Drug Calculations

Half-Life Calculator

Time for a drug level to fall by half, from volume of distribution and clearance; about five half-lives to steady state or washout.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

Turn on JavaScript to use this calculator, or open it in the web app.

Formula

(0.693 * Vd) / Clearance

How it is used

• t½ = 0.693 × Vd ÷ clearance: a larger volume or a lower clearance lengthens the half-life.
• About five half-lives reach steady state on a constant dose, and about five clear a drug after it stops (about 97% gone).
• Kidney or liver failure can lengthen half-life sharply; levels and dose intervals are adjusted to match.

Same formula as the app. Open it in the web app.

Related one-pagers

• CEN: ED Safety, Disaster and Toxic ExposuresHalf-life about 4.5 h on room air, 1.5 h on 100% O₂, 20 min hyperbaric.
• Vasopressors and InotropesHalf-life · About 2 min · About 2.3 h; longer in AKI
• BurnsCO: 100% O2. COHb half-life about 4 to 6 h on room air, 1 h on 100% O2, 20 to 40 min hyperbaric.
• Right Heart Failure and Pulmonary HypertensionNever interrupt chronic IV epoprostenol (half-life 6 minutes or less) or treprostinil (about 4 h; labels): no stopping, flushing, bolusing, blood draws or other drugs in the line. If it fails, restart through a peripheral IV within minutes and call the PH team; rebound PH can kill.

Drip cards with a dose ↔ mL/h converter

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For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.