Free interactive tool

Nuclear Fallout Arrival Time Estimator

Roughly how soon fallout could reach you after a nuclear explosion, from your distance and the wind, and what the guide says to do with that time. It deliberately errs on the early side: if in doubt, get inside now.

From the guide Nuclear Event Survival: fallout and wind →

Go in, stay in, tune in. Official instructions on BBC radio or from the emergency services come first. Fallout can arrive within minutes close to the explosion, and you cannot see or smell it. This estimate is only a rough guide.

Your situation

Distance from the explosion

Wind speed

From a forecast, or: light breeze 5–10 mph, moderate 15–25, strong 30–50.

Where are you, compared with the wind?

Downwind Crosswind Upwind Don't know

Downwind: the wind blows from the explosion towards you.

Type of explosion

Ground burst, or don't know Air burst

A ground burst throws up a dirty, brown-grey mushroom cloud with a stem joined to the ground. If you can't tell, assume a ground burst.

Time of the explosion

Earliest likely–

Most likely around–

Your timeline

From the moment of the explosion, using the 7-10 rule: every 7-fold increase in time cuts the radiation from fallout to a tenth.

How the estimate works

Close in (within about 50 km of a large ground burst), fallout comes down from the spreading cloud itself, whatever the wind at ground level: the guide's table for about 1 megaton gives 2–3 minutes at 5 km, 4–6 at 10 km, 15–25 at 25 km and 30–60 at 50 km.
Further out, fallout drifts with the wind: arrival is about the distance divided by the wind speed. Winds a few kilometres up, where the cloud is, are usually faster than at the ground, so it can come sooner: the earliest figure allows for winds half as fast again.
Wind direction can change, rain can bring fallout down in "hot spots", and hills and cities change local patterns. Smaller weapons make a lower cloud, but the advice is the same.