Quick answer: A fire hydrant's real capacity is found with a fire flow test, not from the pipe size. Read the static pressure, open a hydrant to flow water and read the residual pressure, and measure the pitot pressure at the flowing outlet. The flow is Q = 29.83 × C × d² × √P (US gpm), then corrected to the flow available at 20 psi using the NFPA 291 formula. Skip the maths with the fire hydrant flow calculator.
Why a hydrant's flow isn't just its pipe size
A 100 mm hydrant on a strong ring main can deliver several thousand litres a minute; the same hydrant on a dead-end 80 mm branch might barely make half that. Flow depends on the available pressure and the friction in the supplying main, both of which only reveal themselves when water is actually moving. That is what a flow test measures: how much the pressure sags when you draw a known flow, so you can predict the flow available at the pressure your fire system needs.
The fire flow test, step by step
- Static pressure — fit a cap gauge to a test hydrant and read the pressure with no flow. Call it Ps.
- Flow a second hydrant — open a nearby hydrant fully. Hold a pitot tube in the centre of the outlet stream and read the pitot pressure P (psi).
- Residual pressure — while the second hydrant flows, read the cap gauge on the first hydrant again. Call it Pr (it will be lower than static).
The pitot flow formula
The flowing hydrant's discharge is:
Q (gpm) = 29.83 × C × d² × √P d = outlet diameter (inches, usually 2.5") P = pitot pressure (psi) C = outlet discharge coefficient
| Outlet type | Discharge coefficient C |
|---|---|
| Smooth / rounded | 0.90 |
| Square / sharp-edged | 0.80 |
| Projecting into barrel | 0.70 |
Multiply gpm by 3.785 for litres/min.
Available flow at 20 psi (NFPA 291)
The test flow rarely equals the flow you can use. Correct it to a standard 20 psi (1.4 bar) residual:
Q₂₀ = Qₜ × (hₕ / h₌)^0.54 h₌ = Pₛ − Pₕ (pressure drop while flowing) hₕ = Pₛ − 20 (drop down to the target residual)
Worked example
Static 65 psi, residual 52 psi while flowing, pitot 22 psi at a 2.5″ smooth outlet (C = 0.9):
Qₜ = 29.83 × 0.9 × 2.5² × √22 ≈ 787 gpm h₌ = 65 − 52 = 13 psi ; hₕ = 65 − 20 = 45 psi Q₂₀ = 787 × (45/13)^0.54 ≈ 1,540 gpm (≈ 5,800 L/min)
So this hydrant can supply about 1,540 gpm at 20 psi — the number you compare against the building's required fire flow.
Common mistakes
- Reading pitot pressure off-centre or too close to the outlet — hold it about half the outlet diameter back, in the middle of the stream.
- Using the wrong discharge coefficient — a projecting outlet flows ~20% less than a smooth one at the same pitot reading.
- Quoting the raw test flow instead of the flow at 20 psi — only the corrected figure is usable for design.
- Testing at low-demand hours — test when the main is under normal or peak domestic draw for a realistic worst case.
FAQ
What is a good fire hydrant flow rate? For most municipal hydrants a delivery of 1,000–1,500 gpm (3,800–5,700 L/min) at 20 psi is considered good; anything under ~500 gpm is a weak hydrant.
How do I convert pitot pressure to flow? Use Q = 29.83 × C × d² × √P, or enter the readings into the fire hydrant flow calculator.
Why 20 psi? It is the minimum residual pressure that keeps the water main from collapsing and pumper suction cavitating, so fire-flow availability is universally quoted at 20 psi.