What is a drug half-life?

A drug's half-life is the time required for the concentration in the body to decrease by half. After one half-life, 50% remains; after two, 25% remains; and so on.

How many half-lives until a drug is fully cleared?

After 5 half-lives, about 3% of the original dose remains, which is generally considered clinically insignificant. After 7 half-lives, less than 1% remains.

Is this tool medically accurate?

This tool models first-order (linear) pharmacokinetics, which applies to most drugs. Some drugs follow non-linear kinetics and may behave differently. Always consult a healthcare professional for medical decisions.

What units should I use?

You can use any consistent unit for dose (mg, mcg, g, etc.). The half-life and elapsed time should both be in the same time unit (hours is the default, but you can use minutes or days as long as you're consistent).

Does the calculator account for absorption time?

No. This calculator assumes the drug is already fully absorbed and models elimination only. For oral drugs, peak concentration occurs after the absorption phase, which varies by formulation.

Half Life Drug Calculator

Name: Half Life Drug Calculator
Availability: InStock
Author: Nham Vu

Enter a drug dose and its half-life to see how much remains in your body over time.

Drug Parameters

Initial Dose

Half-Life (hours)

Elapsed Time (hours) — optional

Quick Examples

Enter dose and half-life to see results

Elimination Schedule

Half-Lives	Time Elapsed	Remaining	% Left

Summary

Enter a drug dose and its half-life to see how much remains in your body over time.

How it works

Enter the initial dose (in any unit, e.g. mg).
Enter the drug's half-life in hours.
Optionally enter a time elapsed in hours to calculate concentration at that exact moment.
The calculator applies the formula: C = C₀ × (0.5)^(t / t½).
A table shows remaining concentration at each half-life interval up to 10 half-lives.
A bar chart visualizes how the concentration drops over time.

Use cases

Estimate how long a medication stays active in your system.
Understand pharmacokinetics for drug interaction research.
Calculate safe re-dosing windows for medications.
Determine roughly when a drug will be below detectable levels.
Educational demonstration of exponential decay in pharmacology.
Plan medication timing around procedures or other treatments.

Frequently Asked Questions

Last updated: 2026-06-14 · Reviewed by Nham Vu