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Pharmaceutical Tools

Drug Half-Life Calculator

Calculate remaining plasma concentration, time to reach a target level, or view a full elimination table — all from the drug's half-life.

Core Formula

Ct = C0 × (0.5)t / t½

Ct = concentration at time t  |  C0 = initial concentration
t = elapsed time  |  t½ = drug half-life (same time unit)

Remaining Conc.

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% Remaining

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%

Half-lives elapsed

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half-lives

0% remaining 100% remaining

Time needed

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Half-lives required

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half-lives

% Reduction

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%

Half-lives Elapsed time Concentration % Remaining Status

Half-Life Pharmacology Facts

50%

after 1 half-life

25%

after 2 half-lives

12.5%

after 3 half-lives

6.25%

after 4 half-lives

3.125%

after 5 half-lives

4–5×

half-lives to steady state

After 5 half-lives, less than 3.125% of the original dose remains — the drug is considered clinically eliminated. This same principle governs the time to reach steady state: after 4–5 half-lives of consistent dosing, plasma levels plateau at the steady-state concentration.

Clinical example

Warfarin has a half-life of approximately 36 hours. After 3 days (72 hours): 72 ÷ 36 = 2 half-lives have elapsed → 25% of the drug remains. Full washout (5 half-lives) requires approximately 7.5 days (180 hours).

How to Use This Calculator

1
Remaining concentration: Enter C0, drug half-life, and elapsed time (in matching units). The result shows remaining concentration, % remaining, and a visual bar.
2
Time to target: Enter C0, your target concentration, and half-life to find how long until the drug drops to that level.
3
Elimination table: Enter C0 and half-life to generate a row-by-row table showing concentration and % remaining at each half-life interval from 1 to 7.

Frequently Asked Questions

Drug half-life (t½) is the time required for the plasma concentration of a drug to decrease by 50%. It determines dosing interval, time to steady state, and how long a drug remains in the body after cessation.
After 4–5 half-lives, less than 3–6% of the drug remains and it is considered clinically eliminated. For example, a drug with a 6-hour half-life is essentially cleared within 24–30 hours.
Half-life guides dosing frequency (typically every 1–2 half-lives), predicts when steady state is reached (~4–5 half-lives), defines washout periods before switching medications, and helps time therapeutic drug monitoring tests for accurate trough or peak levels.
Half-life is a pharmacokinetic (PK) property — it describes plasma concentration decay. Duration of action is a pharmacodynamic (PD) property — it depends on whether the concentration remains above the minimum effective concentration (MEC). A drug may be active at concentrations well below the initial peak if they still exceed the MEC.
Approximately 4–5 half-lives after starting consistent dosing at a fixed interval. At steady state, drug input equals drug elimination, so plasma levels fluctuate predictably within a therapeutic window.
Yes. Renal impairment extends the half-life of renally-cleared drugs, increasing accumulation and toxicity risk — dose reduction or interval extension is required. Hepatic impairment extends the half-life of hepatically-metabolised drugs. eGFR (creatinine clearance) guides renal adjustments; Child-Pugh or MELD score guides hepatic adjustments.

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