Shelter Tier Breakdown
Three classification tiers describe how well a shelter reduces accumulated fallout dose. Use the tier to decide how long you can safely stay and what to do next.
Tier 1: Inadequate
Range: PF under 10 (less than 90 percent dose reduction) - below FEMA adequate fallout shelter standard
Typical examples: Wood-frame single-story home above ground, vehicle, mobile home, exterior position in any building.
What it means: You are getting some protection, but a person outside would receive less than 10 times your dose. Acute radiation symptoms are possible at high fallout intensities. Plan to relocate to a better shelter within the first few hours if fallout is heavy.
Recommended actions:
- Move to the lowest floor of your building (preferably basement)
- Get to the interior, away from all exterior walls and windows
- Add mass: stack books, full water containers, furniture, mattresses against exterior walls
- If a multi-story concrete building is within 5 minutes, consider relocating there before fallout arrives
- Seal windows and doors with any plastic and tape available
Tier 2: Some Protection
Range: PF 10 to 39 (90 to 97 percent dose reduction) - below FEMA adequate fallout shelter standard but meaningful protection
Typical examples: Home basement, brick or masonry house basement interior, ground floor of a multi-story concrete building.
What it means: Meaningful protection for the high-intensity first 24 to 72 hours of fallout, but below the FEMA "adequate" threshold of PF 40. Most healthy adults can shelter here for the critical first window without acute radiation sickness, provided the shelter is sealed reasonably and you stay deep inside. Relocating to a multi-story basement or adding interior mass can reach Tier 3.
Recommended actions:
- Stay sheltered for at least 24 to 72 hours (the most intense fallout period)
- Conserve water, food, and battery power
- Monitor emergency broadcasts (battery radio) for fallout advisories
- Add interior mass between you and the nearest exterior wall to push toward Tier 3
- Limit time near exterior windows even if sealed
Tier 3: FEMA Adequate
Range: PF 40 or higher (over 97.5 percent dose reduction) - meets FEMA adequate fallout shelter standard
Typical examples: Basement of a 3-or-more-story concrete building, purpose-built fallout shelter, deep interior of large commercial or institutional building, dedicated underground bunker.
What it means: Meets the FEMA adequate fallout shelter standard, which traces to the 1962 Office of Civil Defense national shelter survey threshold of PF 40. Safe to shelter for the full 7 to 14 day fallout decay window. Purpose-built shelters can reach PF 250 or higher. Total dose accumulated over two weeks remains below the threshold for radiation health effects.
Recommended actions:
- Plan to shelter for 7 to 14 days (full fallout decay window)
- Ration water and food for extended sheltering
- Establish a sanitation plan (sealed containers, designated area)
- Keep entry and exit minimal; decontaminate outer clothing before re-entry if you must go outside
- Maintain a battery radio rotation to preserve power across multiple days
How to Improve Your Shelter's Protection Factor
Most homes start at Tier 1 or low Tier 2. Modest preparation moves shelters up one or two tiers, which can mean the difference between survivable and dangerous accumulated dose.
Move below grade if possible
A basement is the single biggest PF improvement available in most homes. Below-grade walls surrounded by earth provide 5 to 10 times more shielding than above-ground walls.
Get to the interior core
Move as far from exterior walls and roofs as possible. In a one-story home, choose the most interior bathroom or closet. In a larger building, pick the most central room on the lowest floor.
Add mass between you and the outside
Stack heavy books, full water containers, dirt-filled boxes, mattresses, or furniture against exterior walls of your shelter room. Every inch of dense material reduces gamma dose roughly 50 percent.
Seal openings
Cover windows, doors, vents, and HVAC intakes with 6-mil plastic sheeting and duct tape. This blocks fallout particle ingress (the dominant inhalation and skin contamination risk) but does not significantly increase gamma shielding.
Build an expedient shelter
FEMA describes "expedient" shelters built inside a basement using interior doors propped to form a lean-to, with mass piled on top. A door covered with 18 inches of books, soil, or water reaches PF 200 or more.
How These Numbers Are Calculated
The calculator uses a flat lookup of eight published scenarios. Each scenario maps to a specific protection factor (or PF range) documented in a published primary source. The calculator returns the midpoint of the range and shows the full range alongside it for transparency. Earlier versions of this calculator used a building-type by floor-location matrix that produced 16 cells where only 4 were directly cited; the other 12 were physics-based interpolations sold as cited values. That structure has been removed in favor of cited-only scenarios.
A single-story wood frame house at ground level provides PF 2 to 4 because the wall and roof mass is insufficient to attenuate the energetic gamma radiation that dominates fresh fallout. A wood frame house basement reaches PF 10 to 20 because earth surrounding the below-grade walls absorbs most of the gamma flux. A basement inside a multi-story concrete or brick building reaches PF 200 to 300 because earth, dense walls, and additional building floors above all contribute shielding. These values are documented in OSTI report 1880931 ("US Fallout Shelter," Dillon, Schwefler, Chinn; Lawrence Livermore National Laboratory; March 2022) and FEMA Planning Guidance for Response to a Nuclear Detonation (3rd edition, May 2022).
Sealing windows, doors, and vents with plastic sheeting and tape is reported as a separate qualitative recommendation in the calculator output. Sealing reduces fallout particle ingress (which causes inhalation dose and skin contamination), but sealing does NOT change the gamma protection factor. The protection factor measures gamma shielding from building mass, which is a different physical mechanism than particle filtration. Earlier versions of this calculator combined the two into a single multiplier; that was incorrect and has been removed.
Output PF is clamped to the range 1 to 1500. Dose reduction percentage is derived as (1 minus 1 divided by PF) times 100. Tier classification follows the FEMA "adequate fallout shelter" standard, which traces to the 1962 Office of Civil Defense national shelter survey that used PF 40 as the adequacy threshold: Tier 1 below PF 10 (inadequate), Tier 2 from PF 10 to 39 (some protection, below FEMA standard), Tier 3 at PF 40 or higher (FEMA adequate). The "purpose-built shelter, modern target" scenario uses PF 1,000, which derives from U.S. Army Field Manual FM 3-05.70 (2002) half-value-layer data: 10 HVLs of shielding (for example, 56 cm of concrete or 84 cm of packed dirt) reduces gamma dose by a factor of 1,000.
Primary Sources
Every scenario in the calculator corresponds to one or more of the following published primary sources. Each is publicly available and searchable by name.
- FEMA Planning Guidance for Response to a Nuclear Detonation, 3rd edition (May 2022), Federal Emergency Management Agency. Latest edition; no 4th edition exists as of 2026.
- OSTI report 1880931, "US Fallout Shelter" by Michael B. Dillon, Callen Schwefler, and I. Chinn, Lawrence Livermore National Laboratory, March 2022. Replaces an earlier mis-cited OSTI ID 1845378, which is a different report ("Distances to Prompt Effects for a Nuclear Device," Sandia 2022) not about shelter PFs.
- U.S. Army Field Manual FM 3-05.70 (2002), half-value layer data for shielding material thicknesses, cited via Health Physics Society "Ask the Experts" Q&A 14423.
- 1962 Office of Civil Defense National Fallout Shelter Survey methodology, which established the PF 40 "adequate fallout shelter" threshold still used by FEMA today.
Frequently Asked Questions
What protection factor do I need to survive fallout?
For the most intense 24 to 72 hour period after fallout arrives, FEMA recommends seeking shelter with a PF of at least 10 (Tier 2). For extended sheltering through the full 7 to 14 day decay window, PF 40 or higher (upper Tier 2 or Tier 3) is preferred. The "7-10 rule" of fallout decay means each 7-fold increase in time reduces radiation 10-fold; after 49 hours, intensity is roughly 1 percent of arrival levels.
Is a home basement enough for nuclear fallout?
For most fallout intensities, yes. A typical home basement provides PF 10 to 20 (Tier 2), which reduces dose by 90 to 95 percent. This is adequate for the critical first 24 to 72 hours. To improve further, move to the center of the basement (away from above-grade exposed walls if any) and stack mass against any exterior basement walls or windows. Walk-out and partial basements protect less than fully below-grade basements.
How long should I stay in a fallout shelter?
Minimum 24 to 72 hours covers the highest-intensity period. For heavy fallout zones, 7 to 14 days allows radiation to decay to safer levels (per the 7-10 rule). The exact duration depends on your distance from ground zero, wind direction, and shelter PF. Monitor emergency broadcasts for guidance specific to your area. Even brief exposure during the first 48 hours is significantly more dangerous than the same exposure days later.
Does sealing my windows really help against radiation?
Sealing helps against fallout particle ingress (preventing radioactive dust from entering your shelter and contaminating your skin, lungs, and food) but does NOT significantly block gamma radiation already deposited outside. Gamma radiation passes through air and most building materials; only mass (concrete, earth, water, books) attenuates it meaningfully. Sealing is one layer of protection. Mass shielding is the dominant layer.
What is the difference between blast shelter and fallout shelter?
Blast shelters protect against the initial detonation (overpressure, thermal radiation, prompt radiation, flying debris). They are typically deep underground or heavily reinforced. Fallout shelters protect against radioactive particles deposited by the wind in the hours and days after detonation. A fallout shelter needs mass overhead and around you, but does not need to withstand blast pressure. Most home basements function as fallout shelters but not blast shelters. The closer you are to a likely target, the more you need both.