Medication Dosage Calculator
One Multiplication, Considerable Consequence
Weight-based dosing is the most common arithmetic in clinical medicine and the most consequential multiplication most people never think about. A prescriber reads a rate in mg/kg from the drug's labelling, multiplies by the patient's weight, and a dose falls out. The formula is trivial. Getting the inputs right — the correct rate for the correct indication, the correct weight convention, the correct units on both sides — is where the real work lies, and where errors concentrate.
The Formula
This calculator is deliberately generic. It is not tied to any medication, applies no maximum dose limit, and knows nothing about indication, renal or hepatic adjustment, or drug-specific rules. It multiplies the rate you supply by the weight you supply. Everything that makes a dose correct rather than merely arithmetically consistent lives outside this calculation.
Computed Dose Reference
Every cell below is the product of the row weight and the column rate:
| Weight | 5 mg/kg | 10 mg/kg | 15 mg/kg | 25 mg/kg |
|---|---|---|---|---|
| 5 kg | 25 | 50 | 75 | 125 |
| 10 kg | 50 | 100 | 150 | 250 |
| 20 kg | 100 | 200 | 300 | 500 |
| 40 kg | 200 | 400 | 600 | 1000 |
| 70 kg | 350 | 700 | 1050 | 1750 |
| 90 kg | 450 | 900 | 1350 | 2250 |
The rates shown are illustrative arithmetic, not recommendations for any drug. A real mg/kg rate comes from the medication's own labelling for the specific indication.
Where the Errors Live
- Pounds entered as kilograms — a 2.2× overdose, and the single most common weight-based dosing error. Divide pounds by 2.20462 first.
- Per-dose versus per-day — a rate written as mg/kg/day divided into three doses is a very different number from mg/kg per dose. The units are the instruction.
- Ceiling doses ignored — many drugs stop scaling above a defined weight, so linear multiplication overshoots in larger patients. This calculator applies no such ceiling.
- Actual versus adjusted weight — some agents dose on ideal or adjusted body weight rather than actual, particularly in obesity.
- Concentration confusion — a dose in mg still has to become a volume in mL, which is a second calculation with its own failure modes.
Why mg/kg at All?
Weight is a reasonable first-order proxy for the volume a drug will distribute into, so scaling by mass keeps concentration in a comparable range across body sizes. It is not the only defensible denominator: cytotoxic agents are usually dosed per square metre of body surface area, on the reasoning that surface area tracks metabolic rate better than mass does. The choice of denominator is itself a pharmacological judgement, made drug by drug.
Where This Calculation Appears
- Paediatrics — nearly all children's dosing is weight-based, because the weight range across childhood is enormous.
- Infusions and induction agents — anaesthetic and critical care dosing is routinely expressed per kilogram.
- Anticoagulation — several agents use weight-based regimens.
- Checking your own arithmetic — an independent recalculation is a standard safety practice, and this tool is fit for that narrow purpose.
How to Use This Calculator
- Enter the Dose Rate (mg per kg) taken from the medication's own labelling.
- Enter the Weight (kg). Confirm the unit before you type — if you have pounds, convert first by dividing by 2.20462.
- Select Calculate. The result is the total dose in mg, with the multiplication shown so it can be checked at a glance.
Related Calculations
For the historical ratio-based approach to children's doses, see the Pediatric Dosage Calculator. Where the correct denominator is surface area rather than mass, use the Body Surface Area (BSA) Calculator.