Prescriptions are rarely written as "give the child 400 milligrams." They are written as "20 mg/kg/day, divided BID," and turning that instruction into a number you can measure with an oral syringe is where dosing mistakes happen. This medication dosage calculator converts body weight and a per-kilogram-per-day recommendation into the total daily dose, the milligrams in each administration, the liquid volume in milliliters, and the number of tablets — so you can compare the result against the bottle label or the written order before a single dose is given.
The model applies to any oral medicine dosed on body weight: antibiotics such as amoxicillin, fever reducers, and many other pediatric and adult drugs. Enter the weight in kilograms, the recommended mg/kg per day, how many times the medicine should be taken, and the strength of the bottle or tablet in front of you, and every figure updates instantly.
How to Use
- Enter the Body Weight in kilograms. If your scale reads pounds, divide by 2.2 first — a 44-pound child weighs about 20 kg. Weight-based dosing uses kilograms, not age, because two seven-year-olds can weigh 22 kg or 40 kg.
- Enter the Daily Dose per kg from the drug label or the clinician's order — for example, 20 for amoxicillin. Never guess this number; it is set by the manufacturer and the prescribing clinician.
- Choose Doses per Day: 1 for once daily, 2 for BID (twice), 3 for TID (three times), 4 for QID (four times), or 6 for every four hours. Splitting the daily total evenly is the default rhythm.
- Add the Liquid Strength in mg/mL. A suspension labeled "250 mg per 5 mL" is 50 mg/mL. Set this to 0 when the medicine comes as tablets and you only need milligram figures.
- Optionally enter the Tablet Strength in milligrams (for example, 200) to see how many tablets equal one dose. Set to 0 to skip tablet results.
The result card then shows the total daily milligrams, the milligrams in each administration, the liquid milliliters per dose, the total milliliters used over 24 hours, and the tablet count per dose.
The Dosing Formula
Weight-based oral dosing follows one short chain of arithmetic:
- Total daily mg = body weight (kg) × recommended mg/kg/day
- Mg per dose = total daily mg ÷ number of doses per day
- Liquid mL per dose = mg per dose ÷ mg per mL of the suspension
- Tablets per dose = mg per dose ÷ mg per tablet
Worked example: a child weighing 44 lb (20 kg) prescribed amoxicillin at 20 mg/kg/day. The daily total is 20 × 20 = 400 mg per 24 hours. Split BID, that is 200 mg per dose, twice daily. With a 50 mg/mL suspension, each 200 mg dose draws up to 4.0 mL, and the whole 24-hour course uses 8.0 mL. Compare that against the 5 mL oral syringe in the box before pouring anything.
If the recommended daily dose is 0 — for instance, while verifying a setting — the engine returns zero across the board rather than inventing a dose. If body weight is entered as zero or a negative number, the calculator refuses to return a result, because a negative kilogram reading can never produce a safe dose.
Why Weight Matters More Than Age
Clinicians dose most oral children's medicines on kilograms because age bands are too rough. An 8-year-old can weigh anywhere from 22 kg to 45 kg, and a dose fixed by age would overdose one child and underdose the other. This is also why the box label often lists dosing by weight brackets ("20 kg and over") rather than by years. Converting pounds to kilograms — pounds divided by 2.2 — is therefore not an optional step; it is the entry point for the whole calculation.
The same arithmetic works for adults on liquid medicines, people with feeding tubes, and anyone adjusting a suspension to a tablet strength: the milligram target is fixed, and the question is only how much of your bottle or pill delivers that target.
Examples
Example 1 — Child with an ear infection. A 44 lb (20 kg) child is prescribed amoxicillin at 20 mg/kg/day, split twice daily. The calculator gives a 400 mg daily total, 200 mg every 12 hours, and 4.0 mL per dose of a 50 mg/mL suspension. Each 12-hour administration delivers roughly 50% of the daily total, which is the expected split for BID dosing. The caregiver marks the syringe at 4.0 mL morning and evening.
Example 2 — Adult tablets. A 60 kg adult is prescribed a medicine at 10 mg/kg/day, three times daily, with 200 mg tablets. The daily total is 600 mg; each dose is 200 mg, which equals exactly one 200 mg tablet taken three times a day. No liquid conversion is needed because the tablet strength already matches the per-dose target.
Example 3 — Switching from suspension to tablets. A 30 kg child on a 15 mg/kg/day regimen, split twice daily, needs 225 mg per dose from a 75 mg/5 mL suspension (15 mg/mL), which draws up to 15.0 mL per dose. If the clinician switches to 100 mg tablets instead, the calculator reports 2.25 tablets per dose — a signal that the tablet strength does not fit the prescribed target, and a reason to confirm the order rather than splitting tablets awkwardly.
FAQ
How do I calculate medication dosage by weight?
Multiply the patient's weight in kilograms by the recommended mg/kg/day value from the label or prescription to get the total daily milligrams, then divide by the number of doses per day. For liquid medicine, divide the mg per dose by the mg/mL strength of the suspension to get milliliters. This calculator performs all three steps at once.
What does "mg/kg/day" mean?
It means milligrams of medicine for every kilogram of body weight, given over a full 24-hour period. A prescription of 20 mg/kg/day for a 20 kg child therefore calls for 400 mg in total across the day — not 400 mg in a single administration. The daily amount is then split by the dosing frequency, such as twice daily (BID) or three times daily (TID).
How do I convert pounds to kilograms for dosing?
Divide the weight in pounds by 2.2. For example, 44 lb ÷ 2.2 = 20 kg, and 110 lb ÷ 2.2 = 50 kg. Always use kilograms in weight-based dosing, because the mg/kg recommendation assumes a kilogram body weight; entering pounds by mistake would produce a dose more than twice too high.
How do I convert milligrams to milliliters for a liquid medicine?
Read the bottle's concentration, usually printed as "250 mg per 5 mL" — that is 50 mg/mL. Divide the target milligrams per dose by that number. A 200 mg dose at 50 mg/mL is 4.0 mL. If the label gives "125 mg per 5 mL," the strength is 25 mg/mL, and the same 200 mg dose becomes 8.0 mL.
What do BID, TID, QID, and q6h mean?
BID means twice daily (two doses per 24 hours), TID means three times daily, QID means four times daily, and q6h means every six hours (four doses). These frequencies simply divide the total daily milligrams into equal administrations; the mg/kg/day total itself does not change.
How much amoxicillin do I give a 44-pound child?
At the common 20 mg/kg/day amoxicillin regimen, a 44 lb (20 kg) child receives 400 mg per day. Split BID, that is 200 mg every 12 hours — about 4.0 mL per dose of a standard 250 mg/5 mL (50 mg/mL) suspension. Exact regimens vary by infection, so always follow the bottle label and the prescribing clinician's instructions.
Is this calculator safe for newborns and babies?
The arithmetic is correct for weight-based oral dosing, but newborn and infant dosing often uses age-adjusted or maximum single-dose limits that this general tool does not apply. For children under three months, premature infants, or any high-risk medication, confirm the dose directly with the prescribing clinician, a pediatric pharmacist, or a poison-center line before administration.
Disclaimer
This medication dosage calculator is provided for informational purposes only and is not medical advice. It performs the standard weight-based arithmetic from the inputs you enter; it does not know the patient's diagnosis, kidney or liver function, allergies, other medicines, age-adjusted limits, or the specific product's label. Recommended mg/kg values vary by drug, formulation, and condition, and liquid suspensions differ in strength between manufacturers. Always confirm the dose, frequency, and duration directly with a licensed clinician or pharmacist, and read the actual bottle label before giving any medicine.