- Gage CB, Preuss CV. Dose Calculation. In: StatPearls. StatPearls Publishing; 2023. https://www.ncbi.nlm.nih.gov/books/NBK430836/ (retrieved 2026-09-29)
- Toney-Butler TJ, Nicolas S, Wilcox L. Dose Calculation Ratio and Proportion Method. In: StatPearls. StatPearls Publishing; 2023. https://www.ncbi.nlm.nih.gov/books/NBK499884/ (retrieved 2026-09-29)
- National Institute of Standards and Technology. NIST Special Publication 811, Appendix B (1 lb = 0.45359237 kg). https://www.nist.gov/pml/special-publication-811