Rex says
The fast lane for the math you almost remember from school. Type the numbers, get the answer, move on with your day.
Try a scenario
Click to load — tweak from there.Inputs
Result
Remaining quantity
12.5000
Percent remaining
12.50
Amount decayed
87.5000
Half-lives elapsed
3.00
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter starting quantity.
- 2Enter half-life (hours).
- 3Enter elapsed time (hours).
- 4Read your remaining quantity on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Exponential decay: every half-life cuts the remaining amount in half. Works for isotopes, drug clearance, and anything with a fixed decay rate.
Worked example
Using the values the calculator loads with:
Inputs
- Starting quantity: 100
- Half-life: 12 hours
- Elapsed time: 36 hours
Results
- Remaining quantity: 12.5
- Percent remaining: 12.5
- Amount decayed: 87.5
- Half-lives elapsed: 3
What each field means
Inputs
- Starting quantity
- The starting quantity used in the calculation. Starts at 100 so you have a working example on load.
- Half-life (hours)
- The half-life used in the calculation, measured in hours. Starts at 12 hours so you have a working example on load.
- Elapsed time (hours)
- The elapsed time used in the calculation, measured in hours. Starts at 36 hours so you have a working example on load.
Results
- Remaining quantity
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Percent remaining
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Amount decayed
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Half-lives elapsed
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the radioactive half-life calculator work?
Exponential decay: every half-life cuts the remaining amount in half. Works for isotopes, drug clearance, and anything with a fixed decay rate. The underlying maths is: N = N₀ × (1/2)^(t ÷ t½)
What do I need to enter?
3 values: starting quantity, half-life, and elapsed time. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the remaining quantity result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Results are rounded for display; the underlying calculation keeps full precision.
Who is this tool for?
It's built for students, developers, and anyone who needs the answer right now without setting up a spreadsheet.
Accuracy and limitations
- Results are rounded for display; the underlying calculation keeps full precision.
- Very large or very small inputs may hit floating-point limits in the browser.
- Inputs outside the accepted range are clamped rather than rejected.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Half-Life Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/half-life
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/half-life" target="_blank" rel="noopener">Half-Life Calculator — RevenueLab</a> (2026).</p>
Source: [Half-Life Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/half-life) (2026).