About the Medication Half-Life Calculator
Every medicine has an elimination half-life — the time it takes for the amount in the body to fall by half. Knowing it answers common questions such as "how long does this medication stay in my system?", "when will it be essentially gone after I stop?" and "how long until a regular dose reaches a stable level?". This calculator turns a half-life into those answers using standard first-order (exponential) elimination.
Enter the half-life from the product information or a pharmacology reference, the time since the last dose, and — if the drug is taken regularly — the dosing interval. You get the percentage still in the body, the number of half-lives elapsed, the time to roughly 97% elimination (five half-lives), the time to approach steady state and the accumulation ratio, plus a chart of the decline.
Results are expressed as a percentage of a dose, not in milligrams, and the tool does not suggest doses or timing. Real half-lives vary between people with age, kidney and liver function, interacting drugs and genetics, and some drugs have active metabolites that last much longer than the parent drug. Use it for understanding, and ask a pharmacist or prescriber before changing how you take any medicine.
With the default inputs, the amount remaining (of last dose) is 6.25%. Change any value above to recalculate instantly.
How to use the medication half-life calculator
- 1Choose whether you will enter times in hours or days.
- 2Enter the drug’s elimination half-life from its label or a trusted reference.
- 3Enter the time since the last dose.
- 4If the medicine is taken regularly, enter the dosing interval; otherwise leave it at 0.
- 5Read the percent remaining, the 5-half-life clearance time and the steady-state estimate.
Formula and method
Most drugs follow first-order kinetics: a constant fraction is eliminated per unit time, so the amount halves every half-life. After t hours the fraction left is (½) raised to the number of half-lives elapsed. After 5 half-lives about 3.1% remains (97% eliminated), which is why "five half-lives" is the usual rule of thumb for clearance.
With repeated doses at a fixed interval τ, each dose adds to what remains of earlier ones. Levels rise until elimination per interval equals the amount given, which takes the same 4–5 half-lives regardless of dose size. The accumulation ratio R compares steady-state levels with those after the first dose. The chart uses an idealised instant-absorption model; real absorption, distribution and active metabolites change the shape.
- t
- Time since the dose
- t½
- Elimination half-life
- τ
- Dosing interval
- R
- Accumulation ratio at steady state
Worked examples
6-hour half-life, 24 hours after the last dose
24 ÷ 6 = 4 half-lives, so ½⁴ = 6.25% of the last dose remains. Five half-lives (30 hours) clears about 97%. Taken every 8 hours, levels accumulate to 1 ÷ (1 − ½^(8/6)) ≈ 1.66× the first-dose level.
36-hour half-life, 4 days after a once-daily drug is stopped
96 ÷ 36 = 2.67 half-lives, leaving about 15.7% of the last dose. A drug with this half-life needs about 180 hours (7.5 days) to approach steady state and accumulates to roughly 2.7× the first-dose level.
Single dose, 12-hour half-life, 3 days later
72 ÷ 12 = 6 half-lives, so ½⁶ = 1.56% remains and 98.44% has been eliminated. Because it was a single dose there is no accumulation ratio.
Frequently asked questions
How many half-lives until a drug is out of your system?+
About 4–5 half-lives. After 4 half-lives 6.25% remains and after 5 about 3.1%, which is usually treated as clinically negligible. Drug tests may still detect metabolites for longer.
How long does it take to reach steady state?+
Steady state is approached after roughly 4–5 half-lives of regular dosing, whatever the dose. A drug with a 24-hour half-life therefore takes about 4–5 days to reach stable levels.
Why does my medication last longer than its half-life suggests?+
Active metabolites, slow release formulations, reduced kidney or liver function, older age and interacting drugs can all prolong how long a medicine acts. The published half-life is an average for typical adults.
Does this calculator tell me when to take my next dose?+
No. It shows how a drug is eliminated in general terms only. Dose timing and amounts must come from your prescriber, pharmacist or the medicine’s instructions.
What is the accumulation ratio?+
It is how much higher drug levels are at steady state compared with after the first dose. It depends only on the dosing interval relative to the half-life: dosing once per half-life gives a ratio of 2.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.