Drug Half Life Calculator
See how much of a drug remains after time passes. Enter the starting amount (mg), the half-life in hours, and the elapsed time to get the amount left, the percent remaining, and the time to clear roughly 97%.
Example: with Starting amount (mg) 500 · Half-life (hours) 4 · Time elapsed (hours) 12 → Amount remaining: 62.5 mg remaining.
- Percent remaining12.5% of the dose left
- Half-lives elapsed3 half-lives elapsed
- Near-complete clearance≈ 20 h to clear ~97% (5 half-lives)
Computed by the calculator below using its default values. Change any input to see your own numbers.
For first-order kinetics, amount remaining = dose × (1/2)^(time ÷ half-life). About 5 half-lives clears roughly 97% of a dose.
How a drug clears over time
Most drugs follow first-order kinetics: a fixed fraction leaves each time interval, so a constant half-life keeps halving whatever is present. Start with 500 mg and a 4-hour half-life, and after 4 hours you have 250 mg, after 8 hours 125 mg, after 12 hours 62.5 mg — three half-lives, or one-eighth left. The formula is amount = dose × (1/2) raised to the number of half-lives elapsed.
That is also why clinicians use the rule of thumb that about five half-lives clears a drug: five halvings leave roughly 3% behind. The same math running the other way explains why it takes about five half-lives of regular dosing to build up to a steady state in the body.
How it’s calculated
Amount remaining = dose × (1/2)^(t ÷ t½); percent remaining = (1/2)^(t ÷ t½) × 100. Half-lives elapsed = t ÷ t½. Near-complete clearance ≈ 5 × t½ (about 96.9% gone). Assumes first-order, single-compartment kinetics.
Real drugs vary with dose, organ function, and interactions, and some (alcohol, high-dose phenytoin, aspirin) follow zero-order kinetics where this model does not apply. Educational estimate, not dosing or safety guidance.
Fraction left by half-lives
| Half-lives | Fraction left | Percent |
|---|---|---|
| 1 | 1/2 | 50% |
| 2 | 1/4 | 25% |
| 3 | 1/8 | 12.5% |
| 4 | 1/16 | 6.25% |
| 5 | 1/32 | 3.125% (~cleared) |
Remaining = dose × (1/2)^(t ÷ t½); ~5 half-lives to clear ~97% (first-order kinetics).
Common mistakes
- Subtracting a fixed amount each interval instead of halving — decay is exponential, not linear.
- Assuming the drug is fully gone after one or two half-lives; it takes about five to clear.
- Applying this to zero-order drugs like alcohol, which leave at a constant amount per hour.
- Treating a population half-life as exact when age, kidney, and liver function shift it.
Frequently asked questions
What is the drug half-life formula?
Amount remaining = dose × (1/2)^(time ÷ half-life). With a 4-hour half-life, 500 mg falls to 62.5 mg after 12 hours, which is three half-lives.
How long until a drug is out of my system?
About five half-lives clears roughly 97%. For a 4-hour half-life that is around 20 hours, though individual clearance varies.
Why does it take 5 half-lives to reach steady state?
Each dosing interval adds drug while the body removes a fixed fraction; the build-up and the wash-out mirror each other, both taking about five half-lives.
Can I use this to time my medication?
No. This is an educational model that ignores your individual physiology and drug specifics. For timing, interactions, or stopping a drug, ask a pharmacist or clinician.