Why it matters
Understand
the numbers.
Every figure above has a reason behind it. This is what each one protects against.
Mass stops radiation
Fallout radiation is cut in half by roughly every 2.2 inches of concrete or 3.3 inches of packed earth. Ten halvings gives a protection factor of about 1,000, which is why cover is designed, not guessed.
Time is protection too
Fallout decays quickly at first. By the 7:10 rule of thumb, every sevenfold increase in time cuts the exposure rate about tenfold. The first hours and days below grade matter most.
Air is the system that keeps you there
A sealed space is only habitable with filtered, positive-pressure air: a blast valve, a HEPA filter for particles, activated carbon for gases, and a fan with a hand-crank backup.
Biological threats need a clean line
For airborne illness, the defense is a clean side and a dirty side: filtered supply air, an airlock, a decontamination shower, and an isolation room for anyone who falls ill.
Chemical release needs a seal
Most chemical hazards arrive as gas or vapor. Activated-carbon filtration and a pressurized, gas-tight envelope keep them outside while the event passes.
Minimums are for hours, not months
Swiss civil defense requires at least one square meter per person, for stays of hours to days. For a month or a season, people need room to sleep, cook, wash and stay sane. We plan for that.
Method
How we
calculate.
Planning figures, not guarantees. Final systems are engineered for the site, the climate and the household.
Living area per person scales with the length of stay: 12 sq ft for 72 hours (just above the Swiss minimum of 1 square meter per person), 25 for two weeks, 40 for a month, 70 for three months and 100 for six months. These are Styrkholm planning figures for long stays, not code minimums. About 45 percent goes to sleeping and 55 percent to common space.
Fixed rooms: one toilet per 6 people for stays over three days (one per 15 for short stays), one shower per 10 people for two weeks or more, a 100 sq ft medical bay for six or more people or stays of a month or longer, a 90 sq ft airlock and decontamination room when chemical or biological protection is selected, and a mechanical room sized at 80 sq ft plus 3 per person. Circulation and walls add 15 percent.
Storage holds food for the whole stay at 1.5 lb per person per day, and up to 30 days of water inside at 3 gallons per person per day, with the rest in an outside cistern.
Filtered air follows the ASHRAE 62.2 residential ventilation formula: 0.03 CFM per square foot plus 7.5 CFM per person. That is several times the Swiss emergency minimum of 3 cubic meters per hour per person, for comfort over long stays.
Protection factor is 2 raised to the number of halving layers overhead, using about 2.2 inches per halving for concrete and 3.3 inches for earth. Walls, entry geometry and depth also matter; the final design is engineered.
Power is a planning estimate: 2 kWh a day base load, 0.4 kWh per person, and 1 watt per CFM of filtered air.
- FEMA: Safe room frequently asked questions (P-361 occupant area)
- Swiss Federal Office for Civil Protection: Preparing a shelter (brochure)
- Health Physics Society: Half-value layers for shielding materials
- DHS Center for Domestic Preparedness: The 7:10 rule
- FDA: Potassium iodide FAQ
- Ready.gov: Nuclear explosion (archived)
Questions
Common
questions.
How big should an underground shelter be for a family?
For short stays, codes and civil defense standards use as little as 5 to 11 square feet per person. For a family that may stay below grade for a month or more, we plan roughly 40 to 100 square feet per person once bunks, a common room, sanitation, storage, a medical bay and mechanical space are included.
What does NBC mean?
Nuclear, biological and chemical. An NBC shelter keeps radioactive fallout, airborne pathogens and chemical agents outside, using mass for radiation and a sealed, filtered, positive-pressure air system for particles and gases.
How much earth cover stops fallout?
Each 3.3 inches of packed earth cuts gamma radiation roughly in half, and each 2.2 inches of concrete does the same. About ten halvings, for example 12 inches of concrete under 18 to 24 inches of earth, gives a protection factor near 1,000.
How long would we need to stay below grade after a nuclear event?
Federal guidance is to get inside and stay inside for at least 24 hours unless officials say otherwise. Fallout decays quickly, roughly tenfold for every sevenfold increase in time, but conditions vary and longer stays may be advised. A shelter designed for weeks gives the family that choice.
Is this a substitute for an engineer?
No. It is a planning tool. Every Styrkholm shelter is engineered by licensed professionals, and its filtration and blast-valve equipment is specified to recognized standards.
Private Consultation
Bring us
your numbers.
Send your results with a consultation request and we will walk through them with you, privately and without obligation.
More planning tools
Days of Independence
Enter what your property holds today and see how many days your family could stand on its own, resource by resource.
Open SuppliesSelf-Sufficiency Calculator
Water, food, solar, batteries, fuel and heat for your household, sized for 90 days, 180 days or a full year.
Open HealthMedicine and Nutrition Planner
Build your household person by person: calories, medical conditions, cold storage, medical power and potassium iodide doses.
Open