Drug / Medication Half-Life Calculator
Calculate how much of a drug remains in your body after a given time using its half-life, plus how long until it is ~99% cleared. Free, with the formula shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
| Half-lives | Time (h) | % remaining | Amount (mg) |
|---|
Show the formula & steps
How the Drug Half-Life Calculator Works
This calculator estimates how much of a medication remains in your body after a period of time, using the drug’s half-life. Enter the starting dose, the drug’s half-life in hours, and the time elapsed, and it returns the amount remaining, the percentage remaining, and roughly how long until the drug is nearly gone.
Most drugs are cleared by first-order elimination, meaning a fixed fraction is removed in each equal time interval. That produces a smooth exponential decay that the formula below describes exactly.
The Formula
Remaining = Dose × (1/2)^(time ÷ half-life)
Here time and half-life must use the same unit (hours in this tool). The exponent time ÷ half-life is simply the number of half-lives that have passed. To get the percentage remaining, divide the remaining amount by the dose and multiply by 100.
A drug is conventionally treated as effectively eliminated after 5 half-lives (about 96.9% gone) and about 99% gone after roughly 6.6 half-lives, because 0.5^6.64 ≈ 0.01.
Worked Example
Suppose you take a 500 mg dose of a drug with a 6-hour half-life and want the amount left after 12 hours:
- Number of half-lives = 12 ÷ 6 = 2
- Remaining = 500 × (1/2)² = 500 × 0.25 = 125 mg
- Percent remaining = 125 ÷ 500 × 100 = 25%
- Time to ~99% cleared = 6.64 × 6 ≈ 39.8 hours
Half-Life Decay Reference
| Half-lives elapsed | Fraction remaining | Percent remaining |
|---|---|---|
| 1 | 1/2 | 50% |
| 2 | 1/4 | 25% |
| 3 | 1/8 | 12.5% |
| 4 | 1/16 | 6.25% |
| 5 | 1/32 | 3.13% |
| 6 | 1/64 | 1.56% |
Why Half-Life Matters
Knowing a drug’s half-life helps you understand:
- When effects fade — as concentration drops, so does the therapeutic effect.
- Dosing intervals — drugs are often dosed roughly once per half-life to keep levels steady.
- Washout and interactions — how long to wait before a new drug or a clean drug test.
- Time to steady state — repeated dosing reaches a stable level after about 4 to 5 half-lives.
This tool is for educational estimates only and uses simple single-dose kinetics. It is not medical advice. Always follow the directions from your prescriber and pharmacist.
Frequently asked questions
How do I calculate a drug's half-life elimination?+
Multiply the dose by one-half raised to the power of (time elapsed ÷ half-life). For a 500 mg dose of a drug with a 6-hour half-life, after 12 hours the amount remaining is 500 × 0.5^(12 ÷ 6) = 500 × 0.5^2 = 125 mg.
How many half-lives until a drug is out of your system?+
After 1 half-life 50% remains, after 2 about 25%, after 3 about 12.5%, after 4 about 6.25%, and after 5 about 3.1%. A drug is generally considered eliminated after 5 half-lives (about 97% gone) and roughly 99% gone after about 6.6 half-lives.
What is a drug's half-life?+
A drug's half-life is the time it takes for the concentration or amount in your body to fall by half. Because elimination is first-order, the same fraction is removed in each equal time interval, so the amount decays exponentially.
Does this calculator account for repeated doses?+
No. It models a single dose decaying by first-order elimination. With repeated dosing the drug accumulates until it reaches steady state, which also takes about 4 to 5 half-lives. Use the single-dose result as the building block for that picture.
Can I use this for any medication?+
Yes, as long as you know the drug's elimination half-life, which you can find in the prescribing information. This is an educational estimate using simple first-order kinetics and is not medical advice; always follow your clinician's and pharmacist's guidance.