Drug Half-Life Calculator

A drug's elimination half-life is the time it takes for the amount of the drug in the body to fall by half¹. For most medicines, each half-life removes half of what is left, so the math needs only two numbers: the half-life and the time that has passed.

What Drug Half-Life Measures

Half-life measures how fast the body removes a drug, not how strong it is or how long it works. It depends on how quickly the liver and kidneys clear the drug (clearance) and how widely it spreads into body tissues (volume of distribution)¹. Some medicines also break down into active metabolites, which still have an effect and can last much longer than the drug itself³ ⁴.

The Formula

Amount left = starting amount x 0.5^(time ÷ half-life)

Percent left = 100 x 0.5^(time ÷ half-life)

Time to reach a chosen percent = half-life x log2(100 ÷ percent)

Build-up with repeat doses = 1 ÷ (1 - 0.5^(dosing interval ÷ half-life))

These come from first-order elimination, where a constant fraction of the drug is removed in each unit of time; the last line is the accumulation ratio for equal doses at equal intervals².

How This Calculator Computes Your Result

Enter the half-life (minutes, hours or days), the time since the dose, and a target percent (10% by default). The dose is optional: leave it blank to see percents, or enter it in mg, mcg, g or units to see amounts. The tool converts everything to hours and shows the amount and percent left, the half-lives passed, the time until your target percent remains, and a table and chart of the first 7 half-lives. Results below 0.001% show as "less than 0.001%".

Example: 50 mg of a medicine with a 26 hour half-life, 72 hours later. That is 72 ÷ 26 = 2.77 half-lives, so 50 x 0.5^2.77 = 7.33 mg is left (14.7%). Falling to 10% takes 26 x log2(10) = 86 hours, or 3.6 days.

An optional repeat-dose view (a dosing interval and 2 to 50 doses) shows the level just after each dose, just before the next, and the percent of steady state reached.

What Your Result Means

Each half-life removes half of what is left:

About 90% of a drug is eliminated after roughly 3.3 half-lives, and 94% to 97% after 4 to 5 half-lives¹. That is why "about 5 half-lives" is the usual rule of thumb for a drug to be mostly gone. It is a math rule, not a sign that a medicine has no effect left.

Repeat Doses and Steady State

When a dose is taken before the last one is gone, the new dose adds to what is left. Levels climb with each dose until the amount removed between doses equals the dose, a point called steady state, which takes about 4 to 5 half-lives¹ ². Build-up mainly happens when doses are closer together than about 4 half-lives².

The sertraline label shows this in practice. Its half-life is about 26 hours, and once-daily dosing gives about a two-fold build-up, with steady state reached after one week³. The formula agrees: 1 ÷ (1 - 0.5^(24 ÷ 26)) = 2.12, and 5 half-lives is 130 hours, about 5.4 days. In older adults, sertraline clearance was about 40% lower and steady state took 2 to 3 weeks³.

Example Half-Lives From Drug Labels

These values are quoted from US prescribing information. The third column is the half-life times 5, as math only.

Over-the-counter Drug Facts labels do not list a half-life, so the ibuprofen value comes from a prescription ibuprofen tablet label¹⁰ and the acetaminophen value from an injectable label that combines acetaminophen and ibuprofen⁶.

Why Real Clearance Varies

A label half-life is an average from study groups. Your own can be shorter, or much longer:

• Liver: in mild liver impairment, sertraline exposure was about 3 times higher³. In cirrhosis, diazepam's average half-life rose 2 to 5 fold⁴, and fluoxetine's averaged 7.6 days in one study⁵.

• Kidneys: amoxicillin is mostly removed in urine, and its label says the dose usually needs adjusting when kidney filtration (GFR) is under 30 mL/min⁷. Sertraline, which is mostly broken down first, appeared unaffected by kidney impairment³.

• Age: diazepam's half-life rises about 1 hour for each year of age, from about 20 hours at age 20⁴. That puts 5 half-lives at about 4.2 days at age 20 and 14.6 days at age 70. Newborns clear caffeine far more slowly than adults⁸.

• Other medicines: with diazepam, cimetidine, ketoconazole, fluvoxamine, fluoxetine and omeprazole may lead to increased and prolonged sedation⁴. Cimetidine and ketoconazole slow caffeine elimination, while phenobarbital and phenytoin speed it up⁸.

• Genes: the FDA lists CYP2C19 poor metabolizers as having possibly altered diazepam levels, and CYP2C9 poor metabolizers (or *3 carriers) as possibly having higher ibuprofen levels⁹.

• Extended release forms: these release the drug over hours, but this calculator assumes the whole dose is in the body at once, so it fits them less well.

• Alcohol: the body removes a roughly constant amount over time rather than a constant fraction, so half-life math does not apply¹ ².