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Drug Half-Life Calculator (from Two Levels)

Calculate a drug’s elimination half-life and rate constant (ke) from two plasma levels and the time between them, plus how long until the level falls to a target and the time for ~97% elimination.

First (higher) level

Second (later) level

Time between the two levels

Target level (0 = skip)

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How it works

ke = ln(C₁ ÷ C₂) ÷ Δt · t½ = ln 2 ÷ ke = 0.693 ÷ ke · time to reach a target = ln(C₂ ÷ C target) ÷ ke · remaining after n half-lives = 0.5ⁿ

Most drugs are eliminated by first-order kinetics: a constant fraction leaves the body per unit time, so concentrations fall exponentially and plot as a straight line on a log scale. Two levels drawn during the elimination phase (after absorption and distribution are complete, and with no dose in between) are enough to calculate the elimination rate constant and the half-life. After about 5 half-lives roughly 97% of the drug is gone, the usual rule for when a drug is “out of the system” or when a new dose regimen reaches steady state. It does not apply to drugs with saturable (zero-order) elimination such as phenytoin or alcohol at high levels.

Worked example

A level of 20 mg/L falls to 5 mg/L 12 hours later. ke = ln(20 ÷ 5) ÷ 12 = 1.3863 ÷ 12 = 0.1155 /h, so the half-life is 0.693 ÷ 0.1155 = 6.00 h. About 97% is eliminated after 30.0 h, and the level falls from 5 to 1 mg/L in a further ln(5) ÷ 0.1155 = 13.9 h.

Sources: StatPearls — Physiology, Zero- and First-Order Kinetics · StatPearls — Pharmacokinetics

Frequently asked questions

How do you calculate half-life from two concentrations?

Take the natural log of the ratio of the higher to the lower level, divide by the time between them to get ke, then divide 0.693 by ke. Halving in 6 hours, for example, gives ke = 0.1155 /h and t½ = 6 h.

How long does a drug stay in your system?

Pharmacologically, about 4–5 half-lives until roughly 94–97% is eliminated. Detection on drug tests can be much longer, because tests look for metabolites and are very sensitive.

When should the two levels be drawn?

Both in the elimination phase — after absorption and distribution are complete and with no dose given between them. For IV aminoglycosides and vancomycin that means at least 30–60 minutes after the end of the infusion.

Does half-life depend on the dose?

Not for drugs with first-order (linear) kinetics: the same fraction is removed per hour whatever the level. It does for drugs with saturable metabolism, such as phenytoin, where half-life lengthens as levels rise.

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Educational pharmacokinetic estimate for health professionals only — not medical advice or a prescription. Population parameters vary widely between patients; base real dosing on the product label, measured drug levels, local protocols and a clinical pharmacist.