Drug Half-Life Calculator

See how much of a substance remains in the body over time from its elimination half-life.

Amount at the start	100 mg
Half-life	6 hours
Time elapsed	24 hours

Amount at the start

Half-life

hours

Time elapsed

hours

Remaining

6.25 mg(6.25%)

After 4 half-lives. About 97% is eliminated after 5 half-lives (30 hours), which is why that's often used as 'cleared'. This simple model ignores absorption, repeat doses and individual differences.

Amount over time (hours)

6.3at (24, 6.25)Point at the chart, or Tab to it and use ← →

6.3: (24, 6.25)

About the Drug Half-Life Calculator

A half-life is the time for the amount of a drug in the body to fall by half. After one half-life 50% remains, after two 25%, and after five about 3%. Half-lives vary a lot between people with age, liver and kidney function and other medicines. This is for education only: don't use it to time doses or decide when it's safe to drive or drink.

The formula

Remaining = start × 0.5^(time ÷ half-life)

Worked example

Example: Remaining = 8.381 mg (7.62%)

Inputs: Amount at the start: 110 mg; Half-life: 7 hours; Time elapsed: 26 hours.
Formula: Remaining = start × 0.5^(time ÷ half-life)
Result: Remaining = 8.381 mg (7.62%)

Things to keep in mind

Not medical advice. Ask a pharmacist about your medicines.

Questions people ask

How long until a drug is gone?

About 4–5 half-lives for most practical purposes.

Do repeated doses build up?

Yes, levels reach a steady state after about 4–5 half-lives of regular dosing.