Perform weight-based medication arithmetic from a dose rate and schedule supplied by a qualified prescriber.
Calculator inputs
Enter the available information
Arithmetic only
Calculated result
Result summary
Your result will appear here
Complete every required field, then select the calculate button.
Amount per doseweight × prescribed daily dose ÷ doses per day
Converted weight
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Total daily amount
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Amount per dose
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Doses per day
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Educational estimate only—not medical advice, a diagnosis, or a treatment instruction. This page does not recommend a medicine or dose. Verify every result with a pharmacist or qualified prescriber before administration.
Only calculate from a dose and schedule already verified for the specific medicine and patient.
This dosage calculator performs the arithmetic for a weight-based daily dose that has already been prescribed or supplied by a qualified clinician. It converts body weight to kilograms, converts the entered rate to milligrams per kilogram per day, finds the total milligrams per day, and divides that total by the selected number of administrations.
It does not choose a medicine, decide a dose, account for maximum-dose limits, or confirm that a frequency is safe. Drug dosing depends on the medicine, indication, age, kidney and liver function, concentration, route, allergies and other factors. Use the result only to check arithmetic against an authoritative prescription or medicine reference.
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How to Use This Calculator
Enter the patient weight and select kilograms or pounds. Use a recent, reliable measurement.
Enter the prescribed daily dose rate. The label must say mg/kg/day or g/kg/day; do not enter a rate written per dose.
Select the prescribed frequency. “Twice per day” divides the daily amount into two equal administrations; “every 4 hours” is treated as six administrations in 24 hours.
Select Calculate. Review the converted weight, daily total and amount per administration.
Compare every number and unit with the prescription and medicine-specific maximum. Ask a pharmacist or prescriber to resolve any mismatch before use.
The frequency assumes a full 24-hour schedule with evenly divided amounts. Actual instructions may limit dosing to waking hours, specify unequal amounts or say “as needed.” Those directions cannot be represented by a simple equal split.
Dose per administration (mg) = daily dose ÷ administrations per day
A rate entered in g/kg/day is multiplied by 1,000 to convert grams to milligrams before the calculation. The selected schedules correspond to 1, 2, 3, 4, 6, 8, 12 or 24 administrations each day.
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Variables Explained
Term
Meaning
Weight
Patient body mass entered in kilograms or pounds, then expressed in kilograms for the calculation.
Daily dose rate
The clinician-supplied amount per kilogram for the whole day, in mg/kg/day or g/kg/day.
Administrations per day
How many equal doses the daily total is split into.
Total daily dose
Calculated milligrams for the entire 24-hour day before checking a drug-specific maximum.
Dose per administration
Calculated milligrams in each equal share of the daily total.
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Worked Example
Consider a hypothetical arithmetic check for a 20 kg patient with a prescribed rate of 10 mg/kg/day, divided twice daily. The daily total is 20 kg × 10 mg/kg/day = 200 mg/day. Dividing 200 mg by 2 gives 100 mg per administration. The result is an arithmetic value, not a recommendation that this rate suits any particular medicine or patient.
If the weight is entered as 44.0925 lb, conversion gives about 20 kg and the same result after rounding. If a valid prescription instead states 0.01 g/kg/day, converting 0.01 g to 10 mg also produces 200 mg/day for 20 kg. Unit labels are essential: 10 mg/kg/dose twice daily would mean 200 mg per dose and 400 mg/day, which is different from 10 mg/kg/day.
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Why Use This Calculator?
Read the two outputs separately. “Total daily dose” is the full amount over 24 hours. “Dose” is the equal portion for one administration at the selected frequency. Both are displayed in milligrams. Neither output tells you the liquid volume, number of tablets or safe maximum.
To convert milligrams into millilitres, a verified product concentration is required. For example, 100 mg cannot be converted to a volume unless the label states something such as 50 mg per 5 mL. Different formulations of the same medicine may have different concentrations, so never assume a familiar bottle strength.
Rounding also needs clinical judgment. A calculated 103 mg may not match available tablet sizes or measurement markings. Do not round, split a dosage form or substitute a concentration without medicine-specific guidance from a pharmacist or prescriber.
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Common Mistakes
Confusing mg/kg/day with mg/kg/dose can multiply the intended daily amount.
Using pounds as though they were kilograms makes the result about 2.2 times too large.
Mixing grams and milligrams creates a thousand-fold error.
Selecting a frequency that differs from the written directions changes each divided dose.
Ignoring a medicine-specific daily or single-dose maximum can make mathematically correct arithmetic unsafe.
Do not use this page to select treatment, replace a prescription or respond to an overdose. Contact a clinician, pharmacist or local emergency service as appropriate.
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Frequently Asked Questions
Can this calculator tell me what dose to take?
No. It only checks arithmetic from a dose rate and schedule already supplied by a qualified clinician. It cannot determine whether that instruction is appropriate or safe.
What is the difference between mg/kg/day and mg/kg/dose?
A daily rate covers the whole day and is divided among administrations. A per-dose rate applies at every administration; multiplying it by the frequency gives the daily total.
Why is the result in milligrams rather than millilitres?
Millilitres measure volume. A product-specific concentration, such as milligrams per millilitre, is needed to convert a mass dose into liquid volume.
Does every 4 hours always mean six doses?
Six evenly spaced administrations fit in 24 hours, which is how this arithmetic form treats the choice. Real instructions may have different limits, so follow the written directions.
Can I use a weight measured several months ago?
Use a current reliable weight when dosing depends on body mass. This can be especially important for children and for people whose weight changes quickly.
What should I do if the result differs from the label or prescription?
Do not guess or average the values. Recheck the units and entries, then ask the prescriber or pharmacist to clarify the instruction before administering medicine.
