Weight-Based Dosage
Daily total and per-dose amounts from a mg/kg order and the patient’s weight.
15 mg/kg/day × 70 kg.
Divided into 3 doses a day.
Multiply the ordered milligrams per kilogram by the patient’s weight in kilograms, then divide by the number of doses a day. At 15 mg/kg/day for a 70 kg adult that is 1050 mg per day, given in three doses of 350 mg. Convert pounds to kilograms first.
Why doses are written per kilogram
Many drugs — antibiotics, anticoagulants, most paediatric medication — are dosed against body mass rather than as a fixed amount, because the volume the drug distributes into and the rate the body clears it both scale with size. A single fixed dose that suits an 80 kg adult could be a large overdose in a 12 kg child.
Two forms of order look similar but mean different things. mg/kg/day is the total for 24 hours, which you then divide between doses. mg/kg/dose is the amount given each time, which you multiply by the frequency to find the daily total. Reading one as the other is the single most consequential error in weight-based dosing, and it is why this calculator asks for the daily figure explicitly.
Weight in kilograms, always
Orders are written per kilogram, so a weight recorded in pounds must be converted before anything else: divide by 2.2046. A 154 lb patient is 69.9 kg. Using pounds directly inflates the dose by a factor of about 2.2.
Where an order is written per dose rather than per day, multiply by the frequency to find the daily total instead. 1 kg = 2.2046 lb.
Worked example: 15 mg/kg/day for a 70 kg adult, three times a day
Multiply for the daily amount, then divide by the frequency:
- 1 Record the weight in kilograms. The patient weighs 70 kg. A weight in pounds is divided by 2.2046 first.
- 2 Read the order carefully. The order is 15 mg/kg/day — a daily total, not a per-dose amount.
- 3 Multiply by the weight. 15 mg/kg/day × 70 kg = 1050 mg per day.
- 4 Divide by the number of doses. Three doses a day: 1050 mg ÷ 3 = 350 mg per dose.
- 5 Convert to a volume if it is a liquid. At 100 mg/mL, 350 mg ÷ 100 mg/mL = 3.5 mL per dose.
- 6 Check against the maximum dose. Compare the daily total with the drug reference’s ceiling — weight-based orders can exceed adult maximums in heavy patients.
Daily totals at common mg/kg orders
Total daily dose = mg/kg/day × weight. Divide by the frequency for the amount per dose.
| Weight | 10 mg/kg/day | 15 mg/kg/day | 25 mg/kg/day | 50 mg/kg/day |
|---|---|---|---|---|
| 10 kg | 100 mg | 150 mg | 250 mg | 500 mg |
| 20 kg | 200 mg | 300 mg | 500 mg | 1000 mg |
| 40 kg | 400 mg | 600 mg | 1000 mg | 2000 mg |
| 60 kg | 600 mg | 900 mg | 1500 mg | 3000 mg |
| 70 kg | 700 mg | 1050 mg | 1750 mg | 3500 mg |
| 90 kg | 900 mg | 1350 mg | 2250 mg | 4500 mg |
Ceilings and which weight to use
Weight-based dosing does not scale without limit. Many drugs carry a maximum daily dose that applies regardless of body mass, so a large patient can reach the adult ceiling before the mg/kg arithmetic would suggest. Always compare the calculated total against the reference maximum, and take the lower of the two.
Which weight to use is a clinical decision that varies by drug: some are dosed on actual body weight, others on ideal or adjusted body weight, and some paediatric drugs on body surface area instead. The calculation is the same either way — it is the weight you feed into it that changes.
This tool is a study aid for practising calculations. Doses must be calculated and independently verified by a qualified professional against the order, the drug reference, and local protocol.