MATH · FREE
Cesium-137 Half-Life Calculator
Cesium-137 loses half of its amount about every 30 years. Use the calculator, already set to that half-life, to see how much remains after any number of years.
The half-life
The half-life of cesium-137 is 30.08 years according to NNDC (the EPA lists 30.17 years), so about 30 years. After 30 years half is left, after 60 years a quarter, after 90 years about 12.5%.
Math only. This page says nothing about radiation safety. For health or safety questions, follow your national radiation authority and emergency guidance.
AMOUNT REMAINING
Amount left
Percent left
Half-lives elapsed
| Half-lives | Time | Amount left | Percent left |
|---|
Math only. This page says nothing about radiation safety. For health or safety questions, follow your national radiation authority and emergency guidance.
Worked example: 100 g of cesium-137
Start with 100 g. After 30.08 years 50 g is left, after 60.16 years 25 g and after 90.24 years 12.5 g. For 90 years exactly, 90 ÷ 30.08 ≈ 2.99 half-lives, so N = 100 × (1/2)2.99 ≈ 12.6 g.
Milestones on a 30.08 year half-life
- 10%: 30.08 × log2(10) ≈ 99.9 years.
- 1%: 30.08 × log2(100) ≈ 199.8 years.
- About 0.1%: 10 half-lives, which is 300.8 years.
To find the time to any other level, switch the calculator to "Time to reach an amount". To check a measurement, use "Solve for half-life" with the starting amount, the amount left and the time that passed.
This is decay arithmetic for a single isotope. It does not model contamination, removal from the environment, exposure or health effects.
Sources
- NNDC (Brookhaven National Laboratory), NuDat 3, Cs-137 decay data (accessed October 2026). Half-life 30.08 y (uncertainty 9 in the last digit). This value is used as the default.
- US EPA, Radionuclide Basics: Cesium-137 (accessed October 2026). Lists a half-life of 30.17 years.
Frequently asked questions
What is the half-life of cesium-137?
The Brookhaven National Laboratory's NNDC lists 30.08 years for cesium-137 (uncertainty 9 in the last digit). The US EPA's radionuclide page lists 30.17 years. Evaluations differ slightly in the second decimal, so in everyday terms it is about 30 years. This page starts from 30.08 years.
How long until cesium-137 is down to 1%?
About 200 years. Reaching 1% needs log2(100) ≈ 6.64 half-lives, and 6.64 × 30.08 years ≈ 199.8 years.
How much is left after 100 years?
100 years is 100 ÷ 30.08 ≈ 3.32 half-lives, so (1/2)^3.32 ≈ 10.0% of the starting amount remains.
Does half-life tell me whether a level is safe?
No. Half-life only describes how fast the amount of the isotope falls. Whether a given amount or dose is a hazard is a separate question for radiation protection authorities, and this calculator does not answer it.
Why do two sources give 30.08 and 30.17 years?
Half-life values are measured and re-evaluated over time, and different organizations publish different evaluations. Both are within about 0.3% of each other. For a calculation that spans centuries the difference is small.
Math only. This page says nothing about radiation safety. For health or safety questions, follow your national radiation authority and emergency guidance.
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