Many medicines — especially for children — are not dosed by age. They are dosed by weight. A 5-year-old who weighs 15 kg needs a very different amount than a 5-year-old who weighs 25 kg, and guessing the difference can mean an underdose that does nothing or an overdose that causes harm. This dosage-by-weight calculator takes the recommended mg/kg dose straight from the drug label or your clinician's order and converts it into the milligrams (and milliliters of liquid) to actually give.
Smekys built this tool around the same formula used on the FDA-approved drug labels: multiply the dose per kilogram by the patient's weight, then compare that single dose against the 24-hour maximum. Adjust the sliders and every result updates instantly.
How to Use
- Enter the Body Weight in kilograms. Use the most recent measured weight, not an estimate. If you only have pounds, divide by 2.2 first.
- Type the Dose per kg — the number of milligrams recommended for each kilogram of body weight in one administration. For example, ibuprofen is commonly 10 mg/kg; acetaminophen is 10–15 mg/kg.
- Enter the Safe Max per kg / Day — the 24-hour ceiling on the label, such as 40 mg/kg/day for ibuprofen. This is the line you never cross in a single day.
- Choose how often a dose may be repeated — every 4, 6, 8, or 12 hours. The calculator converts that into doses per day.
- Add the Liquid Strength in mg/mL if you are giving a syrup or suspension. Most children's suspensions are 100 mg per 5 mL, which equals 20 mg/mL. Set this to 0 if you are giving tablets and do not need the mL result.
The result card shows the dose per administration in mg, the liquid dose in mL, the safe 24-hour maximum in both mg and mL, the number of doses that fit in a day, and the actual 24-hour total at the interval you chose. If that actual total climbs close to the ceiling, the tool flags that you are using most of your daily allowance.
The Weight-Based Dosing Formula
The whole calculation comes down to one multiplication:
Dose (mg) = Body Weight (kg) × Dose per kg (mg/kg)
For example, a 70 kg adult on a 10 mg/kg regimen takes 70 × 10 = 700 mg per dose. The safe daily ceiling is the same multiplication against the per-day limit: 70 × 40 = 2,800 mg in 24 hours. If the label says "every 6 hours," that is 24 ÷ 6 = 4 doses per day, and 4 × 700 mg lands exactly at 2,800 mg — 100% of the ceiling. Stretch the interval to every 8 hours (3 doses/day), and the 24-hour total drops to 2,100 mg, about 75% of the limit, which is why labels often allow "every 6 to 8 hours."
For liquids, divide the milligram dose by the concentration: Volume (mL) = Dose (mg) ÷ Strength (mg/mL). A 700 mg dose at 20 mg/mL is 35 mL.
Weight-based dosing matters because the mg/kg rule is what keeps a medicine effective in a 5 kg infant without overdosing a 100 kg adult. Age-based guesses are the opposite: they assume every child of a given age weighs the same, which they do not.
Examples
Example 1 — A 70 kg adult. Enter 70 kg, 10 mg/kg, a 40 mg/kg/day ceiling, repeat every 6 hours, and a 20 mg/mL liquid. The result card shows 700 mg (35 mL) per dose, a 2,800 mg (140 mL) maximum per day, 4 doses per day, and an actual 24-hour total of 2,800 mg — exactly at the safe ceiling, so no extra doses should be added.
Example 2 — A 30 kg child. At 10 mg/kg the single dose is 300 mg (15 mL), the daily ceiling is 1,200 mg (60 mL), and 4 spaced doses use the full allowance. If the child only needs mild relief, dropping to 5 mg/kg gives a 150 mg dose and leaves more headroom before the daily cap.
Example 3 — Checking an interval. Suppose the label allows 10 mg/kg every 6 to 8 hours for a 20 kg patient. At every 8 hours the single dose is 200 mg, 3 doses give 600 mg/day, and a 40 mg/kg/day ceiling means 800 mg/day — you are using 75% of the ceiling. At every 6 hours you would hit 800 mg, 100% of the ceiling. The calculator shows that trade-off before you decide how often to dose.
FAQ
How do I calculate dosage by weight?
Multiply the patient's weight in kilograms by the dose per kilogram stated on the label or prescription. For a 30 kg patient on a 10 mg/kg dose, that is 30 × 10 = 300 mg per administration. Then multiply the weight by the mg/kg/day ceiling to get the 24-hour maximum, and divide 24 by the repeat interval to get doses per day.
Should I use pounds or kilograms?
Use kilograms. Weight-based dosing is defined in mg/kg, so the calculation assumes metric weight. If your scale shows pounds, divide by 2.2 to convert — a 154 lb adult is about 70 kg. Entering pounds directly as if they were kilograms would overestimate the weight and the dose by roughly 55%.
How do I convert a mg dose to mL of liquid?
Divide the milligram dose by the liquid strength in mg/mL. If the bottle says 100 mg per 5 mL, that is 20 mg/mL. A 300 mg dose becomes 300 ÷ 20 = 15 mL. Always read the concentration on the actual bottle, because children's suspensions and adult concentrates can differ by a factor of 4.
What does the safe 24-hour maximum mean?
It is the most medicine a patient should take in any single day, set by the manufacturer. For example, ibuprofen's ceiling is about 40 mg/kg/day. If the calculator's "actual mg per 24 h" equals that ceiling, you are already at the limit — do not add another dose. Going above it raises the risk of stomach, kidney, and liver side effects.
How many doses per day can I give?
Divide 24 by the interval. Every 6 hours is 4 doses/day, every 8 hours is 3, every 12 hours is 2. The number of doses also interacts with the daily ceiling: more frequent dosing is safe only if mg/kg × doses/day stays at or below the mg/kg/day limit.
Is this calculator right for infants and pregnant adults?
No tool replaces a clinician for infants under a year, pregnant or breastfeeding patients, or anyone with kidney or liver disease. Those groups need adjusted doses that depend on more than weight alone. Use the numbers here as a double-check against the prescription, and confirm anything you are unsure about with a pharmacist or doctor.
Disclaimer
This dosage-by-weight calculator is provided for informational purposes only and is not medical advice. It applies a generic mg/kg multiplication to the numbers you enter; actual dosing depends on the specific drug formulation, the patient's age, organ function, other medications, and the instructions on your prescription or the product label. Never use the result to start, stop, or change a medicine without checking a licensed clinician or pharmacist first. Overdosing, especially in children, can be dangerous — when in doubt, call your doctor or poison control.