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Drug Dosage Calculator

Calculate drug dosage based on patient weight. Essential for pediatric dosing, oncology, and weight-based medications.

Reviewed by Chase FloiedUpdated

This free online drug dosage calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Drug Dosage Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.

2

Review your inputs

Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The Drug Dosage Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

Formula Reference

Drug Dosage Calculator Formula

See calculator inputs for the governing equation

Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.

When to Use This Calculator

  • •Use the Drug Dosage Calculator for personal health tracking and wellness monitoring, establishing a baseline and tracking changes over time.
  • •Use it when recording fitness metrics to track progress toward health or athletic goals.
  • •Use it to compare measurements before and after a lifestyle, diet, or training change to quantify the impact.
  • •Use it as a conversation starter before a doctor's appointment, bringing objective data to discuss with a healthcare professional.

About This Calculator

The Drug Dosage Calculator is a free health and wellness calculation tool designed for personal use and general informational purposes. Calculate drug dosage based on patient weight. Essential for pediatric dosing, oncology, and weight-based medications. This calculator provides reference values based on established health screening formulas and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Results may vary based on individual factors not captured by the calculation. Consult a qualified healthcare provider for personalized health guidance. All calculations are performed locally in your browser — no personal health data is transmitted or stored.

About Drug Dosage Calculator

The Drug Dosage Calculator computes medication doses based on patient weight — the standard approach in pediatric medicine and for many adult medications including chemotherapy, antibiotics, and weight-based protocols. Weight-based dosing ensures patients receive proportional amounts of medication appropriate for their body size, avoiding under-dosing in larger patients and over-dosing in smaller patients. This calculator handles single-dose and total daily dose calculations for medications prescribed at a specified mg/kg rate.

The Math Behind It

Weight-based dosing is standard for many medications because pharmacokinetic parameters (absorption, distribution, metabolism, excretion) scale with body size. The basic formula is simple: multiply the dose per kilogram by the patient's weight in kilograms. **When weight-based dosing is used**: - **Pediatrics**: Almost all pediatric medications use mg/kg dosing - **Chemotherapy**: Usually uses BSA (mg/m²) but some drugs use mg/kg - **Antibiotics**: Many have weight-based dosing for children and sometimes adults - **Anesthetics**: Induction agents and muscle relaxants are dosed by weight - **Heparin and other anticoagulants**: Weight-based loading and maintenance doses **Important considerations**: 1. **Dose capping**: Many drugs have maximum doses regardless of weight. For example, acetaminophen might be dosed at 15 mg/kg but capped at 1000 mg per dose in adults. 2. **Obese patients**: For some drugs (especially hydrophilic ones), using actual body weight in obese patients can cause overdose. Adjusted or ideal body weight may be more appropriate. 3. **Lean body mass dosing**: For drugs that distribute primarily to lean tissue, dosing by lean body mass is more accurate than total weight. 4. **Round to available strengths**: Calculated doses often don't match commercial formulations. Round to the nearest practical dose (typically within ±10%). **Always verify**: Weight-based dosing doesn't replace clinical judgment. Always check drug references, consider comorbidities, drug interactions, and organ function.

Formula Reference

Weight-Based Dosing

Dose = Dose_per_kg × Weight

Variables: Dose per kg from drug reference, weight in kg

Worked Examples

Example 1: Pediatric Amoxicillin

20 kg child, 40 mg/kg/day divided into 3 doses

Step 1:Daily total: 40 × 20 = 800 mg/day
Step 2:Per dose: 800 / 3 = 267 mg
Step 3:Round to 250 mg per dose

Prescribe 250 mg three times daily (close to calculated 267 mg).

Example 2: Adult Gentamicin

70 kg adult, 5 mg/kg once daily

Step 1:Single dose: 5 × 70 = 350 mg
Step 2:Daily: 350 mg (once daily)

Prescribe 350 mg IV once daily. Monitor serum levels.

Common Mistakes & Tips

  • !Forgetting to divide total daily dose by frequency when drug is dosed multiple times per day.
  • !Using actual body weight for obese patients when adjusted body weight is more appropriate for the specific drug.
  • !Exceeding maximum dose limits. Always check for dose caps.
  • !Confusing total daily dose with single dose when reading drug references.

Related Concepts

BSA Calculator

For drugs dosed by body surface area (common in oncology).

Drug Half-Life

Determines dosing frequency and washout periods.

Used in These Calculators

Calculators that build on or apply the concepts from this page:

Corticosteroid Conversion Calculator

Drip Rate Calculator (mL/hr)

Drug Half-Life Calculator

IV Flow Rate Calculator

Opioid Equianalgesic Calculator

Frequently Asked Questions

Should I use actual or ideal body weight for obese patients?

It depends on the drug. Lipophilic drugs (benzodiazepines, propofol) generally use actual weight. Hydrophilic drugs (aminoglycosides, neuromuscular blockers) often use ideal or adjusted body weight to avoid overdose.

Why cap doses?

Pediatric doses are calculated by weight, but as children grow into adolescents, the mg/kg calculation can exceed the maximum safe adult dose. Caps prevent this. Always check drug references for maximums.