Enter your Initial Concentration, Target Concentration, and Time Elapsed into the Biological Half-Life Calculator to find your substance's Half-Life — or flip the Calculation Type to solve for any missing variable, plus see Decay Constant and Percent Remaining.
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Decay Constant
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Percent Remaining
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Percent Eliminated
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The half-life of a drug is the time it takes for the concentration of the drug in the bloodstream to reduce to half of its initial value. It's a crucial parameter in pharmacology for determining dosing schedules and drug elimination.
Half-life can be calculated using the formula: t½ = ln(2) / k, where k is the elimination rate constant. Alternatively, if you know the concentrations at two different times, you can use: t½ = (t × ln(2)) / ln(C₀/C).
Drugs like adenosine and succinylcholine have very short half-lives of just seconds to minutes. These are typically used in emergency medicine or anesthesia where rapid onset and offset are required.
Amiodarone has one of the longest half-lives among commonly used drugs, ranging from 25-100 days. This long half-life means it takes months to reach steady-state concentrations and for the drug to be eliminated after discontinuation.
After 5 half-lives, approximately 3.125% of the original drug concentration remains. This is why 5 half-lives is often considered the time needed for complete elimination of a drug from the body.
Biological half-life is affected by kidney function, liver metabolism, age, body weight, genetic factors, and drug interactions. Disease states and individual variations can significantly alter elimination rates.
Half-life is the time for 50% elimination, while mean lifetime is the average time a molecule exists before elimination. Mean lifetime equals 1.44 times the half-life for first-order elimination.