Dosage Calculator

When a doctor writes a weight-based prescription, they are doing something practical: tying the dose to the patient's metabolism. Larger people generally process drugs faster, so a flat dose of 500 mg means something different for a 40 kg child than for a 100 kg adult. Weight-based dosing is how medicine accounts for that difference.

The core calculation is multiplication: body weight in kilograms times the prescribed milligrams per kilogram equals the total milligrams to give. The tricky part is the maximum dose cap. Most medications have an upper ceiling — a point at which giving more adds risk without adding benefit. A 120 kg adult calculated at 15 mg/kg would get 1,800 mg of acetaminophen, which exceeds the safe single dose. The cap exists to catch exactly that case.

For liquid medications, there is a second conversion: total milligrams divided by the concentration of the liquid in mg/mL gives you the volume in milliliters. This is where parents most often make errors — confusing mg (the dose) with mL (the volume to measure). A 160 mg/5 mL bottle and a 500 mg/5 mL bottle require completely different volumes for the same mg dose.

Use this calculator when you have a specific mg/kg figure from a prescription label or a doctor's instructions and need to convert it to a total dose or liquid volume. It is appropriate for common over-the-counter medications, antibiotics, and anti-inflammatories where the dosing is purely weight-based with no adjustment for other factors.

Do not rely on this calculator for chemotherapy, anticoagulants, or any medication where dose is adjusted based on kidney function, liver function, body surface area, or serum drug levels. Those calculations involve additional clinical variables that weight alone does not capture. Similarly, this tool is not appropriate for dosing newborns or premature infants, where drug metabolism is fundamentally different from older children and adults.

This tool also does not calculate dosing intervals or daily totals. If you need to know how many times per day to give a dose, or whether a repeated dose has brought the 24-hour total above a safe threshold, consult the prescribing information or ask a pharmacist.

The most common mistake is confusing the concentration printed on the bottle with the mg/mL figure this calculator needs. A label reading 160 mg/5 mL means 32 mg per mL. Entering 160 instead of 32 would produce a volume five times too small — a significant underdose. Always divide mg by mL yourself before entering the value.

The second mistake is skipping the maximum dose field. For adults, weight-based calculations can easily exceed safe ceilings, especially for medications like acetaminophen where the therapeutic window narrows at high doses. Entering the maximum from the label takes ten seconds and prevents the calculator from handing you a number you should not use.

The third mistake is using an old weight. Children's weight changes fast — a weight from six months ago on a growing toddler can be off by 15 to 20 percent, which translates directly into an equivalent dosing error. For children under five, weigh them at home before calculating.

The formula has two stages. First, convert weight to kilograms if needed: weight in lbs divided by 2.20462 equals weight in kg. Second, multiply: weight in kg times dose per kg equals total dose in mg.

If a maximum dose applies: total dose equals the smaller of (weight kg times dose per kg) and the maximum dose. This is written as: totalDose = Math.min(weightKg * dosePerKg, maxDose).

For liquid volume: volume in mL equals total dose in mg divided by concentration in mg/mL. The concentration must come from the label — it is almost never a round number. A bottle labeled 160 mg per 5 mL has a concentration of 32 mg/mL, not 160. Dividing by the wrong figure is the single most common measuring error in home pediatric dosing.

Weight-based dosing assumes linear pharmacokinetics — that drug clearance scales proportionally with body mass. This holds reasonably well for lean body weight but breaks down in obesity. Many dosing protocols use adjusted body weight (ABW) or ideal body weight (IBW) for obese patients rather than actual body weight, because adipose tissue does not metabolize drugs the same way lean tissue does. Entering actual body weight for a significantly obese adult will produce a dose that is higher than most clinical protocols intend. For those cases, the correct weight to enter is the adjusted or ideal body weight, not the scale weight.