Half life calculator

Find what remains after a chosen time: remaining = initial × 2^(−time/half-life).

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Starting quantity

Elapsed time

More inputs and assumptions

Half-life in the same time unit

Answer

…

Find what remains after a chosen time: remaining = initial × 2^(−time/half-life).

More results

Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Step-by-step calculation

Inputs are substituted below. Each line evaluates the next operation; intermediate values are displayed to 12 significant digits while the calculation uses full precision.

Saved scenario

Save the current answer, then change an input to compare. This clears when you leave.

Formula and assumptions

You supply the half-life; no isotope or drug data is built in. Assumes one constant exponential decay process and no replenishment. This does not calculate a medical dose.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

Scratch calculator

Check another calculation

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Updated 2026-09-26

Using Half life calculator

Find what remains after a chosen time: remaining = initial × 2^(−time/half-life).

You supply the half-life; no isotope or drug data is built in. Assumes one constant exponential decay process and no replenishment. This does not calculate a medical dose.

Next: Exponential decay calculator

A worked example

Example inputs: starting quantity = 100, elapsed time = 10, half-life in the same time unit = 5. Remaining quantity = 25.

Before you use the result

You supply the half-life; no isotope or drug data is built in. Assumes one constant exponential decay process and no replenishment. This does not calculate a medical dose.

Example results for starting quantity

These examples use Elapsed time: 10; Half-life in the same time unit: 5. They are reference calculations, not recommended settings.

Starting quantity	Remaining quantity
50	12.5
100	25
200	50

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ToolOctopus. “Half life calculator.” Updated 2026-09-26. https://tooloctopus.com/half-life-calculator.

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