How big a storm shelter does your family need?

Sizing a tornado shelter is simpler than it looks. FEMA's homeowner guide sets the floor space for a residential tornado shelter at a minimum of 3 square feet per person, which also satisfies the ICC 500 standard for one- and two-family homes. Tornadoes pass quickly, so FEMA treats comfort as a lesser concern for tornado-only shelters. Hurricane shelters, where people may wait out a full day or longer, call for 7 square feet each.

The figure is usable floor area, not the outside dimensions. A bench, a water heater or a shelf inside the unit eats into it, so ask the installer for the net interior space.

Quick table: floor area to people

That "cap" is real. FEMA's residential guidance limits a home safe room to 16 occupants no matter how big it is. Past that you are into community shelter territory, with its own design rules.

Count more than the people

• Guests and neighbors. If yours is the only shelter on the street, think about whether a neighbor or the grandparents might be counting on it, and leave seats for them.
• Pets. A crated dog takes the floor space of a person or more. Measure the crate.
• Mobility. A wheelchair needs room to get in and turn. FEMA points out that stairs into an in-ground shelter may be a problem for anyone using a wheelchair or crutches, which often points toward an above-ground room.
• Supplies. FEMA's guide suggests keeping an emergency kit in the safe room, including a first-aid kit, needed medicines, flashlights, a day's water for tornado use (a gallon per person) and tools to open the door from inside if debris blocks it. Leave a corner for it.

Height and headroom

FEMA's site-built designs top out at 8-foot walls. Prefabricated units vary from model to model, above and below ground. If anyone in the household can't crouch or sit on a low bench for long, ask for the interior height before you choose.

Bigger costs more, but not proportionally

FEMA's new-home estimates put an 8x8 site-built room at roughly $12,700 to $16,100 and a 14x14 at roughly $25,600 to $34,500, door extra. That is about three times the floor area for a bit over double the cost. Prefab steel and in-ground units have their own pricing; the cost page lays it out, and the tax credit caps at $3,000 regardless of size.

Once you know roughly how many people and what kind of unit, an installer can measure the garage or yard and tell you what will physically fit.

Good questions

How many square feet per person does a tornado shelter need?

FEMA and ICC 500 use 3 square feet of floor area per person for a tornado shelter at a one- or two-family home.

How many people fit in a 4x6 shelter?

At 3 square feet each, 24 square feet holds 8 people, though that is snug, especially with children on laps, pets or supplies inside.

Is there a maximum number of people for a home safe room?

Yes. FEMA's residential safe room guidance sets a design limit of 16 occupants regardless of the room's size.