Dosage by Weight Calculator
Single dose, liquid volume, and daily total from weight and mg per kg.
What it does
Weight based dosing scales medicine to body size instead of age brackets. A 20 kg child at 10 mg per kg needs 200 mg per dose, and at 50 mg per mL that measures 4 mL in the syringe. The tool above runs that chain from weight through daily total.
The metric default mirrors a routine pediatric setup: 20 kg, 10 mg per kg, 50 mg per mL, twice daily. One dose is 200 mg, the syringe reads 4 mL, and the day totals 400 mg. Imperial mode converts pounds behind the scenes and lands on the same milliliters.
Each input has one job. Weight sets the scale, the prescribed rate sets milligrams per kilo, bottle strength converts milligrams to milliliters, and frequency builds the day. Mixups almost always come from skipping a link, such as dosing by pounds with a kilo rate.
This page checks arithmetic and nothing else. Labels, kidney and liver limits, maximum single doses, and interactions all overrule the math. Confirm every figure with the prescriber or pharmacist before measuring anything, and use an oral syringe rather than a kitchen spoon.
How to use
- Pick metric (kg) or imperial (lb).
- Type the current body weight.
- Type the prescribed mg per kg rate from the label.
- Type the bottle strength in mg per mL.
- Set doses per day and read dose, mL, and daily total.
Formula
- weight (kg)
- Current body weight; pounds divide by 2.20462 first.
- rate (mg/kg)
- Prescribed milligrams per kilo per dose.
- strength (mg/mL)
- Bottle concentration linking mg to mL.
Single dose math only. Caps, organ limits, and loading doses from the label or clinician stand above these products.
Worked example
Example: 20 kg at 10 mg/kg
Metric defaults: 20 kg, 10 mg/kg, 50 mg/mL, twice daily.
- Dose: 20 x 10 = 200 mg.
- Volume: 200 / 50 = 4 mL.
- Daily: 200 x 2 = 400 mg/day.
- Cross check: 4 mL twice is 8 mL at 50 mg/mL = 400 mg.
- Tool shows 200 mg, 4 mL, 400 mg/day.
Reference table
| Weight | Dose | At 50 mg/mL |
|---|---|---|
| 10 kg | 100 mg | 2 mL |
| 20 kg | 200 mg | 4 mL |
| 30 kg | 300 mg | 6 mL |
| 40 kg | 400 mg | 8 mL |
Limits
- Math aid only, not prescribing advice.
- Ignores max doses and organ based caps.
- Actual versus ideal weight choice follows the drug label.
- Syringe markings round real pours to 0.1 mL or coarser.
Common mistakes
- Dosing pounds with a kilo rate, doubling wrong.
- Reading mg as mL without dividing by strength.
- Using a spoon instead of an oral syringe.
- Forgetting the daily total across split doses.
- Weighing once and dosing that weight for months.
Try it yourself
Quick checks. Answers sit tucked under each question.
15 kg at 5 mg/kg, 25 mg/mL, once daily?
75 mg, 75 / 25 = 3 mL, daily 75 mg/day.
30 kg at 8 mg/kg, 40 mg/mL, 3 times daily?
240 mg, 6 mL, daily 720 mg/day.
44.1 lb at 10 mg/kg?
44.1 lb is 20 kg, so 200 mg per dose.
Questions
How do I convert lb to kg?
Divide pounds by 2.20462. 44.1 lb is 20 kg.
What if the bottle shows percent?
Convert first: 1% w/v is 10 mg per mL.
Can I split a dose across the day?
Only as the label directs; daily totals still cap each part.
Why weigh again each visit?
Children gain fast, and stale weights under or overdose.
Syringe or cup?
Syringe for small volumes; cups misread below 5 mL.
Does this replace the pharmacist?
No. Confirm every dose with the prescriber or pharmacist.
Sources
- FDA: Labeling for Dosing Based on Weight or Body Surface Area
- HHS Guidance Portal: Dose Banding by Weight or BSA