Medication Half-Life Calculator
Remaining drug concentration over time.
Enter your values
Enter the initial dose in mg.
Enter the half-life in hours.
Enter the time elapsed in hours.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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)
At a glance
| What it does | Remaining drug concentration over time. |
|---|---|
| Category | Health |
| Inputs needed | Initial dose, Half-life, Time elapsed |
| Main output | Remaining amount |
| Formula | remaining = dose x (1/2)^(t / half-life) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Initial dose(mg)
- Enter the initial dose in mg.
- Half-life(hours)
- Enter the half-life in hours.
- Time elapsed(hours)
- Enter the time elapsed in hours.
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.
What do I need to enter into the Medication Half-Life Calculator?
Just 3 values: initial dose, half-life, time elapsed. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Medication Half-Life Calculator work out the answer?
Remaining drug amount follows exponential decay, halving during each half-life period. It applies the formula remaining = dose x (1/2)^(t / half-life) and shows the working so you can check each step by hand.
Is the Medication Half-Life Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.