Drug Half-Life Calculator
See how a drug fades after a single dose, how long it takes to clear a target percentage, and how repeated doses build up to steady state — with an interactive decay curve, a 0–10 half-life table, and multiple-dose accumulation math.
Half-life calculator
Single dose and multiple doses in one tool. Everything runs in your browser — nothing is uploaded.
Decay curve
Drug half life chart: fraction of the dose remaining over time, with your selected time marked.
Elimination timeline
| Eliminated | Remaining | Half-lives | Time |
|---|
Drug half life table: amount after each half-life (0–10)
| Half-lives | Time | Remaining % | Remaining mg |
|---|
Build-up to steady state
The drug half life multiple doses accumulation chart below simulates each dose adding to the remainder of the last, up to steady state.
Drug half life table: amount after each half-life (0–10)
| Half-lives | Time | Remaining % | Remaining mg |
|---|
How the math works
First-order elimination: a constant fraction of the drug is cleared per unit time. That is what makes the half-life constant — the drug half life definition behind this page. The drug half life formula for the first order elimination half life model is shown below each case.
Single dose
After one half-life, half remains; after two, a quarter (the percent of drug remaining after 2 half lives is 25%); after three, an eighth. The elimination rate constant is k = ln(2) / half-life ≈ 0.693 / half-life. This is how to calculate drug half life decay for any starting amount and any elapsed time. This page also works as an elimination rate constant calculator: k is shown for every half-life you enter, and how to calculate elimination rate constant by hand is simply k = 0.693 / half-life.
Time to reach a target percentage
Rule of thumb for how many half lives to eliminate a drug: about 4.3 half-lives to 95% eliminated and about 6.6 half-lives to 99% eliminated (how long does it take for 99 percent of a drug to leave your system: 6.6 half-lives). Another way to ask how many half lives until a drug is out of your system: about 5 half-lives leaves only about 3% behind — that is why 5 half lives is the standard rule of thumb. This time-to-target estimate is sometimes called drug washout time.
Multiple doses: accumulation to steady state
R is the drug accumulation factor: how many times larger the steady-state level is versus a single dose. If you are wondering what is the accumulation factor in pharmacokinetics, it is exactly this steady-state-to-single-dose ratio. The drug half life calculator multiple dose mode uses the same k together with your dosing interval to estimate R and the multiple dose steady state peak trough values. Steady state is approached exponentially — about 93.8% after 4 half-lives and 96.9% after 5. The steady state 4 to 5 half lives rule holds because reaching steady state and washing out are mirror images of the same exponential process. The calculator reports time to 90 percent steady state (3.3 half-lives), 95% (4.3), and 97% (5.1). In short, pharmacokinetics half life steady state explained: repeated doses accumulate by the factor R until the amount eliminated per interval equals one full dose.
Scope boundary: what this tool is (and is not)
Half-life means different things in different fields. This page covers one of them.
This tool: drug elimination timing (educational)
Estimates how a drug amount decays over time under simplified first-order elimination, plus multiple-dose accumulation to steady state. It answers “how much is left?” and “how long until X% is gone?” — nothing about whether a dose is right for anyone.
Not this tool: choosing a dose amount
Estimating how much medicine to give (for example weight-based pediatric dosing) is a different calculation with different safety rules. For that kind of estimator see our amoxicillin pediatric dosage estimator, which is likewise educational and not medical advice.
Not this tool: radioactive or diving half-times
Radioactive decay half-lives (physics) and tissue nitrogen half-times in scuba diving models are separate domains with their own formulas — this page does not cover them.
Not this tool: drug testing or withdrawal timing
Detectability in blood, urine, or hair tests depends on test sensitivity cutoffs, metabolites, and individual factors — not on the parent drug’s half-life alone. Do not use this page to predict test results or to time stopping a medicine.
Half-life reference: educational examples
The same presets as the calculator above, including a drug half life caffeine example, an ibuprofen half life example, and a diazepam half life example. Single approximate values for illustration — published references usually give ranges, and the real value varies by person, dose, and formulation.
| Example | Illustrative half-life | Note |
|---|---|---|
| Caffeine | ≈ 5 hours | Commonly cited 3–7 h in healthy adults |
| Ibuprofen | ≈ 2 hours | Commonly cited 1–3 h in healthy adults |
| Acetaminophen | ≈ 2.5 hours | Commonly cited 1–4 h |
| Loratadine | ≈ 8 hours | Reference ranges vary by source |
| Diazepam | ≈ 48 hours | Long; active metabolites extend effects |
| Fluoxetine | ≈ 120 hours (5 days) | Very long; active metabolite even longer |
Frequently asked questions
Drug half life explained in plain-language answers. Educational only — not medical advice.
What is drug half life?
A drug's half-life is the time it takes for the amount of the drug in the body to fall to one-half of its current level, assuming first-order elimination. For example, if 200 mg has a 5-hour half-life, about 100 mg remains after 5 hours and about 50 mg after 10 hours. It is a property of how the body clears the drug, not a timer for when effects wear off.
How do you calculate how much drug is left after a given time?
For first-order elimination, multiply the starting amount by one-half raised to the number of half-lives that have passed: remaining = starting amount x (1/2)^(time / half-life). This calculator applies that formula and also converts the result into milligrams and percent remaining. That is how to calculate remaining drug concentration at any time point.
How many half-lives does it take to clear a drug from the body?
A common rule of thumb is 4 to 5 half-lives: after 4 half-lives about 93.8% is eliminated (6.3% remains), and after 5 half-lives about 96.9% is eliminated (3.1% remains). Mathematically the amount never reaches exactly zero, so 'cleared' is a practical convention, not an absolute. Higher starting doses take longer to fall below any given threshold.
How long does it take for a drug to leave your system?
It depends on the drug's half-life and what you mean by 'leave': roughly 4 to 5 half-lives removes about 94-97% of a single dose. A drug with a 2-hour half-life is mostly eliminated in about 8-10 hours, while one with a 48-hour half-life needs about 8-10 days. Enter the half-life above to get the exact timeline for any target percentage.
Is half-life the same as how long a drug's effects last?
No. Half-life describes elimination; duration of action describes how long effects are felt. Effects often fade well before the drug is fully eliminated because they depend on concentration staying above a therapeutic threshold. Never use half-life alone to decide when it is safe to drive, drink alcohol, or take another dose - ask your pharmacist. So is half life the same as drug duration? No - and the half life vs duration of action distinction matters because effects usually fade well before elimination finishes.
Does taking a larger dose change the half-life?
So does half life change with dose? For most drugs, no: first-order elimination means a constant fraction is cleared per unit time, so the half-life stays the same regardless of dose. A larger dose does take longer to fall below any fixed threshold (for example below 5% remaining). A few drugs, such as alcohol and high-dose aspirin, follow zero-order elimination where a constant amount is cleared per hour instead.
What is the difference between first-order and zero-order elimination?
In first-order elimination, a constant fraction of the drug is cleared per unit time, so the half-life is constant - this applies to most drugs. In zero-order elimination, a constant amount is cleared per unit time, so the half-life changes as the concentration falls. This zero order vs first order elimination distinction is why the constant-half-life math on this page applies to most drugs but not to zero-order ones. This calculator models first-order elimination only.
What is the elimination rate constant (k)?
The elimination rate constant k is the fraction of drug cleared per unit time, calculated as k = ln(2) / half-life, which is about 0.693 / half-life. A drug with an 8-hour half-life has k = 0.0866 per hour, meaning about 8.7% of the remaining amount is cleared each hour. This calculator shows k for every half-life you enter.
What happens when a drug is taken repeatedly - what is the accumulation factor?
With repeated doses, each new dose adds to the remainder of previous ones until elimination balances intake. The accumulation factor R = 1 / (1 - e^(-k x dosing interval)) tells you how many times larger the steady-state level is compared with a single dose. If the dosing interval equals one half-life, R = 2, so the body eventually holds about twice one dose at peak.
How long does it take to reach steady state, and why 4 to 5 half-lives?
Steady state is approached exponentially: after n half-lives you are at 1 - (1/2)^n of the final level - 87.5% after 3, 93.8% after 4, and 96.9% after 5 half-lives. That is the same 4-to-5 rule used for elimination, because reaching steady state and washing out are mirror images of the same exponential process.
What are steady-state peak and trough levels?
At steady state the drug level rises to a peak right after each dose and falls to a trough just before the next one, then repeats. This calculator estimates both as multiples of a single dose: for example, with a half-life of 8 hours dosed every 8 hours, the peak settles at about 2.0x one dose and the trough at about 1.0x one dose.
What is the half-life of caffeine?
Caffeine's half-life in healthy adults is often cited around 5 hours, but it varies widely - roughly 3 to 7 hours - with age, pregnancy, smoking, liver function, and other medications. This tool includes it only as an editable educational example: the preset fills in 5 hours, and you should change it to match your own reference. For how long does caffeine stay in your system: about 4 to 5 half-lives - roughly 20 to 25 hours when the caffeine half life hours value used is 5.
What is the half-life of ibuprofen?
Ibuprofen's elimination half-life is often cited around 2 hours in healthy adults, with references commonly giving a range of about 1 to 3 hours. Like caffeine, it is included here only as an editable educational example - actual values vary by person and formulation, so treat the preset as a starting point, not a medical fact. The ibuprofen half life most often cited is about 2 hours, so for how long does ibuprofen stay in your system, 4 to 5 half-lives is roughly 8 to 10 hours for most of a dose.
Can this calculator tell me when to take my next dose? Is it medical advice?
No. This is an educational estimator that shows the mathematics of first-order elimination; it is not medical advice and cannot account for your age, liver or kidney function, other medicines, or the specific formulation you take. Never start, stop, or change a dose based on this page - always follow your prescription label and your doctor's or pharmacist's instructions. For how long to wait before a drug is cleared from the body, only your prescriber or pharmacist can advise. Questions like can you take another dose after one half life cannot be answered by half-life math alone - dosing intervals are set by the prescriber.
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