Drug Half Life Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Greenblatt DJ. Elimination half-life of drugs: value and limitations Annual Review of Medicine (1985)See 1 more source
Smith DA, Beaumont K, Maurer TS, Di L. Relevance of Half-Life in Drug Design. Journal of Medicinal Chemistry (May 2018)
Last updated: Jun 10, 2022

Our drug half-life calculator is an easy tool to discover the dosage of a drug that is still present in a patient's system. The half-life of a drug describes the process of its elimination.

Read on to discover the half-life of a drug and how to calculate the half-life of a medication. We'll also present plenty of useful examples.

This form of half life is not to be mistaken with the decay of radioactive elements! ☢️

How to use the drug half-life calculator?

In order to use our 1/2 life calculator you'll need the following data:

  1. The half time of a given drug - this input can be either in minutes, hours, or days. The calculator will set the unit of the result automatically.

    If you don't know the half life of the drug, take a look at our table below.

  2. Dosage - the amount of the drug administered at the very start.

  3. Your results will be shown in a table that consists of at least 5 half-life cycles:

    • Time - the amount of time that has passed since the first administration of a drug;

    • Dosage - the amount of drug left in the patient's body at the given time; and

    • Concentration - the part of the original concentration still present in the patient's body.

What is the half-life of a drug?

In pharmacology, when we talk about the half life of a drug we usually mean the time it takes a drug's concentration in the blood plasma to reduce by half.

💡 The half life of a drug describes the amount of time it will take half of the administered drug to be eliminated.

Why do we need it?

The half-life of a drug helps us assess the potency and toxicity of a drug over a given amount of time. If we know the elimination time, we can easily evaluate how often the drug should be administered to maximize its effect and minimize its adverse reactions.

The concentration of a drug at a given moment crucial in drugs with narrow tolerance range, e.g., medicines that can prolong the patient's QT and consequently lead to heart arrhytmias.

🕒 The half life of a drug is the complete opposite of its doubling time.

How to calculate a half-life of a drug?

Our drug elimination half life calculator uses the following equation:

Dosage(t) = Dosage(0) * 0.5(t/T)


  • T - the half life of a drug;
  • t - time that has passed since the first, original administration of the drug;
  • Dosage(t) - the amount of drug present in a patient's body after time t; and
  • Dosage(0) - the amount of drug administered at the very beginning.

Half-life of medication - examples

Don't know the half-life of a given drug? Are you looking for an Adderall half-life calculator, or any other specific drug calculations? We're here to help!


Half time


1-4 h


9-14 h


26 - 107 days


14 - 47


4 h


21-37 h


36-48 h


2-3 min


3-12 h


20-50 h


3-6 h


14-41 h


3-6 h


2-8 h


1-2 h


3-10 h


8 h


1-7 h


3-5 h


81-117 h


2-6 h


5-6 h


7-11 h

Valproic Acid

9-16 h


20-60 h

Łucja Zaborowska, MD, PhD candidate
Half life
Time (min)Dosage (mg)Concentration
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