# Dosage by weight calculator

> Dose by weight from a mg/kg rate and body weight, capped at the single and daily maximum, with the liquid volume in mL for a given strength.

Interactive version: https://www.calcopenly.com/health/dosage-by-weight-calculator
Subject: Health and fitness calculators

A weight-based dose is body weight in kilograms times the prescribed rate in mg/kg, given either per dose or per day and divided into doses. The result is capped at the maximum single dose and at the daily maximum divided by the number of doses, whichever is lower. For a liquid it is converted to millilitres with the product strength: dose × mL ÷ mg.

It is used to check a calculated dose against a prescription or product label. With the defaults, 20 kg at 15 mg/kg gives 300 mg, which is 9.38 mL of a 160 mg per 5 mL liquid, taken 4 times a day for 1,200 mg in total.

The rate, the maximums and the strength must come from the prescription or the label; the calculator holds no dosing information for any medicine. The BNF for Children notes that a calculated child's dose should not exceed the adult maximum, which is why the caps apply.

## Inputs

- **Weight**
- **The mg/kg figure is** (options: Per dose, Per day, split into doses)
- **Dose rate**
- **Doses per day**
- **Maximum single dose**: From the label or prescription; leave blank if none is given.
- **Maximum per day**
- **Liquid strength: amount**: E.g. 160 mg in 5 mL. Leave both blank for tablets.
- **Liquid strength: per volume**

## Results

- Calculated dose (mg) — main result
- Liquid volume per dose (mL)
- Every
- Total per day (mg)
- Weight-based dose before limits (mg)
- Limit applied

## Formula

$$
\text{dose} = \min\!\Big(w \cdot r,\ \text{max}_{\text{single}},\ \frac{\text{max}_{\text{day}}}{n}\Big),\qquad V = \text{dose} \times \frac{\text{mL}}{\text{mg}}
$$

## Worked examples

### 20 kg at 15 mg/kg, 160 mg/5 mL

- Weight: 20 kg
- The mg/kg figure is: Per dose
- Dose rate: 15 mg/kg
- Doses per day: 4
- Maximum single dose: 1000 mg
- Maximum per day: 4000 mg
- Liquid strength: amount: 160 mg
- Liquid strength: per volume: 5 mL
- **Calculated dose: 300 mg**
- **Liquid volume per dose: 9.38 mL**
- **Total per day: 1,200 mg**
- **Limit applied: None**
- **Every: 6 h**
- Checked against: Python 3.8 decimal: 20 × 15 = 300 mg; 300 × 5 / 160 = 9.375 mL; 300 × 4 = 1200 mg; 24 h / 4 = 21600 s

### Edge: single-dose cap reached

- Weight: 80 kg
- The mg/kg figure is: Per dose
- Dose rate: 15 mg/kg
- Doses per day: 4
- Maximum single dose: 1000 mg
- Maximum per day: 4000 mg
- Liquid strength: amount: 100 mg
- Liquid strength: per volume: 1 mL
- **Weight-based dose before limits: 1,200 mg**
- **Calculated dose: 1,000 mg**
- **Liquid volume per dose: 10.00 mL**
- **Limit applied: Maximum single dose**
- Checked against: Python 3.8 decimal: min(80 × 15, 1000) = 1000 mg; 1000 × 1 / 100 = 10 mL

### Edge: daily cap reached

- Weight: 50 kg
- The mg/kg figure is: Per dose
- Dose rate: 20 mg/kg
- Doses per day: 4
- Maximum single dose: 1000 mg
- Maximum per day: 3000 mg
- Liquid strength: amount: 160 mg
- Liquid strength: per volume: 5 mL
- **Weight-based dose before limits: 1,000 mg**
- **Calculated dose: 750 mg**
- **Total per day: 3,000 mg**
- **Liquid volume per dose: 23.44 mL**
- **Limit applied: Maximum daily dose**
- Checked against: Python 3.8 decimal: min(50 × 20, 1000, 3000 / 4) = 750 mg; 750 × 5 / 160 = 23.4375 mL

### Per-day rate split three ways, no maximums

- Weight: 12 kg
- The mg/kg figure is: Per day, split into doses
- Dose rate: 40 mg/kg
- Doses per day: 3
- Liquid strength: amount: 250 mg
- Liquid strength: per volume: 5 mL
- **Calculated dose: 160 mg**
- **Total per day: 480 mg**
- **Liquid volume per dose: 3.20 mL**
- **Every: 8 h**
- Checked against: Python 3.8 decimal: 12 × 40 / 3 = 160 mg; 160 × 5 / 250 = 3.2 mL; 24 h / 3 = 28800 s

### BNFC example: 10 kg at 8 mg/kg, max 300 mg

- Weight: 10 kg
- The mg/kg figure is: Per dose
- Dose rate: 8 mg/kg
- Doses per day: 1
- Maximum single dose: 300 mg
- Liquid strength: amount: 160 mg
- Liquid strength: per volume: 5 mL
- **Calculated dose: 80 mg**
- **Limit applied: None**
- Checked against: BNF for Children, prescribing in children: a 10 kg child should receive 80 mg

### BNFC example: 40 kg at 8 mg/kg, max 300 mg

- Weight: 40 kg
- The mg/kg figure is: Per dose
- Dose rate: 8 mg/kg
- Doses per day: 1
- Maximum single dose: 300 mg
- Liquid strength: amount: 160 mg
- Liquid strength: per volume: 5 mL
- **Calculated dose: 300 mg**
- **Weight-based dose before limits: 320 mg**
- **Limit applied: Maximum single dose**
- Checked against: BNF for Children, prescribing in children: a 40 kg child should receive 300 mg (rather than 320 mg)

## Questions

### How do you calculate a dose by weight?

Multiply body weight in kilograms by the dose rate in mg/kg. A 20 kg child at 15 mg/kg gets 20 × 15 = 300 mg per dose. If the rate is per day, divide by the number of doses: 12 kg at 40 mg/kg/day in 3 doses is 480 mg a day, or 160 mg per dose. Convert pounds to kilograms first; 44 lb is 19.96 kg.

### How do you work out how many mL of liquid medicine to give?

Divide the dose in mg by the strength in mg per mL. A 300 mg dose of a 160 mg per 5 mL liquid is 300 × 5 ÷ 160 = 9.375 mL, about 9.4 mL. The FDA's 2011 guidance for over-the-counter liquid medicines asks for a measuring device marked in the same units as the label directions, which is why an oral syringe or the supplied cup is used rather than a kitchen spoon.

### Why is a child's weight-based dose capped?

Because a heavier child's mg/kg dose can exceed the adult dose. The BNF for Children advises that calculated doses should not exceed the adult maximum: at 8 mg/kg with a 300 mg maximum, a 10 kg child receives 80 mg, but a 40 kg child receives 300 mg rather than 320 mg. This calculator applies both a single-dose and a daily maximum and shows which limit, if any, reduced the dose.

### What is the difference between mg/kg per dose and per day?

A per-dose rate gives the amount for each dose; a per-day rate gives the total for 24 hours, which is then divided by the number of doses. At 40 mg/kg, a 12 kg child would get 480 mg per dose on a per-dose reading but 160 mg per dose if the 40 mg/kg is per day in three doses, a threefold difference. Prescriptions state which applies, often as mg/kg/dose or mg/kg/day.

### How accurate is the dosage by weight calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “20 kg at 15 mg/kg, 160 mg/5 mL” is checked against Python 3.8 decimal: 20 × 15 = 300 mg; 300 × 5 / 160 = 9.375 mL; 300 × 4 = 1200 mg; 24 h / 4 = 21600 s.

### Where does the method come from?

BNF for Children — General guidance, prescribing in children: calculated doses should not exceed the adult maximum (worked example 8 mg/kg, max 300 mg); US FDA — Dosage delivery devices for orally ingested OTC liquid drug products (guidance for industry, 2011).

## Sources

- [BNF for Children — General guidance, prescribing in children: calculated doses should not exceed the adult maximum (worked example 8 mg/kg, max 300 mg)](https://bnfc.nice.org.uk/)
- [US FDA — Dosage delivery devices for orally ingested OTC liquid drug products (guidance for industry, 2011)](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/dosage-delivery-devices-orally-ingested-otc-liquid-drug-products)

_Note: medical information, not professional advice._