Planning guide

How long to stay in a fallout shelter

Fallout is most dangerous in the first hours and days, and it fades fast. The 7:10 rule of thumb is how emergency planners estimate that drop.

The short answer
  • By the 7:10 rule, every sevenfold increase in time cuts the exposure rate about tenfold.
  • Emergency guidance is to expect to stay inside at least 24 hours unless authorities say otherwise.
  • Official guidance in a real event decides when to come out.

The 7:10 rule

The DHS Center for Domestic Preparedness teaches the 7:10 rule of thumb: for every sevenfold increase in time after a detonation, the exposure rate drops by about a factor of ten. A rate measured at hour one is about a tenth as high at hour seven, and about a hundredth at hour 49, roughly two days.

What it means for planning

The early drop is steep, which is why getting below ground quickly matters most. After that, the rate keeps falling, but more slowly. Conditions vary with distance, weather and the size of the event, and authorities may advise a longer stay, so plan supplies for longer than the minimum.

Supplies for the stay

  • Water at one gallon per person per day, at a minimum
  • Food that needs little water or cooking
  • Powered ventilation with filtration, and a hand crank
  • A battery or hand-crank radio for official instructions
  • A decontamination area at the entry to wash off fallout

Questions

How long should you stay in a fallout shelter?

Emergency managers say to expect at least 24 hours inside unless authorities tell you otherwise, and longer if they advise it. Listen for official instructions on a radio before coming out.

Does the 7:10 rule apply to every event?

It is a rule of thumb for fallout from a nuclear detonation. Real conditions vary, which is why emergency managers measure rather than rely on it alone.

Sources
  1. DHS Center for Domestic Preparedness: the 7:10 rule of thumb
  2. Prince William County, Virginia, Office of Emergency Management: Nuclear blast
  3. Ready.gov: Water (archived)

CUBE Bunker prices are preliminary until engineering for the cube system is complete and are confirmed in writing after a site visit. This guide is general information, not engineering, legal or medical advice. Updated 2026-11-02.