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Pharmacokinetics Calculator

Half-Life Calculator: Drug Elimination & Steady State

Calculate remaining plasma concentration, time to reach a target level, or view a full elimination table — all from the drug's half-life. Built for PK study design, washout planning, and therapeutic drug monitoring.

Quick Answer

Drug half-life (t½) is the time for plasma concentration to fall by 50% under first-order elimination. This calculator estimates remaining concentration, time to a target level, and elimination tables from t½. Pharma teams use half-life for dosing interval selection, steady-state timing (4–5 half-lives), crossover trial washout, therapeutic drug monitoring sample windows, and renal or hepatic impairment PK planning.

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)

Half-Life Calculator

Calculate remaining concentration, time to reach a target level, or generate an elimination table from drug half-life.

Concentration and elapsed time

Remaining Conc.

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% Remaining

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%

Half-lives elapsed

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half-lives

0% remaining 100% remaining
Target concentration

Time needed

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Half-lives required

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half-lives

% Reduction

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%

Elimination table inputs
Half-lives Elapsed time Concentration % Remaining Status

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.

Half-Life Elimination Reference

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. The same principle governs 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).

Pharma & clinical trial context

Half-life is a foundational input in Phase 1 pharmacokinetic study design, protocol washout rules, and population PK modeling. Sponsors use t½ to define crossover washout periods (typically ≥5 half-lives), estimate time to steady state before sparse or rich PK sampling, and plan renal or hepatic impairment cohort timing when clearance is reduced and effective half-life lengthens.

Half-life links directly to other PK calculators on this site. Use the Loading Dose Calculator when rapid target concentration is needed for drugs with long half-lives; the Maintenance Dose Calculator when clearance and target Css drive ongoing dosing; the Clearance Calculator to relate t½ to CL and Vd (t½ = 0.693 × Vd / CL); and the AUC Calculator to quantify exposure from concentration–time data collected after steady state or during washout.

Therapeutic drug monitoring protocols specify trough sampling at the end of the dosing interval and peak sampling per label-defined post-dose windows. For crossover trials, residual drug above ~3% of prior exposure after fewer than 5 half-lives can confound period comparisons — use the elimination table mode to document washout adequacy in protocol appendices and investigator training.

Evidence & sources

Frequently Asked Questions

Drug half-life (t½) is the time required for plasma drug concentration to decrease by 50% under first-order elimination. It is a core pharmacokinetic parameter that guides dosing frequency, predicts time to steady state, defines washout before switching therapies, and helps plan therapeutic drug monitoring. Half-life reflects the combined effects of clearance and volume of distribution.
After each half-life, concentration falls by half: 50% after 1, 25% after 2, 12.5% after 3, 6.25% after 4, and 3.125% after 5. Clinically, a drug is considered effectively eliminated after 4–5 half-lives. For example, a drug with a 6-hour half-life is essentially cleared within 24–30 hours after the last dose.
Steady state is reached after approximately 4–5 half-lives of consistent dosing at the same interval. At steady state, the rate of drug input equals the rate of elimination, so average plasma concentration plateaus. A drug with a 12-hour half-life dosed twice daily typically reaches steady state within 48–60 hours. The maintenance dose sets Css; half-life sets how quickly Css is approached.
Half-life is a pharmacokinetic property describing how quickly plasma concentration declines. Duration of action is a pharmacodynamic property — how long the drug produces a clinical effect — and depends on whether concentration remains above the minimum effective concentration (MEC). A drug may still be clinically active at concentrations well below the peak if they exceed the MEC.
Yes. Renal impairment extends the half-life of renally cleared drugs, increasing accumulation and toxicity risk. Hepatic impairment extends the half-life of hepatically metabolised drugs. Both often require dose reduction or interval extension. Creatinine clearance or eGFR guides renal adjustments; Child-Pugh or MELD score guides hepatic impairment PK studies and label dosing.
For first-order elimination, t½ = 0.693 / ke, where ke is the elimination rate constant (units of time⁻¹). A larger ke means faster elimination and a shorter half-life. Half-life can also be expressed as t½ = 0.693 × Vd / CL when clearance (CL) and volume of distribution (Vd) are known from the same compartment model.
When the dosing interval (τ) is shorter than the half-life, each dose adds to residual drug from prior doses and accumulation occurs. The accumulation ratio approaches 1 / (1 − e^−keτ) at steady state. Drugs with long half-lives relative to dosing interval reach higher Css for the same maintenance dose. Renal or hepatic impairment lengthens half-life and increases accumulation risk.
Crossover and drug–drug interaction studies typically require a washout of at least 5 half-lives of the prior drug so that less than ~3% of the previous exposure remains. Protocols may specify longer washouts for narrow therapeutic index drugs, active metabolites, or irreversible binding. This calculator helps estimate elapsed time and remaining concentration for washout planning.
The elimination half-life of the drug in plasma is the same regardless of route when describing the same elimination phase. IV dosing shows the true distribution and elimination profile immediately. Oral dosing may include an absorption phase, so the apparent terminal half-life can reflect absorption-limited kinetics in some cases. Bioavailability and absorption rate affect peak timing but not the underlying elimination rate constant for most drugs.
First-order kinetics means a constant fraction of drug is eliminated per unit time, so half-life remains constant regardless of concentration. This calculator assumes first-order elimination: Ct = C0 × (0.5)^(t/t½). Zero-order or saturable elimination (e.g., phenytoin at high levels) does not follow a fixed half-life, and simple half-life math may not apply.
Trough samples are drawn immediately before the next dose, when concentration is lowest in the dosing interval. Peak samples are drawn after absorption, often 1–2 hours post-dose for many oral drugs or per label-specific timing. For drugs with long half-lives, timing is less critical; for short half-lives, sampling must match the protocol-defined window. Steady-state sampling requires 4–5 half-lives of consistent dosing first.
Drugs are commonly dosed at intervals of approximately one half-life to two half-lives to limit peak–trough fluctuation while maintaining therapeutic exposure. Shorter intervals increase fluctuation but may be needed for short half-life drugs; longer intervals are used when half-life is long. Maintenance dose rate is driven by clearance; half-life primarily guides interval selection and time to steady state.

Related Calculators

LD Loading Dose Calculator MD Maintenance Dose Calculator CL Clearance Calculator AUC AUC Calculator F Bioavailability Calculator

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