Calculator · Residential shelter planning · Clarksville, TN
How big a storm shelter do you need? Size calculator
Count the people, enter the interior size from a model’s spec sheet, and see how the usable floor area compares with the minimum FEMA P-320 and ICC 500 state for a home tornado shelter. It is arithmetic on your numbers, not a rating.
Get a Clarksville storm shelter quoteHow to use the calculator
Enter the number of people, then the interior length and width of a unit you are considering. Subtract any fixed benches or fixtures, and add any extra floor area you want to plan for, such as the footprint of a wheelchair or walker you have measured. Nothing is sent anywhere, and the result is only as good as the spec-sheet numbers you enter.
Storm shelter floor-area calculator
The minimum used here is 3 square feet of usable floor area per person for a tornado shelter serving a one- or two-family home, stated in FEMA P-320 (December 2024) and consistent with ICC 500. The 16-occupant limit is also from P-320. The values shown are an example household and a 4 × 6 ft interior; replace them with yours.
Results below use the example values shown. Turn on JavaScript to change them.
- Minimum usable area for these people
- 12.0 sq ft3 sq ft per person
- Minimum plus your extra area
- 12.0 sq ft
- Interior floor area you entered
- 24.0 sq ft
- Usable area after fixtures
- 24.0 sq ft
- Usable area per person
- 6.0 sq ft
- Most people covered at the minimum
- 8 peopleCapped at 16, the residential limit in P-320
- Against the minimum. The usable area you entered is at least the 3 sq ft per person minimum stated in FEMA P-320 and ICC 500. The manufacturer’s listing and your permit office decide the rated capacity.
- With your extra area. The usable area also covers the extra floor area you chose to plan for.
Planning arithmetic only. It does not rate a shelter, confirm a model’s listed capacity, give a price or decide whether a unit complies with ICC 500 or your local code. Use usable floor area from the manufacturer, not outside dimensions.
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Worked examples: minimum area for common household sizes
On phones, each row is shown with its column labels.
| Household | Minimum usable area | An interior that reaches it before fixtures |
|---|---|---|
| 2 people | 6 sq ft | About 2 × 3 ft |
| 4 people | 12 sq ft | About 3 × 4 ft |
| 6 people | 18 sq ft | About 3 × 6 ft |
| 8 people | 24 sq ft | About 4 × 6 ft |
| 12 people | 36 sq ft | About 6 × 6 ft |
| 16 people (residential limit) | 48 sq ft | About 6 × 8 ft |
The minimum is a floor, not a comfort target
FEMA P-320 explains that tornadoes are not typically long-lasting, so comfort is not the primary concern and at least 3 square feet per person should be large enough for a one- or two-family home. For a hurricane safe room, which may be occupied for 24 hours or more, P-320 gives 7 square feet per person. Clarksville households are planning for tornadoes, so 3 is the figure used here.
Standards explainer
What this calculator does not tell you
The unit’s rated capacity
The design occupant capacity comes from the manufacturer’s listing or the sealed plans, and your permit office reviews it.
Whether everyone can get in
Door width, threshold height and turning room are separate checks. FEMA P-320 asks you to consider the accessibility needs of everyone who will use the shelter.
Space for mobility aids
The calculator only adds an area you measure and enter. P-320 does not give a per-device allowance, so ask the manufacturer how a wheelchair or walker fits.
Seating and fixtures
Hard fixtures and furnishings reduce the usable floor area. Ask for the net usable floor space, not the gross footprint.
Site, foundation and permit
Fit on the floor says nothing about the slab, anchors, flood map or permit. See the planning checklist.
Sources and further reading
Reviewed 2026-10-14
- FEMA P-320, Taking Shelter from the Storm (December 2024)
Sections 3.4 and 3.5: 3 sq ft per person for tornado, 7 sq ft for hurricane, 16-occupant limit, usable-space note and accessibility siting. Read directly October 14, 2026.
- ICC 500-2023 storm-shelter standard (publisher page)
Common questions
How many square feet per person does a storm shelter need?
At least 3 square feet of usable floor area per person for a tornado shelter serving a one- or two-family home. FEMA P-320 gives this figure and says it also meets the ICC 500 minimum for residential tornado storm shelters.
How big of a storm shelter do I need for a family of six?
At least 18 square feet of usable floor area, which is 6 people at 3 square feet each. Fixtures, seating and mobility aids reduce usable space, so compare the manufacturer’s usable area and listed capacity rather than outside dimensions.
Is a 4 x 6 storm shelter big enough for four people?
By arithmetic, yes: 4 × 6 ft is 24 square feet, which covers 8 people at the 3 square foot minimum before fixtures. Benches, seating and mobility aids reduce that, so confirm the model’s net usable area and its listed capacity.
How many people can fit in a residential storm shelter?
No more than 16. FEMA P-320 says the design occupant capacity of a residential safe room must be no more than 16, regardless of its dimensions. Real capacity is set by the model’s listing or sealed plans.
Does a wheelchair need extra space in a storm shelter?
Plan extra space, but there is no per-device figure in FEMA P-320 to apply. Measure the wheelchair or walker you use, enter that floor area in the calculator, and ask the manufacturer how it fits through the door and inside.
Should I buy a bigger shelter than the minimum?
That is a household choice. The minimum is the smallest area FEMA and ICC 500 state, and extra room can help with seating, mobility aids and pets. Ask each provider to quote the same capacity and any larger size separately.