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Drug Half-Life Calculator

Health

Estimate how much of a drug remains in the body after a given time, based on its half-life. Educational pharmacokinetics tool, not medical advice.

12,000
0.5500
01,000

Amount Remaining

250
Percent Remaining
50.00%
Number of Half-Lives Elapsed
1
Guidance
This is a simplified single-compartment model — actual elimination can vary by individual metabolism, organ function, and the specific drug's pharmacokinetics.

This calculator computes your Amount Remaining, Percent Remaining, Number of Half-Lives Elapsed, Guidance from the values you enter.

Inputs
Initial AmountHalf-LifeTime Elapsed
Outputs
Amount RemainingPercent RemainingNumber of Half-Lives ElapsedGuidance

What is a Drug Half-Life?

The Drug Half-Life Calculator models how much of a substance theoretically remains in the body after a given period, based on its half-life — the time it takes for the amount to reduce by half. This is a foundational pharmacokinetics concept: most drugs follow a predictable exponential decay pattern, where the same fraction is eliminated in each successive half-life period.

This calculator is an educational pharmacokinetics tool and is not medical advice. It uses a simplified single-compartment exponential model, which approximates but doesn't precisely capture every drug's actual elimination behaviour. It does not provide dosing timing recommendations — those depend on a drug's specific therapeutic window and should come from a doctor, pharmacist, or product label.

How to use this Drug Half-Life calculator

  1. Enter the Initial Amount in mg.
  2. Enter the Half-Life in hours for the substance you're modelling (check the specific drug's prescribing information for this value).
  3. Enter the Time Elapsed in hours since the initial amount.
  4. Review the Amount Remaining, Percent Remaining, and Number of Half-Lives Elapsed.
  5. Read the Guidance field for context on the "5 half-lives" elimination rule of thumb.

Formula & Methodology

Remaining Amount = Initial Amount × 0.5^(Elapsed Time ÷ Half-Life)

Worked example — for 500 mg with a 24-hour half-life, after 48 hours have elapsed:

Half-lives elapsed = 48 ÷ 24 = 2
Remaining = 500 × 0.5² = 500 × 0.25 = 125 mg (25% remaining)

This is a simplified educational model — actual elimination varies by drug and individual, and this tool does not provide dosing timing recommendations.

Frequently Asked Questions

Half-life is the time it takes for the amount of a drug in the body to reduce by half, following a predictable exponential decay pattern for most medications. A drug with a 6-hour half-life will have roughly half its peak amount remaining after 6 hours, a quarter after 12 hours, and so on.
After about 5 half-lives, a drug is generally considered clinically eliminated from the body — roughly 97% gone — though trace amounts can persist longer and individual metabolism varies. This '5 half-lives' rule of thumb is widely used in pharmacology.
This is a simplified single-compartment exponential model that works reasonably well for many drugs, but some medications follow more complex multi-compartment kinetics or non-linear elimination, where this simple formula is less accurate. It's an educational approximation, not a precise clinical model.
Half-life values are typically listed in a medication's prescribing information or package insert, and vary significantly between drugs — from under an hour to several days or longer. Always use the specific published value for the drug you're interested in.
Factors like age, liver and kidney function, genetics, and other medications being taken can all affect how quickly an individual actually metabolises and eliminates a drug, meaning the population-average half-life used here is an approximation rather than a guarantee for any specific person.
No — this tool models how much of a substance theoretically remains based on half-life alone; it does not provide dosing timing recommendations, which depend on the specific drug's therapeutic window and should come from a doctor, pharmacist, or the product label.
No — this simplified model assumes the initial amount is already fully absorbed and starts decaying immediately, which doesn't account for absorption time (particularly relevant for oral medications) or different routes of administration.
Half-life measures how quickly a drug's total amount in the body decreases, while duration of action refers to how long the drug's clinical effect lasts — these aren't always the same, since a drug can remain clinically effective even after significant elimination has occurred.
Students studying pharmacology, researchers, and curious individuals often use half-life calculations to understand general drug elimination concepts — it's a foundational idea in pharmacokinetics taught in health science courses.
No — clinical dosing decisions use more sophisticated pharmacokinetic models that account for absorption, distribution, metabolism, and elimination together. This tool is for general educational understanding of the half-life concept, not clinical decision-making.
Also known as
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