Medication Dosage Calculator
Weight-based medication dosing calculates the correct amount of a drug for an individual patient by multiplying the prescribed dose in milligrams per kilogram by the patient's body weight. This method is standard in pediatric medicine, oncology, and any specialty where body size strongly influences how a drug is absorbed and cleared. To use this calculator, enter the patient's weight in kilograms or pounds, the dose prescribed by the clinician in mg per kg, and how many times per day the medication is to be taken. The calculator returns the single-dose amount and the total daily dose, both in milligrams. These figures are for informational purposes only: always verify the result with a licensed pharmacist or prescribing provider before administering medication. Clinical dosing decisions also depend on renal and hepatic function, concomitant medications, allergies, and patient-specific factors that this tool cannot assess. For IV medications or drugs with narrow therapeutic windows, dosing must be confirmed by a qualified healthcare professional. The FDA requires drug labeling to specify pediatric dosing information where it exists; the NIH MedlinePlus database provides approved dosing information for many common drugs.
Single dose: -- mg — Daily total: -- mg
How the calculation works
Weight-based dosing uses two steps: first, convert the patient's weight to kilograms if given in pounds; second, multiply weight by the prescribed dose per kilogram to get the single dose. Multiply the single dose by the number of daily administrations to get the daily total.
If lb: weight_kg = weight_lb × 0.453592
Single dose (mg) = weight_kg × dose_per_kg
Daily total (mg) = single_dose × doses_per_day
Worked example
Patient weight 70 kg, prescribed dose 5 mg/kg, frequency 3 times per day:
- Weight is already in kg: 70 kg
- Single dose = 70 × 5 = 350.00 mg
- Daily total = 350 × 3 = 1,050.00 mg
When is weight-based dosing used?
Weight-based dosing is standard practice in the following clinical settings:
- Pediatrics: most drugs for children are dosed in mg/kg because children's organ maturity and body composition differ from adults.
- Oncology: chemotherapy agents are often dosed per kg or per m2 of body surface area to maximize efficacy and minimize toxicity.
- Anticoagulation: heparin and low-molecular-weight heparins are weight-based to balance clotting prevention with bleeding risk.
- Anesthesia: induction agents and neuromuscular blockers are weight-adjusted to ensure appropriate depth and duration.
- Infectious disease: aminoglycosides (e.g., gentamicin) use weight-based dosing with monitoring of serum levels.
Important dosing considerations
This calculator uses actual body weight. For obese patients, some drugs use ideal body weight or adjusted body weight instead. The prescribing clinician determines which weight to use based on the drug's pharmacokinetic properties. Never substitute this calculator's output for a pharmacist's verification, especially for medications with narrow therapeutic windows such as digoxin, warfarin, or aminoglycosides.
| Input | Example value | Calculation | Result |
|---|---|---|---|
| Weight | 70 kg | As entered | 70 kg |
| Dose per kg | 5 mg/kg | 70 × 5 | 350.00 mg/dose |
| Daily frequency | 3 times/day | 350 × 3 | 1,050.00 mg/day |
Medication dosage calculator: frequently asked questions
What is a weight-based medication dose?
A weight-based dose is a prescribed amount of medication calculated per kilogram of a patient's body weight. This approach ensures that dosing accounts for differences in body size, particularly important for children, elderly patients, and people with very low or very high body weight. The prescriber specifies the dose as mg per kg, and the pharmacist or caregiver multiplies it by the patient's actual weight.
Why is weight-based dosing used?
Weight-based dosing is used because drug distribution, metabolism, and elimination in the body depend on body mass. A flat dose that works for an average adult may be insufficient for a larger person or toxic for a smaller one. Pediatric dosing almost always uses a per-kg calculation because children's pharmacokinetics differ significantly from adults. Many antibiotics, chemotherapy agents, and anticoagulants are also dosed this way.
How do I convert pounds to kilograms?
To convert pounds to kilograms, multiply the weight in pounds by 0.453592. For example, 154 lb multiplied by 0.453592 equals approximately 69.85 kg. This calculator performs the conversion automatically when you select pounds as the input unit.
What is the difference between single dose and daily total dose?
The single dose is the amount of medication taken at one time (for example, one tablet or one injection). The daily total dose is the sum of all doses taken over 24 hours. If a patient takes 350 mg three times a day, the single dose is 350 mg and the daily total is 1,050 mg. Prescriptions typically specify both the dose per administration and the frequency.
Is this calculator a substitute for a pharmacist or doctor?
No. This calculator is a tool for educational and informational purposes only. Always verify medication doses with a licensed pharmacist or prescribing healthcare provider before administering any drug. Dosing can depend on renal function, hepatic function, drug interactions, patient age, and many other clinical factors that this calculator does not consider.
Official sources
- FDA drug labeling guidance: FDA Prescription Drug Labeling Resources.
- NIH MedlinePlus drug information: MedlinePlus Drug Information.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only, not medical advice.