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NFPA 13
Validated against the standard.
PDF export
Pro-only. Generate professional reports.
$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NFPA 13 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
A sprinkler design is only valid if the water supply can meet it. This app turns a hydrant flow test into an available-supply curve and compares it to the system demand, the final check in any NFPA 13 design.
How it works
- Enter the static pressure, the residual pressure, and the flow (from the pitot reading at the flowing hydrant).
- The app projects the supply curve using the standard hydrant equation (flow varies with the square root of pressure drop).
- It overlays the system demand point so you can see the available margin at the required flow.
Worked example
Static 70 psi, residual 55 psi at 1,000 GPM flowed. The app projects how much water is available at the design residual — say 20 psi — and confirms it exceeds the sprinkler demand of, for example, 600 GPM.
Related calculators
Build the demand with the sprinkler hydraulic calculator; size a pump to close any gap with the fire pump calculator.
Water supply FAQ
What is a flow test?
A hydrant flow test records static pressure, residual pressure while flowing, and the flow rate. From those three values you can project how much water is available at any pressure.
How is the supply curve built?
Using the hydrant equation, where flow is proportional to the square root of the pressure drop. The app plots the curve from your test data.
How do I know the supply is adequate?
Plot the system demand point on the same graph. If it falls under the available supply curve, the supply meets the demand; if not, you need a tank or fire pump.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Water Supply on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.