Article — Dosage Calculator
Dosage calculator: weight-based mg/kg dosing
This dosage calculator is educational only and is not medical advice. Drug dosing depends on factors a calculator cannot see: age, renal and hepatic function, allergies, drug interactions, brand-specific concentration, and the route of administration. Always confirm every dose with a licensed prescriber or pharmacist before giving any medication. Pediatric and high-risk doses must be double-checked.
A dosage calculator converts a weight-based prescription such as 10 mg/kg into a concrete dose in milligrams, then divides by the labeled concentration to give the volume in milliliters. The two ingredients are body weight in kilograms and the drug concentration printed on the label.
Most modern prescriptions for children, and many for adults, are written as mg per kilogram of body weight. The number 10 mg/kg does not tell a nurse how much liquid to pour or how many tablets to crush. It tells the math: multiply the rate by the patient's weight, then divide by the concentration of the bottle in front of you.
Weight-based dosing replaced age-based rules (Clark's Rule from the early 1900s, Young's Rule before that) once routine scales made accurate weights cheap and fast. Age was always a proxy for weight, and a poor one — two five-year-olds can differ in weight by 50 percent. Today every drug reference, from Lexicomp to the British National Formulary for Children, leads with mg/kg figures.
What a dosage calculator does
A dosage calculator handles four conversions in one screen. First, it normalizes weight to kilograms — pounds get divided by 2.20462. Second, it multiplies weight by the prescribed dose to get milligrams per administration. Third, it divides milligrams by the labeled concentration in mg/mL to get a volume. Fourth, it multiplies by the daily frequency to give 24-hour totals.
None of those steps is hard on a single patient. The reason hospitals still see dosing errors is that each step is a chance to drop a decimal, swap a unit, or read 120 mg/5 mL as 120 mg/mL. A calculator removes the arithmetic; the clinician still owns the clinical judgment.
The mg/kg dosage formula
The weight-based dose in milligrams equals weight in kilograms times the dose rate in mg/kg. The volume in milliliters equals that dose divided by the concentration in mg/mL.
Dose (mg) = Weight (kg) × mg/kgVolume (mL) = Dose (mg) ÷ mg/mLDaily dose = Dose × times-per-daykg = lb ÷ 2.20462Dosage worked example
A 30 kg child needs amoxicillin oral suspension at 40 mg/kg/day, divided into three doses. The bottle reads 250 mg per 5 mL.
Daily dose: 30 × 40 = 1,200 mg. Single dose: 1,200 ÷ 3 = 400 mg. Concentration: 250 mg/5 mL = 50 mg/mL. Volume per dose: 400 ÷ 50 = 8 mL. The nurse pours 8 mL three times a day. If the label is misread as 250 mg/mL, the calculated volume drops to 1.6 mL — one-fifth of the prescribed dose.
The Institute of Medicine has estimated that preventable medication errors injure roughly 1.5 million people in the United States every year. Dose-calculation mistakes — most often unit confusion and decimal-point shifts — are among the most common types.
Concentration and volume
Concentration is the bridge between milligrams and milliliters. Pediatric liquid medications are usually labeled in mg per 5 mL because a 5 mL measuring spoon is the household teaspoon. Adult injectables are labeled in mg/mL. The dosage calculator always wants mg/mL, so divide any mg-per-5-mL label by 5 before entering it.
If a vial says 100 mg/2 mL, the concentration is 50 mg/mL. If a syrup says 120 mg/5 mL, the concentration is 24 mg/mL. The arithmetic is tiny; the consequence of skipping it is a five-fold dosing error.
Pediatric dosage considerations
Children are not small adults — pharmacokinetics change with age. Neonates (0–28 days) have immature liver and kidney function and need protocols built for them, not adult rules scaled down. Infants and toddlers metabolize many drugs faster per kilogram than adults, so mg/kg rates are often higher in pediatrics than in adult medicine.
Always weigh the child the day of the prescription if possible. Estimated weights from age tables are a fallback for emergencies, not a default. A four-year-old can range from 13 to 22 kg — almost a two-fold difference in calculated dose.
When checking a pediatric dose, work it both ways: calculate from weight and mg/kg, then back-calculate the mg/kg from the volume the parent is told to give. If the two numbers do not match, stop and recheck the label.
Common dosage mistakes
- Pound-kilogram confusion = 2.2× overdose if pounds are entered as kilograms
- mg/5 mL misread = 5× error in volume when treated as mg/mL
- Decimal-point shift = 10× error, most dangerous in pediatrics
- Daily total vs single dose = giving the daily amount once
- Frequency drift = "three times a day" interpreted as "every three hours"
- Strength assumption = using the wrong concentration of a multi-strength drug
- Adult dose for child = ignoring mg/kg and using adult tablet strength
When dosage uses BSA
Some drugs scale better with body surface area than with weight. Oncology agents, certain biologics, and a handful of cardiovascular drugs are dosed in mg/m². The Du Bois formula from 1916 is still standard: BSA (m²) = 0.007184 × height(cm)^0.725 × weight(kg)^0.425. A 30 kg child of 130 cm has a BSA of roughly 1.04 m². At 25 mg/m², that is 26 mg per dose.
This calculator covers weight-based mg/kg dosing only. BSA dosing belongs in a dedicated tool because the formula is multi-step and the protocols are drug-specific. For chemotherapy, the BSA is usually capped at 2.0 m² regardless of the patient's actual surface area; this is a safety convention rather than a strict rule, and it varies by protocol.
The Du Bois formula from 1916 was derived from just nine subjects. It still anchors modern BSA dosing because attempts to replace it — the Mosteller, Haycock, and Boyd formulas — give answers that agree within 5 percent for most adults.
Dosage quick reference
- 1 kg = 2.20462 lb
- 1 teaspoon = 5 mL (use a syringe, not a kitchen spoon)
- 120 mg/5 mL = 24 mg/mL (typical pediatric paracetamol)
- 100 mg/5 mL = 20 mg/mL (typical pediatric ibuprofen)
- 250 mg/5 mL = 50 mg/mL (common amoxicillin)
- Daily dose ÷ frequency = single dose
- Round volumes to the nearest 0.1 mL with an oral syringe
Two safety habits make weight-based dosing safer in practice. Always state weights in kilograms on the prescription itself, never pounds. And always print the concentration of the dispensed bottle on the label so the pharmacist's math and the parent's measurement use the same denominator.