Medication Dosage Calculator
Many medications — particularly antibiotics, analgesics, and pediatric drugs — are dosed by body weight in mg/kg. This calculator converts pounds to kilograms and computes the single dose and total daily dose based on your prescriber's mg/kg instruction.
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Formula
Dose (mg) = Weight (lbs) × 0.453592 × Dose (mg/kg)
The conversion factor 0.453592 converts pounds to kilograms (1 lb = 0.453592 kg). Multiplying weight in kg by the mg/kg dose gives the total milligrams for a single administration. Multiplying by doses per day gives the total daily dose, which is the figure most commonly used to compare against maximum daily dose limits specified in prescribing guidelines.
How to use the Medication Dosage Calculator
- 1
Enter your body weight
Value should be in lbs.
- 2
Enter your prescribed dose
Value should be in mg/kg.
- 3
Enter your doses per day
- 4
Read your results instantly
Results update in real time as you type.
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Weight-based dosing in clinical practice
Weight-based dosing ensures that patients of different sizes receive pharmacologically equivalent amounts relative to their body mass — the primary volume through which most drugs distribute. It is standard for pediatric patients (where body weight varies enormously), renally cleared drugs (dose adjusted by kidney function and weight), oncology drugs (many chemotherapy agents are dosed by body surface area, derived from weight and height), and antibiotics where underdosing creates resistance risk and overdosing creates toxicity.
Important safety disclaimer
This calculator is a mathematical aid only. It does not account for renal or hepatic impairment, age-related dosing restrictions, maximum dose caps, drug interactions, allergy history, or any other clinical factor that affects appropriate dosing. Always follow your prescriber's specific instructions and consult a pharmacist before administering any medication. Never adjust prescribed doses without medical guidance, even if a weight-based calculation suggests a different amount.
Tips & Insights
Always verify against the maximum daily dose
Most medications have an absolute maximum daily dose that applies regardless of weight. A calculated dose above the maximum should be capped at the maximum — verify with your pharmacist.
Use actual body weight or adjusted body weight as directed
For obese patients, some drugs use ideal body weight or an adjusted body weight rather than actual weight to avoid toxicity. Your prescriber will specify which weight to use.
Check the label for mg vs. mg/kg
Some medications list standard doses in mg (e.g., 500 mg twice daily) rather than mg/kg. The mg/kg designation on a prescription specifically means weight-based dosing is required.
Worked Examples
160 lb patient, amoxicillin 20 mg/kg, twice daily
Single dose: 1,454 mg; Daily: 2,907 mg
44 lb child, ibuprofen 10 mg/kg, three times daily
Single dose: 200 mg; Daily: 600 mg
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Frequently Asked Questions
What is mg/kg dosing?
mg/kg means milligrams of medication per kilogram of body weight. A 10 mg/kg dose for a 70 kg person is 700 mg per administration.
Should I use my actual weight or ideal weight?
It depends on the medication. Your prescriber or pharmacist will specify. Many drugs use actual body weight; some use ideal body weight or adjusted body weight for obese patients.
Can I use this for pediatric dosing?
Yes, this calculator works for any weight. However, pediatric dosing has additional considerations including age-based restrictions and formulation availability. Always confirm pediatric doses with a pharmacist.
What if my calculated dose differs from what was prescribed?
Do not adjust based on a calculator. Discuss any discrepancy with your prescriber or pharmacist — there may be a clinical reason (renal adjustment, maximum cap, different weight basis) for the prescribed dose.
Is this calculator appropriate for chemotherapy dosing?
No. Chemotherapy is typically dosed by body surface area (BSA, in m²), not simple weight-based mg/kg, and requires specialized oncology pharmacists and clinical oversight.
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