算BuildCalc

Fire Water Tank Calculator

Indoor plus outdoor plus sprinkler, converted to tank volume.

Quick answer

With the default inputs: Indoor fire demand 20 L/s · Outdoor fire demand 30 L/s · Fire duration 2 h etc.Effective tank volume 468.0 m³

Edit the inputs below to recalculate.

Inputs

Length LDepth HWidth W
Volume uses the effective depth only, excluding freeboard and the sub-low-water portion.
L/s
10-40 L/s by occupancy and volume
L/s
15-40 L/s; reducible with a reliable dual municipal supply
h
2 h for residential and office, 3 h for process plants, 3-6 h for warehouses
L/s
20-30 L/s for ordinary hazard
h
Normally 1 h
L/s
Only if the mains can be relied on continuously; otherwise 0
m
From overflow to low-water level, excluding freeboard

Result

Effective tank volume
468.0m³
Indoor volume144.0m³
Outdoor volume216.0m³
Sprinkler volume108.0m³
Gross demand468.0m³
Make-up credit0.0m³
Effective volume468.0m³
Volume per compartment (2)234.0m³
Plan area at design depth133.7m²
Approximate square side11.6m

Effective volume is indoor, outdoor and sprinkler demand over the fire duration, less water that can be reliably made up during the fire. The 3.6 factor is 1 L/s for 1 h = 3600 L = 3.6 m³. Three cautions: the make-up credit needs a mains supply guaranteed continuous — dual feed with adequate diameter, never a single feed; tanks above 500 m³ should be split into two compartments and above 1000 m³ into two tanks so half the volume stays available during cleaning; and this is the effective volume, so structural volume also needs freeboard (about 0.3 m), the sub-low-water portion required by pump suction, and wall thickness.

Formula

Indoor volumeV = flow × duration × 3.6
Outdoor volumeV = flow × duration × 3.6
Sprinkler volumeV = flow × duration × 3.6
Make-up creditV = make-up flow × duration × 3.6
Effective volumeV = indoor + outdoor + sprinkler − make-up
Plan areaA = V ÷ effective depth
20 L/s indoor and 30 L/s outdoor for 2 hours plus 30 L/s sprinkler for 1 hour, no make-up: 144.0, 216.0 and 108.0 m³, 468.0 m³ in total; at 3.5 m depth that needs 133.7 m² of plan area.

FAQ

Why is 1 L/s for one hour 3.6 m³?
Because 1 L/s × 3600 s = 3600 L = 3.6 m³. It recurs throughout fire volume calculations, so memorise it. Watch the units: codes give litres per second and durations in hours, and multiplying gives litres, which must then be divided by 1000. The 3.6 factor handles both steps.
When can make-up water be deducted?
Only when all hold: the mains can supply continuously for the whole fire duration, from two independent sources or two ring-main connection points, and the make-up pipe diameter and pressure are verified for that flow. A single feed, or a supply needing booster pumps, cannot be credited. Many projects simply ignore make-up and size for the worst case; crediting water that may not arrive is a serious safety failure.
Why split the tank?
So cleaning, inspection and refilling never interrupt supply. Tanks are cleaned and disinfected periodically, usually annually, which means emptying — undivided, the whole system is dry and a fire in that window has no water. Code requires compartments above 500 m³ and two separate tanks above 1000 m³, each able to run independently with its own suction pipe and valves. Each compartment should also hold at least three minutes of output from the largest pump so starting cannot empty it.

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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.