Medication Half-Life Calculator
Remaining drug concentration over time.
Enter your values
ⓘ Live results
Results
- Remaining amount
- 25 mg
- Percent remaining
- 25%
- Time to ~99% cleared
- 39.9 hours
Quick answer
Remaining drug amount follows exponential decay, halving during each half-life period.
remaining = dose x (1/2)^(t / half-life)
How to use the Medication Half-Life Calculator
- 1Enter the initial dose, the drug's half-life and time elapsed since the dose.
- 2This assumes simple first-order elimination, which approximates but doesn't perfectly match every drug's real pharmacokinetics.
- 3This tool is for educational understanding only — never use it to make actual dosing or timing decisions without consulting a pharmacist or physician.
Worked example
Using the values the calculator loads with:
Inputs
- Initial dose100 mg
- Half-life6 hours
- Time elapsed12 hours
Results
- Remaining amount25 mg
- Percent remaining25%
- Time to ~99% cleared39.9 hours
Frequently asked questions
How long until a drug is fully gone?
By convention, about 4-5 half-lives clear roughly 94-97% of a drug from the body.
Does this apply to all medications?
Only those following first-order (exponential) kinetics; some drugs have more complex elimination.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.