Fire Water Storage Arithmetic Checker
Check a fire water store against its engineer-approved design basis. The tool compares the volume the fire pumps can actually draw with the larger of the approved minimum and the approved flow sustained for the approved duration — and shows a deficit as a deficit.
What this tool estimates
Whether the usable fire water volume — the gross fire reserve less the water the pumps cannot draw — covers the larger of the approved minimum and the approved combined flow over the approved duration, and the surplus or deficit in m³.
What it cannot decide
- The required flow, which systems run together, the duration or any minimum volume — those come from your approved design basis and the authority. The page holds no NBC or state table.
- Whether the fire scheme, the store or the building complies, or whether an approval will be granted.
- Suction and vortex conditions, tank compartments, pump selection, or how reliable a refill source is.
- Anything about domestic or process water: fire storage is counted on its own.
Result
A refill rate is typed but the approval box is not ticked, so no refill is counted.
The approved refill rate equals or exceeds the design flow. Check both entries against what the authority accepted.
| Step | Value |
|---|
| Break-even | Value |
|---|
| Input moved | −10% | As entered | +10% |
|---|
A surplus means only that the arithmetic closes on the design basis you entered. It is not a compliance decision: the approved fire scheme and the authority decide the storage.
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
Usable volume, not gross volume
A fire tank is usually specified, built and checked as a gross volume. What counts in a fire is the volume the pumps can draw before they lose suction — and that is smaller by everything below the suction or below the level at which a vortex starts pulling air. A store that measures right when full can still be short once that water is taken off, which is why this checker asks for the unusable volume separately and refuses a fire reserve declared as shared with domestic or process water.
The requirement side is yours to bring: the combined flow, the duration and any minimum all come from the design basis your fire engineer prepared and the authority accepted. Where the guide behind this tool goes further — draw-off levels, shared tanks, compartments — read fire water storage: gross volume against usable capacity.
How this calculator works — and where it stops
What it calculates
Whether a fire water store holds the usable volume its engineer-approved design basis asks for: the required usable volume (the larger of the approved minimum and the approved combined flow sustained for the approved duration), the usable volume the store offers (the gross fire reserve less the water the fire pumps cannot draw), and the surplus or deficit between them in m³.
Inputs
- Approved combined flow — the total flow the approved design basis requires at the same time, in L/min, m³/h or L/s. Which systems run together is the design basis's decision, not the tool's.
- Approved duration, in minutes — how long the approved basis says that flow must be sustained.
- Approved minimum usable volume — any minimum the approved scheme or the authority states, in m³, litres or kL. Enter 0 only if the basis sets no separate minimum; left blank, the storage is not assessed.
- Gross fire-reserve volume — fire water only. For a tank shared with domestic or process water, only the volume below the lowest domestic or process draw-off; a volume declared as shared is refused.
- Unusable (dead) volume — the water the fire pumps cannot draw, from the engineer's tank and suction details.
- Refill, only if the approving authority accepted it: the approved inflow rate, and whether it reduces the flow-based volume only or counts against the whole requirement. Without the approval tick, a refill rate is ignored.
Method
- Flow-based volume (m³) = approved combined flow × approved duration — L/min × min ÷ 1,000, or m³/h × min ÷ 60.
- Required usable volume (m³) = the larger of the approved minimum and the flow-based volume. The result says which one governs.
- Available usable volume (m³) = gross fire-reserve volume − unusable volume.
- Gap (m³) = available − required. A negative gap is a deficit and is shown as one; it is never rounded to zero.
- Approved refill, only when ticked: refill volume = approved inflow × duration. Against the flow-based volume only, it reduces that volume before the comparison with the minimum — so it can never reduce a minimum that governs. Against the whole requirement, it is added to the available volume.
- Break-even figures: the largest unusable volume and the smallest gross volume at which the gap would be zero, how long the available volume lasts at the approved flow, and the highest flow it sustains for the approved duration (both without refill).
- Sensitivity: the gap recomputed with each input 10% lower and 10% higher, one at a time.
Assumptions
- Every design-basis value — flow, which systems run together, duration and minimum — is the user's, from an engineer-approved basis. The tool holds no NBC table, state minimum, concurrency rule or default duration.
- The gross volume is fire water only. Domestic and process water are not counted as fire storage.
- Volumes are those of the store the fire pumps draw from, at full level, as built or as drawn. 1 m³ = 1,000 litres = 1 kL; 1 L/min = 0.06 m³/h.
Limitations
- Does not decide the required flow, which systems run together, the duration or any minimum volume — the approved fire design and the authority do.
- Does not check the suction arrangement, vortex conditions, tank compartments, pump selection, how reliable a refill source is, or the tank's structure.
- A surplus means only that the arithmetic closes on the basis entered. It is not a compliance decision, an approval or a statement that the store is adequate.
- No refill is counted unless the user ticks that the authority approved it and enters it.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Approved combined flow 2,000 L/min
- Approved duration 60 min
- Approved minimum usable volume 150 m³
- Gross fire-reserve volume 170 m³ (fire water only)
- Unusable volume 25 m³
- No refill counted
Working
- Flow-based volume = 2,000 L/min × 60 min ÷ 1,000 = 120 m³.
- Required usable volume = the larger of 150 m³ (approved minimum) and 120 m³ = 150 m³ — the minimum governs.
- Available usable volume = 170 − 25 = 145 m³.
- Gap = 145 − 150 = −5 m³: a deficit of 5 m³.
- Break-even: the unusable volume would have to be 20 m³ or less, or the gross fire reserve 175 m³ or more. At 2,000 L/min the 145 m³ lasts 72.5 minutes.
Result. A deficit of 5 m³ against the design basis entered. The flow-based volume (120 m³) would have been covered; it is the approved minimum the store falls short of.
Sensitivity — what moves the answer
- While the approved minimum governs, the flow and the duration do not move the answer: 10% either way on either leaves the deficit at 5 m³ (the flow-based volume moves between 108 and 132 m³, still under 150 m³).
- The minimum moves it one for one: at 135 m³ the store shows a 10 m³ surplus; at 165 m³, a 20 m³ deficit.
- The unusable volume counts as much as the gross volume, cubic metre for cubic metre: 22.5 m³ of dead volume gives a 2.5 m³ deficit, 27.5 m³ gives 7.5 m³. A 10% error in the gross volume (153 or 187 m³) swings the result from a 22 m³ deficit to a 12 m³ surplus.
How engineers use the result
As an arithmetic check on a fire water store against its approved design basis — before a tank is cast, when an existing store is reused for an extension, or when the basis changes (sprinklers added, a longer duration) — so the discussion starts from usable volume, not gross.
When a professional design must replace it
Always before relying on it: the fire engineer confirms the design basis, establishes the level the pumps actually draw down to, and checks the store against the scheme the authority accepted. If any design-basis value is uncertain, the result is not an assessment.
Sources
- National Building Code of India 2016 (BIS) — Part 4, Fire and Life Safety — the framework fire water storage is designed within; this tool applies no clause, table or minimum from it
- BIS booklet: guide for using NBC 2016 — a BIS guide to using the Code; not a substitute for the Code or the approved scheme
- Volume = flow × time; 1 m³ = 1,000 litres; 1 L/min = 0.06 m³/h — definitions; no code clause is applied by this tool
Questions people ask before using it
What do I need before using this checker?
The engineer-approved design basis for the store: the combined flow that must be supplied at the same time, how long it must last, and any minimum usable volume the approved scheme or the authority states. For the tank, the gross volume of the fire reserve and the volume the fire pumps cannot draw, from the tank and suction drawings or a measurement. A refill rate only if the authority accepted one, and how it counts. If a design-basis value is missing, the page does not assess the storage.
Which site conditions could change the result?
Where the fire-pump suction sits above the tank floor and the level at which the pumps start drawing air — together they set the unusable volume; domestic or process outlets that could draw down a shared tank; a compartment out of service for cleaning; tank dimensions as built rather than as drawn; and any change to the design basis, such as sprinklers added or a building extended, which can raise the combined flow or the duration.
What should I send an engineer for a project-specific review?
The approved fire scheme and its design basis, the tank general-arrangement drawings with the suction, overflow and inlet levels, as-built dimensions if they differ, the pump schedule, details of any make-up or refill source and what the authority accepted about it, and the CSV or JSON saved from this page.
Request a project-specific fire-system design review
The tank figure above is yours with no form. If you want a fire engineer to check it against your fire scheme, pump schedule and the authority's position, send a request. The scope of that review is confirmed with you before any design work is commissioned; nothing is sent until you press the button.
Request saved
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