# Drug Half-Life Calculator

Calculate how much medication remains in your body over time. Enter the dose, half-life, and elapsed time to see drug elimination kinetics.

## What this calculates

Estimate how much of a medication remains in your system after a given time period. This calculator uses first-order elimination kinetics to model drug clearance based on the drug's half-life.

## Inputs

- **Initial Dose** (mg) — min 0.01, max 10000 — The amount of drug administered
- **Half-Life** (hours) — min 0.1, max 5000 — The drug's elimination half-life in hours
- **Time Elapsed** (hours) — min 0, max 10000 — Time since the dose was taken

## Outputs

- **Remaining Amount** — Amount of drug still in the body
- **Percentage Eliminated** — Percentage of the dose that has been eliminated
- **Half-Lives Elapsed** — Number of half-lives that have passed
- **Time to 97% Elimination** — Time until approximately 97% of the drug is eliminated (~5 half-lives)
- **Time to 97% Elimination (Days)** — Same duration expressed in days

## Details

A drug's half-life is the time it takes for the concentration of the drug in the body to decrease by half. After one half-life, 50% remains. After two half-lives, 25% remains. After five half-lives, approximately 97% of the drug has been eliminated, which is generally considered complete clearance.

The formula used is: Remaining = Initial Dose x (0.5)^(time/half-life). This assumes first-order elimination kinetics, which applies to most medications at therapeutic doses. Some drugs follow zero-order kinetics (alcohol, aspirin at high doses), where the elimination rate is constant rather than proportional to concentration.

IMPORTANT DISCLAIMER: This calculator is for educational purposes only. Real drug elimination is affected by liver function, kidney function, age, body composition, drug interactions, and genetic factors. Do not use this calculator to make medication timing decisions without consulting a healthcare provider.

## Frequently Asked Questions

**Q: Why does it take 5 half-lives to clear a drug?**

A: After each half-life, the remaining amount is halved: 50% after 1, 25% after 2, 12.5% after 3, 6.25% after 4, and 3.125% after 5 half-lives. At 5 half-lives, approximately 97% of the drug has been eliminated, leaving only about 3%. This residual amount is generally considered clinically insignificant for most drugs, which is why 5 half-lives is the standard benchmark for complete elimination.

**Q: What factors affect a drug's half-life?**

A: Several factors can alter how quickly a drug is eliminated: liver function (most drugs are metabolized by the liver), kidney function (many drugs are excreted by the kidneys), age (elderly patients often have slower clearance), body fat percentage (lipophilic drugs accumulate in fat tissue), drug interactions (some medications inhibit or induce enzymes that metabolize other drugs), and genetic variations in metabolic enzymes (pharmacogenomics).

**Q: Does the dose affect the half-life?**

A: For most drugs following first-order kinetics, the half-life remains constant regardless of dose. Whether you take 100 mg or 500 mg, the time to eliminate 50% is the same. However, some drugs (notably alcohol and phenytoin) follow zero-order kinetics at certain concentrations, where the elimination rate is fixed and the effective half-life increases with higher doses.

**Q: How do I find a drug's half-life?**

A: Drug half-lives are published in pharmacology references, drug package inserts, and medical databases. Common examples include ibuprofen (2-4 hours), acetaminophen (2-3 hours), amoxicillin (1 hour), diazepam (20-100 hours), and fluoxetine (1-6 days). Your pharmacist can provide the half-life for any prescription medication.

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Source: https://vastcalc.com/calculators/health/drug-half-life
Category: Health & Fitness
Last updated: 2026-04-21