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Chemistry

First-Order Reaction Half-Life Calculator

Calculate half-life and remaining fraction from a first-order rate constant and elapsed time.

Explicit units and model assumptionsLocal calculations with worked examples
Chemistry workspace
Chemistry workspace

Check the units, concentration basis and assumptions before interpreting the result.

See the method

Set up your calculation

Runs on your device

Your chemistry result

Your result

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Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter rate constant, elapsed time.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

Half-life = ln(2)/k. Remaining fraction after t seconds = exp(−kt), for first-order kinetics.

A WORKED EXAMPLE

Using rate constant = 0.1 1/s, elapsed time = 10 s, the result is 6.93147181 s. Change these example inputs to match your task; use the method above to check each step.

Understanding your result

For an ideal first-order reaction it does not. Zero-order and second-order reactions follow different half-life relationships.

What to keep in mind

For first-order processes with a constant positive rate constant. Not a medication dosing calculator. Educational calculation only; use validated procedures for laboratory work.

Reference: OpenStax Chemistry 2e — chemical quantities and models

Common questions

Does the half-life depend on starting concentration?

For an ideal first-order reaction it does not. Zero-order and second-order reactions follow different half-life relationships.

Are my inputs uploaded?

No. This calculation runs in your browser without an account or an external API.

Methodology maintained by ClarityKit. How these tools are built and checked.