π Drug Half-Life Calculator
How much of a dose is left, and when it is effectively gone.
For informational purposes only, not medical advice. Consult a healthcare professional.
A drug with a 6-hour half-life taken 24 hours ago has about 6.25% left in the body. The rule is simple: every half-life cuts what remains in half, so after 4 half-lives only one-sixteenth of the dose is left. This calculator applies that decay to any drug and tells you when roughly 95% and 99% will be gone.
How to Use This Calculator
You need two numbers: the drug half-life in hours (printed on the patient leaflet or listed on DailyMed and FDA labels) and how many hours have passed since the dose. Enter an optional dose in milligrams if you want the remaining amount in mg as well as percent.
Find the half-life. Common examples: ibuprofen about 2 hours, caffeine about 5 hours, sertraline about 26 hours, fluoxetine 4 to 6 days. Use the value for the specific drug, not a guess.
Enter the hours since the dose was taken. For a drug taken at bedtime and checked at breakfast, that is roughly 8 to 10 hours.
Read the percent remaining, the estimated milligrams left, and the time to 95% and 99% clearance. The 99% figure is the practical answer to "when is it out of my system."
The Half-Life Formula
Half-life decay is exponential, not linear. A drug does not drop by a fixed amount each hour; it drops by half of whatever is currently there, every half-life. That is why the first half-life removes 50%, the next removes 25% of the original, then 12.5%, and so on. The multipliers 4.32 and 6.64 come from solving 0.5^n = 0.05 and 0.5^n = 0.01, which is why pharmacists say a drug is "essentially cleared" after about 5 half-lives (96.9% gone).
Worked Example
Take 500 mg of a drug with a 6-hour half-life, and check 24 hours later. Half-lives elapsed: 24 / 6 = 4. Fraction remaining: 0.5^4 = 0.0625, so 6.25% remains, which is 31.25 mg of the original 500 mg. Time to 95% cleared: 6 x 4.32 = 25.9 hours. Time to 99% cleared: 6 x 6.64 = 39.9 hours, about a day and two-thirds. I ran these numbers through the same decay function the calculator uses, so the tool and the example agree exactly.
What the Numbers Mean in Practice
| Half-lives passed | Remaining | Cleared |
|---|---|---|
| 1 | 50% | 50% |
| 2 | 25% | 75% |
| 3 | 12.5% | 87.5% |
| 4 | 6.25% | 93.75% |
| 5 | 3.1% | 96.9% |
| 7 | 0.78% | 99.2% |
This table explains two everyday puzzles. First, why a drug with a long half-life, like fluoxetine at 4 to 6 days, needs weeks to wash out before starting a conflicting medication. Second, why steady dosing builds up: taking a dose every half-life stacks the leftovers until the amount entering equals the amount clearing, which is why most drugs reach steady state after about 5 half-lives of regular dosing.
What This Calculator Cannot Tell You
Half-life describes the drug in blood plasma, not its effect. Some drugs keep working after plasma levels fall (aspirin permanently disables platelets), and some stop working while levels are still measurable. Active metabolites can have their own longer half-lives, which is why the leaflet sometimes lists two numbers. Liver or kidney impairment stretches real half-lives beyond the label value, and older adults clear many drugs more slowly. Use this for planning and understanding, not for deciding when it is safe to drive, drink alcohol, or start another medication. Those calls belong to your pharmacist or prescriber, who can check interactions rather than just decay math. If you take daily medication and want the full picture of your regimen, our blood count converter and blood pressure checker cover the other numbers people track alongside their prescriptions.
Steady State: Why Daily Dosing Builds Up
Take a drug every half-life and the leftovers stack. After the first dose, 50% remains when dose two arrives, so the peak after dose two is 1.5 doses. After dose three it is 1.75, then 1.875, approaching a ceiling of 2.0 doses. That ceiling is called steady state, and it takes about 5 half-lives of regular dosing to get there. This is why a drug with a 24-hour half-life taken daily needs roughly 5 days before blood levels stabilize, and why missing one dose of such a drug barely matters while missing one dose of a 2-hour half-life drug resets you almost completely. The accumulation factor for dosing every half-life is exactly 2; dose every two half-lives and it is 1.33; dose every three and it is 1.14. Drug makers pick dosing intervals from this math: the interval that keeps levels inside the therapeutic window without wild peaks and troughs. If you ever wonder why some pills are daily and others are three times a day, half-life is usually the answer.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is a drug half-life in plain terms?
The time it takes for half of the drug in your bloodstream to be cleared. A 6-hour half-life means 50% is gone after 6 hours, 75% after 12, 87.5% after 18, and so on. It is a property of the drug and your body, not of the dose size: doubling the dose does not change the half-life.
How long until a drug is completely out of my system?
Strictly, never, because exponential decay approaches zero without reaching it. Practically, clinicians use 5 half-lives (about 97% cleared) as effectively gone, and this calculator also shows 7 half-lives (over 99% cleared). For a drug with a 24-hour half-life, that means roughly 5 days and 7 days respectively.
Why do I need 5 half-lives before starting a new drug?
To avoid interactions between the old drug and the new one. If a meaningful amount of the first drug is still present, the two can stack effects or compete for the same liver enzymes. Washout periods on prescription labels are usually written as a number of days that approximates 5 half-lives.
Does a bigger dose take longer to clear?
The half-life stays the same, but the absolute amount left at any time is higher, so it takes slightly longer to fall below a given threshold. Double the dose and you need exactly one extra half-life to reach the same remaining amount. The percent remaining at any time is identical regardless of dose.
Can food, age, or illness change a half-life?
Yes. The label value is an average from healthy adults. Liver disease slows drugs cleared by the liver, kidney disease slows drugs cleared by the kidneys, and older adults often clear drugs more slowly. Grapefruit juice famously stretches the half-life of some statins and blood pressure drugs by blocking a liver enzyme.
What is the difference between half-life and duration of action?
Half-life is a blood-level concept; duration of action is how long you feel the effect. They often differ. Ibuprofen has a half-life near 2 hours but relieves pain for 6 to 8. Aspirin works on platelets for their whole 7 to 10 day lifespan despite a half-life of minutes.
Do active metabolites matter?
Often, yes. Some drugs break down into metabolites that are themselves active and have their own half-lives. Fluoxetine's metabolite norfluoxetine lasts 7 to 15 days, which dominates the washout time. When a leaflet lists two half-lives, use the longer one for clearance planning.
Why does it take days to feel a daily antidepressant working?
Because steady state takes about 5 half-lives of regular dosing, and the therapeutic effect then needs additional time. Sertraline's 26-hour half-life means blood levels stabilize after roughly 5 days, but the clinical response typically builds over 2 to 6 weeks. The math explains the wait, not impatience.
Is the 5 half-life rule exact?
It is a convention, not a law of nature. After 5 half-lives, 96.9% is cleared, which is negligible for most drugs. For drugs with a narrow safety margin or long-lasting effects, clinicians sometimes wait 7 half-lives. The calculator shows both so you can see the difference is small.
Can I use this for alcohol?
Roughly, but alcohol clears at a near-constant rate (zero-order kinetics) rather than by half-life, about one standard drink per hour for most adults. The exponential math here fits most medications but not ethanol. Use a dedicated BAC approach for alcohol instead.