Also in: Disaster Response and Recovery
Storm Safe Room Occupant Capacity
A tornado safe room packs people at 5 sq ft each and a hurricane safe room gives them 20.
Example
You enter
- Safe room type community_tornado
- Gross floor area (sq ft) 4800
- Usable area method net
- Net usable area for method 2 (sq ft) 4800
- Bed or stretcher spaces needed (community only) 0
You get
- Usable area (ft²) 4800
- Simple capacity count 960
- Occupant capacity count 955
- Standing count 950
- Wheelchair spaces count 5
Checked against published worked examples from: FEMA.
Details, formula, and sources
ScopeBecause one storm lasts minutes and the other a day or more. FEMA P-361 publishes the densities: community tornado rooms at 5 sq ft standing or seated, 10 per wheelchair, and 30 per bed or stretcher; community hurricane rooms at 20, 20, and 40; residential rooms at 3 or 5 sq ft for tornadoes and 7 or 10 for hurricanes (one- and two-family or other residential). Usable area is less than the footprint: reduce the gross by at least 50% for concentrated furnishings or fixed seating, 35% for unconcentrated furnishings, or 15% for open plan, or enter the net area once walls, columns, and fixed equipment are subtracted. Every 200 occupants or portion thereof in a community room need a wheelchair space at the wheelchair density, so adding people adds wheelchair spaces, and the calculator solves for the largest total that fits. That reproduces FEMA's own example: a 4,800 sq ft classroom holds 960 at 5 sq ft, but 955 once five wheelchair spaces are included. A 6,000 sq ft multipurpose room with unconcentrated furnishings has 3,900 sq ft usable and holds 776 for a tornado (772 standing plus 4 wheelchair) but 195 for a hurricane -- the same room, a factor of four apart. Hurricane safe rooms may not count restrooms at all. A residential safe room above 16 occupants is routed to the community criteria. It does not design the structure for the ICC 500 wind speeds and debris impact, or address siting, egress, ventilation, sanitation, lighting, or travel time. Code occupant loads for normal use still apply and are posted alongside. FEMA P-361, ICC 500, the safe room designer, and the AHJ govern.
Formula or tableusable area = gross x (1 - 0.50, 0.35, or 0.15) or the entered net area; community rooms: the largest total T with (T - W - B) x standing + W x wheelchair + B x bed <= usable area, W = ceil(T / 200); residential rooms: floor(usable area / density), routed to the community criteria above 16 occupants.
Source and editionFEMA P-361, Safe Rooms for Tornadoes and Hurricanes, Part B, Section B5.2.1: Table B5-1 (community tornado 5 / 10 / 30 sq ft), Table B5-2 (community hurricane 20 / 20 / 40), Table B5-3 (residential tornado 3 or 5, hurricane 7 or 10), one wheelchair space per 200 occupants, and the 50% / 35% / 15% usable-area reductions, with ICC 500 Sections 502 and 503 cited by number. The safe room designer and the AHJ govern.
Free accessFEMA P-361 is a free federal publication; the densities and reductions are also printed in FEMA's Safe Room Technical Review job aid. No ICC 500 text is reproduced.
What governsEstimate. AHJ and licensed professional govern.
API field namesroom_type, gross_area_sqft, usable_method, net_usable_area_sqft, bed_spaces_count, usable_area_sqft, simple_capacity_count, occupant_capacity_count, standing_count, wheelchair_spaces_count
- Community densities tornado 5 / 10 / 30 sq ft, hurricane 20 / 20 / 40 sq ft (standing, wheelchair, bed)FEMA P-361 Tables B5-1 and B5-2; FEMA Safe Room Technical Review (2022)
- One wheelchair space per 200 occupants or portion thereof, in community safe roomsFEMA P-361 Section B5.2.1
- Usable area reductions at least 50% concentrated or fixed seating, 35% unconcentrated, 15% open planFEMA P-361 Section B5.2.1; FEMA Safe Room Technical Review (2022)