Field tool NUC // 7-10 rule
Fallout Decay Calculator
Enter one dose-rate reading and when it was taken. The calculator uses the standard t-1.2 decay approximation behind the 7-10 rule to estimate the dose rate hours, days and weeks after a nuclear detonation.
| Time after detonation | Estimated dose rate | vs. your reading |
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What the 7-10 rule says
For every sevenfold increase in time after a detonation, the dose rate from fallout drops by about a factor of ten. Seven hours after the blast it is roughly 10% of the one-hour level. After 49 hours (about two days) it is about 1%. After two weeks it is about 0.1%.
That is why official guidance is to get inside, stay inside and stay tuned, for at least 24 hours unless officials say otherwise. The first hours carry most of the danger. Our file on how long nuclear fallout lasts explains the physics in full.
The math behind the calculator
The calculator uses the Way-Wigner approximation for mixed fission products: dose rate at time t equals the reading multiplied by (t / treading)-1.2. Seven to the power of -1.2 is about 0.097, which is where the 7-10 rule comes from.
Limits you should know
- It is least reliable in the first half hour and after several months.
- It describes fallout from a nuclear weapon. Reactor accidents such as Chernobyl decay far more slowly because of long-lived isotopes like cesium-137.
- It cannot tell you your dose. Only a calibrated survey meter or dosimeter can do that.
Where to shelter while you wait: fallout shelter in an apartment and how to survive a nuclear attack.
Frequently asked questions
What is the 7-10 rule for nuclear fallout?
For every sevenfold increase in time after a detonation, the dose rate from fallout drops about tenfold. Seven hours after a blast it is about 10% of the one-hour level, and after about two days (49 hours) about 1%.
How accurate is a fallout decay calculation?
The t to the power of -1.2 approximation behind the 7-10 rule is a planning estimate for mixed fission products. It is least reliable in the first half hour and after several months, and real readings vary with weather and weapon type. Always follow official instructions.
How long should you shelter after a nuclear explosion?
Ready.gov advises staying inside for at least 24 hours unless officials tell you otherwise. The first hours carry the highest dose rates, which is why getting inside quickly and staying put matters most.