How do I work out a dosage by weight?
Enter the prescribed dose in mg/kg, the body weight, and the liquid strength on the label to work out the milligrams and milliliters per dose. The calculator does the arithmetic only and suggests no doses. For information only. This is not medical advice or a diagnosis. Do not use it to make treatment decisions without a clinician. No licensed clinician has reviewed this page.
- Each dose
- 300
Each dose is 300 mg, 1,200 mg per day.
- Total per day
- 1,200
- Volume per dose (mL)
- 9.38
Each dose: 300. Each dose is 300 mg, 1,200 mg per day.
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
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.
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.
Method: Per dose: mg/kg × weight (kg), or mg/kg per day × weight ÷ doses per day. Volume (mL) = dose (mg) ÷ (label mg ÷ label mL).
- 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
Each example is checked against the calculator on every build.
- The prescribed dose is mg/kg per dose, Prescribed dose (mg/kg) 15, Weight 44.09 lb, Doses per day 4, Liquid strength: mg 160, Liquid strength: per mL 5 gives Each dose 300, Total per day 1,200, Volume per dose (mL) 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
- The prescribed dose is mg/kg per day, Prescribed dose (mg/kg) 40, Weight 44 lb, Doses per day 2, Liquid strength: mg 250, Liquid strength: per mL 5 gives Total per day 798.32, Each dose 399.16, Volume per dose (mL) 7.98.
- The prescribed dose is mg/kg per dose, Prescribed dose (mg/kg) 0.1, Weight 154.3 lb, Doses per day 1, Liquid strength: mg 10, Liquid strength: per mL 1 gives Each dose 7, Total per day 7, Volume per dose (mL) 0.7.
- The prescribed dose is mg/kg per dose, Prescribed dose (mg/kg) 5, Weight 26.46 lb, Doses per day 3 gives Each dose 60, Total per day 180.
Medical disclaimer
This calculator is for information and education only. It is not medical advice, and its result is not a diagnosis. Do not use it to start, stop, or change a treatment without a clinician who knows your health history. No licensed clinician has reviewed this page. This site's editors checked the formulas and worked examples against published sources. If you think you have a medical emergency, call your local emergency number.
When this does not apply. The page does arithmetic on a dose that a prescriber, a pharmacist, or the drug label gives you. It does not know any drug, does not check a dose against a maximum, and does not replace the dosing device and instructions that come with a medicine.
The calculator does arithmetic on the numbers you type. It has no drug list, and it does not suggest, check, or limit a dose.
How the dose is worked out
With the weight W in kilograms (pounds × 0.45359237) and N doses per day:
- mg/kg per dose: each dose = mg/kg × W; per day = each dose × N.
- mg/kg per day: per day = mg/kg × W; each dose = per day ÷ N.
- Volume per dose: volume (mL) = each dose (mg) × label mL ÷ label mg. This is the "desired over have times quantity" formula (D ÷ H × Q).
Give both parts of the liquid strength (mg and mL) or neither; with only one, there is no answer. Each amount is worked out from unrounded values and then rounded to 2 decimal places, halves up, read from the decimal value (within 1e-9, relative, of a half counts as the half).
Inputs the calculator accepts
Dose 0.0001 to 1,000 mg/kg; weight 0.3 to 350 kg; 1 to 24 doses a day; label strength 0.001 to 100,000 mg in 0.01 to 10,000 mL.
Worked examples by hand
15 mg/kg per dose, 20 kg, 4 doses a day, 160 mg per 5 mL. Each dose: 15 × 20 = 300 mg. Per day: 300 × 4 = 1,200 mg. Volume: 300 × 5 ÷ 160 = 9.375 = 9.38 mL.
40 mg/kg per day, 44 lb, 2 doses, 250 mg per 5 mL. 44 × 0.45359237 = 19.958 kg. Per day: 40 × 19.958 = 798.32 mg. Each dose: ÷ 2 = 399.16 mg. Volume: 399.16 × 5 ÷ 250 = 7.98 mL.
0.1 mg/kg, 70 kg, once a day, 10 mg per 1 mL. 0.1 × 70 = 7 mg; 7 × 1 ÷ 10 = 0.70 mL.
5 mg/kg per dose, 12 kg, 3 doses, no liquid strength. 60 mg per dose, 180 mg per day.
Other questions people ask
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.