What it is
A generic arithmetic helper: given a weight and a mg/kg dose, it returns the milligrams per dose, applies an optional maximum single-dose ceiling, and — given a concentration (mg/mL) — converts the result to a volume (mL). It deliberately does the arithmetic only: it does not know the drug, the indication, or renal/hepatic adjustments. You supply a correct mg/kg; it multiplies without error.
Formula
$$ Dose,(mg) = weight,(kg) \times dose,(mg/kg\ per\ dose) $$
If a maximum is entered and the product exceeds it, the dose is capped at the maximum. With a concentration:
$$ Volume,(mL) = \frac{Dose,(mg)}{concentration,(mg/mL)} $$
When to use
At the point of prescribing or checking a weight-based pediatric dose when you have already looked up the correct per-dose mg/kg in a formulary and want fast, error-free multiplication and a syrup-volume conversion.
The two traps — read before you use it
Trap 1 — mg/kg PER DOSE, not per day. Many formulary entries are written as mg/kg/DAY divided into N doses. If you type the daily figure into a per-dose field, you give the whole day’s dose every single administration — an N-fold overdose. Divide the daily dose by the number of doses first. Example: high-dose amoxicillin 90 mg/kg/day in 2 divided doses = 45 mg/kg per dose — enter 45, never 90.
Trap 2 — the mg/5 mL concentration trap. Pediatric syrups are labelled per 5 mL, e.g. “250 mg/5 mL”. The calculator wants mg per 1 mL. 250 mg/5 mL = 50 mg/mL, not 250. Enter the wrong number and the volume is off by 5x — a five-fold dosing error in either direction.
Worked example
An 18 kg child, amoxicillin 45 mg/kg per dose (i.e. 90 mg/kg/day ÷ 2), syrup 250 mg/5 mL → 50 mg/mL:
- Dose = 18 x 45 = 810 mg per dose
- Volume = 810 / 50 = 16.2 mL per dose
If you had wrongly entered 90 (the per-day figure) you would have calculated 1620 mg — double. If you had entered 250 as the concentration you would have given 3.24 mL — one-fifth of the correct volume.
Pitfalls
- No maximum entered? At higher weights a per-kg dose can quietly exceed the adult ceiling. Confirm an absolute single-dose cap exists and enter it.
- The tool does not validate the drug or the mg/kg — it cannot catch a wrong dose, a contraindication, or a renal/hepatic adjustment. Garbage in, garbage out.
- Capping is a single-dose ceiling, not a daily maximum — frequency still matters.
- Always reconcile the final number against a current pediatric formulary (BNFc / IAP / Lexicomp).
Run it: Weight-Based Dose
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.