Medication half-life calculator
See how much of a substance is left after a given time, from a half-life you already know. General information only, with no dosing advice.
The details
Change a value and the result updates.
How it works
The amount left is the starting amount × 0.5 to the power of the time passed ÷ the half-life. With a 6 hour half-life, 12 hours is 2 half-lives, so 25% is left. With a 24 hour half-life, 3 days is 3 half-lives, so 12.5% is left.
This assumes one dose that clears at a steady rate. Regular doses build up differently and some medications do not follow this pattern.
What is left after each half-life
| Half-lives | Left |
|---|---|
| 1 | 50% |
| 2 | 25% |
| 3 | 12.5% |
| 4 | 6.25% |
| 5 | 3.13% |
About 5 half-lives is the point commonly cited for when most of a single dose has gone. It is a rough guide and not a rule for any one person.
This is general information and not medical advice. Do not change a dose or when you take it because of this result. Ask your pharmacist or doctor about your own medication.
Common questions
What does half-life mean?
It is the time the body takes to clear half of the amount of a substance. After one half-life 50% is left, and after two 25% is left.
Why is 5 half-lives often mentioned?
After 5 half-lives about 3% is left, so it is commonly cited as a rough point when most of a single dose has gone. It is a rule of thumb and not a limit that applies to everyone.
Does this tell me when to take my next dose?
No. It is arithmetic for general information. Follow the directions from your doctor or pharmacist and do not change a dose or its timing because of this result.
Why might the real figure be different?
Half-life varies from person to person. Age, liver and kidney function and other medications can all change it, and regular doses build up differently to a single dose.
Keep track with Cadence
Cadence is a medication reminder app for iPhone and Apple Watch. It keeps your doses on the schedule you were given.
More free tools: Missed dose time checker, Medication schedule maker, Liquid medicine dose converter, All free tools