How this calculator works
The NFA training estimate uses floor area in square feet divided by three, multiplied by the involvement fraction and burning-floor count. Add one quarter of the single-floor fully involved base for each separately counted exposed floor or exterior side. Do not count a burning floor again as an exposure. The Iowa method instead divides enclosed cubic feet by 100 and assumes full involvement; it does not use the NFA involvement or exposure inputs. These historical approximations omit occupancy, fire load, sprinklers, construction and delivery limitations.
Formula
NFA base = L(ft) x W(ft) / 3 US GPM; total = base x [involvement x burning floors + 0.25 x (interior exposed floors + exterior exposed sides)]. Iowa flow = L(ft) x W(ft) x H(ft) / 100 US GPM.
Worked example
A 30 x 60 ft footprint has NFA base 600 US GPM. At 50% involvement on one floor without exposures, the estimate is 300 US GPM. With a 10 ft height, the Iowa estimate is 180 US GPM.
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Frequently asked questions
Can these methods size a code-required fire tank?
No. Use the applicable adopted standard, local fire authority requirements and qualified design. This page is a training arithmetic tool.
Why do the two methods disagree?
They were developed from different assumptions. The Iowa volume relation and the NFA area relation do not represent interchangeable design standards.
How are exposures counted?
In this training model, each additional exposed floor above the fire or exterior side adds 25% of the single-floor base. Several threatened buildings on one side remain one exterior side.