Fallout Decay Calculator: How Fast Fallout Radiation Fades

Fallout radiation fades quickly at first and then more slowly. Federal planning guidance describes this with one formula and a rule of thumb. This tool applies them. It cannot tell you the radiation level where you are. An instrument reports the level at one spot. Only officials can tell you when it is safe to go outside.

How much has it faded?

Time after the detonation: 24 hours

About 2.2% of the level one hour after the detonation.

A low percentage is not a safety verdict. If the starting level was very high, a small share of it can still be dangerous.

Estimate a later reading

Only useful with a real reading from an instrument or from officials, taken after fallout has stopped arriving at that spot. You usually cannot tell this yourself, so unless officials have told you, treat the estimate as unusable. FEMA notes readings can rise rapidly at first while fallout settles, so an estimate from an earlier reading can be far too low. A reading indoors does not tell you the level outdoors. Do not go outside to take one. We never supply a starting level, because no one can know it from a website.

Your reading (any unit)

Taken at (hours after detonation)

Estimate at (hours after detonation)

Enter a reading to see an estimate.

This is a rule of thumb, not a measurement, and it does not tell you when to leave. Follow officials.

The curve as a table

Time after detonation	Share of the one-hour level
1 hour	100%
2 hours	44%
4 hours	19%
7 hours	9.7%
12 hours	5.1%
24 hours	2.2%
2 days	0.94%
3 days	0.59%
7 days	0.21%
2 weeks	0.091%
30 days	0.037%
3 months	0.010%

The formula

The federal planning guidance gives the decay of fission products from a nuclear explosion as:

R(t) = R₁ × t^−1.2

R(t) is the dose rate t hours after the explosion, and R₁ is the dose rate at one hour. Time counts from the explosion, not from when fallout lands.

The 7-10 rule

The same guidance gives a shortcut: for every sevenfold increase in time after the detonation, the radiation rate falls about tenfold. Starting from the one-hour level:

At 7 hours, about one tenth.
At 49 hours (about 2 days), about one hundredth.
At 343 hours (about 2 weeks), about one thousandth.

These round numbers come from the rule of thumb. The formula itself gives 9.7%, 0.94% and 0.091%, which is why the calculator shows slightly different figures.

How far to trust it

The guidance quotes the rule as accurate to within about 25 percent up to about 2 weeks, and within a factor of two up to about 6 months. The calculator stops at 6 months for that reason.
It describes how a rate changes over time, not what the rate is. Fallout is uneven, and the guidance warns that the residual radioactivity may still be at hazardous levels even after several days.
The guidance tells responders not to use the rule in place of real measurements. Neither should you.

What this means for sheltering

This fast early decay is why officials ask people to stay put. Ready.gov says to remain in the most protective location for the first 24 hours unless there is an immediate hazard, such as fire, a gas leak, building collapse or serious injury, or authorities say it is safe to leave. CDC also says to shelter for at least 24 hours. Federal planners note that sheltering in the first few hours is a critical protective measure, and that the danger can extend tens of miles downwind early on.

See how to find adequate shelter and what fallout maps can tell you.