How to Use Drug Half-Life Calculator

About this tool

Elimination half-life, rate constant and amount remaining over time.

How to use Drug Half-Life Calculator

1. Choose half-life from two levels, from k, k from half-life, or amount remaining.
2. Enter the concentrations and times, k, or the half-life.
3. Read the result and the table of half-lives.

Worked example

Levels of 24 mg/L at 1 h and 9 mg/L at 7 h give k = 0.163 per hour and a half-life of 4.24 hours.

How long until a drug is gone?

About 5 half-lives remove roughly 97% of the drug. The same time is needed to reach steady state with regular dosing.

Does this work for every drug?

It assumes first-order (exponential) elimination. Drugs such as phenytoin and alcohol at high levels do not follow it.

Put this guide into practice

• Drug Half-Life Calculator — Elimination half-life, rate constant and amount remaining over time.