Online Tool Store
Dosage by Weight Calculator
Calculate prescribed weight-based doses in mg/kg or mg/kg/day with daily frequency, kg/lb conversion, and optional liquid mg/mL to mL volume.
This works right here on your device — nothing gets sent out.
Result
Fill in the form and press Calculate to see the single dose, daily total, and calculation steps.
Single dose
- mg
Daily total
- mg
- Calculated daily dose
- Supplied maximum daily dose
Values used
Calculation steps
Calculations use the unrounded kilogram value. Displayed numbers are rounded for reading only; no practical or administration rounding has been applied.
Formula
single dose = weight(kg) × dose(mg/kg) daily dose = single dose × doses/day mL = dose(mg) ÷ concentration(mg/mL)
- Single dose (mg) = weight (kg) × dose (mg/kg/dose). If the prescription is mg/kg/day, the daily total is weight × dose and is divided by doses per day.
- Total daily dose = single dose × number of doses per day.
- Concentration is entered as mg and mL (e.g. 250 mg / 5 mL = 50 mg/mL) to convert a dose in mg into millilitres.
- If you enter a maximum daily dose, the tool warns when the calculated total exceeds it. It never caps or changes the dose.
How it works
- Enter the patient weight and choose kg or lb. Pounds are converted to kilograms by dividing by 2.20462262185, and the converted value is shown.
- Enter the prescribed amount and pick its basis: mg/kg/dose (per single dose) or mg/kg/day (per whole day). Nothing is assumed - you must choose.
- Choose how many doses are given per day (q24h, q12h, q8h, q6h, q4h, or a custom number).
- Optionally enter a liquid concentration such as 250 mg / 5 mL and a maximum daily dose to compare against.
- Press Calculate to see the single dose, the daily total, the volume per dose in mL when a concentration is given, and every step of the arithmetic.
Formulas
Weight-based dosing multiplies a prescribed amount per kilogram by the patient's weight in kilograms. The result depends on whether the prescription is written per dose or per day.
mg/kg/dose: single dose (mg) = weight (kg) × mg/kg/dose mg/kg/dose: daily total (mg/day) = single dose × doses per day mg/kg/day: daily total (mg/day) = weight (kg) × mg/kg/day mg/kg/day: single dose (mg) = daily total ÷ doses per day Liquid: mg/mL = concentration mg ÷ concentration mL Liquid: volume per dose (mL) = single dose (mg) ÷ mg/mL Weight: kg = lb ÷ 2.20462262185
Example: 25 kg at 40 mg/kg/day in 3 doses is 25 × 40 = 1,000 mg/day, so 1,000 ÷ 3 = 333.33 mg per dose. With 250 mg / 5 mL (50 mg/mL), that is 333.33 ÷ 50 = 6.67 mL per dose. These numbers show the arithmetic only. The calculator does not choose, check, or recommend any medication or dose.
FAQ
What is weight-based dosing?
Weight-based dosing means a prescriber writes a dose as an amount of medicine per kilogram of body weight, such as mg/kg. The calculator multiplies that prescribed amount by the patient's weight to get the total in milligrams. It performs the multiplication only - it does not decide which medicine or amount is appropriate.
What is the difference between mg/kg/dose and mg/kg/day?
mg/kg/dose is the amount for each single dose. mg/kg/day is the total for the whole day, which is then divided across the doses. For a 25 kg patient, 10 mg/kg/dose given 3 times a day is 250 mg per dose and 750 mg/day, while 10 mg/kg/day split into 3 doses is only about 83.33 mg per dose and 250 mg/day. Mixing them up changes the answer by the number of doses, so the calculator makes you choose and never guesses.
How do I calculate a dose by weight?
Convert the weight to kilograms, then multiply by the prescribed mg/kg figure. For mg/kg/dose the product is the single dose; multiply by doses per day for the daily total. For mg/kg/day the product is the daily total; divide by doses per day for the single dose. The Calculation steps panel shows each of these lines with your numbers.
How do I convert mg to mL for a liquid medicine?
Divide the concentration's mg by its mL to get mg per mL (for example 250 mg / 5 mL = 50 mg/mL), then divide the dose in mg by that figure. A 333.33 mg dose at 50 mg/mL is 6.67 mL. The calculator shows the unrounded mathematical result and does not round to a practical measuring amount - follow the label, prescriber, or pharmacist for how to measure.
Why must pounds be converted to kilograms?
Weight-based doses are written per kilogram, so the weight must be in kilograms before multiplying. Using pounds directly would overstate the result by about 2.2 times. The calculator divides pounds by 2.20462262185 (1 kg = 2.20462262185 lb) and shows the converted kilograms so you can check it.
How does dosing frequency change the per-dose amount?
For a mg/kg/day prescription, the daily total is fixed and is shared between the doses, so more doses per day means a smaller amount each time (1,000 mg/day is 1,000 mg once daily, 500 mg twice daily, 333.33 mg three times daily). For a mg/kg/dose prescription, each dose stays the same and more doses per day increases the daily total instead. q24h, q12h, q8h, q6h, and q4h correspond to 1, 2, 3, 4, and 6 doses per day.
What does the maximum daily dose field do?
If you enter a maximum daily dose in mg, the tool compares the calculated daily total with it and shows a clear warning when the total is higher. It never lowers or caps the result, and it does not contain any built-in maximums - it only compares against the number you supply.
Can this tool tell me what dose to give or take?
No. It is an arithmetic aid only and does not prescribe or recommend medication doses. Verify the prescribed dose, units, frequency, concentration, and maximum dose with the medication label, prescriber, or pharmacist before use. It also does not account for age, kidney or liver function, other medicines, or any clinical factors.
Is my information stored or sent anywhere?
No. All calculations run in your browser and nothing is uploaded or saved. If you use Copy share link, the values you entered are placed in the link's web address, so anyone you send it to can see them.
How we compare
| Feature | Online Tool Store | Manual arithmetic |
|---|---|---|
| mg/kg/dose and mg/kg/day modes | Both, chosen explicitly | Easy to mix up |
| Pounds-to-kilograms conversion | Automatic, shown | Extra step |
| Frequency split (q24h to q4h or custom) | Built in | Manual |
| mg to mL from concentration | Optional, with steps | Extra step |
| Compare against your own maximum | Warning, no capping | Manual |
| Step-by-step arithmetic | Shown and copyable | Written by hand |
| No upload or account | Yes | Yes |
| Chooses or recommends a dose | No | No |
This is an arithmetic aid only. It does not prescribe or recommend doses, and results should be verified with the medication label, prescriber, or pharmacist.
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