Pipe Flow Rate Calculator
Flow goes with the square of the bore.
Work out Pipe Flow Rate. Flow goes with the square of the bore. Shows the working, not just the answer.
The bore, not the nominal size — 15 mm copper has about 13.6 mm of bore
Water at 20 °C is 1.004 × 10⁻⁶
Through a 15 mm bore at 1.5 m/s
15.9 L/min
954 L/h · flow is turbulent
Flow scales with the square of the bore, which is why one size up carries 1.96× as much at the same velocity. That is usually the answer to a weak-flow complaint, and a larger pump rarely is — pushing more through a narrow pipe just raises velocity, noise and friction loss.
How the Pipe Flow Rate Calculator works
Flow through a pipe from its bore and the velocity, with the Reynolds number and a warning above the velocity domestic pipework is normally kept below. One size up carries roughly twice as much.
Also known as: litres per minute through a pipe · what pipe size for this flow rate · why is my water pressure low · flow velocity in copper pipe
Flow goes with the square of the bore
Cross-sectional area is proportional to the square of the diameter, so a 22 mm pipe carries about 2.15 times what a 15 mm one does at the same velocity. Stepping up one size is not a marginal improvement — it is roughly a doubling.
This is why upsizing a pipe usually fixes a weak-flow complaint that a larger pump does not. Pushing more through a narrow bore raises velocity, noise and friction loss rather than delivering much more water at the outlet.
It also means limescale matters more than its thickness suggests. A thin deposit around the inside of a pipe reduces the bore slightly and the flow substantially, which is why hard-water areas see gradual flow decline that has no single obvious cause.
Velocity limits, and what they are protecting
Domestic pipework is generally kept below about 2 m/s. Above that, flow becomes audible, water hammer on sudden closure gets worse, and erosion of copper at elbows accelerates measurably over years.
Water hammer is the pressure surge when flow stops abruptly — a moving column of water has momentum, and stopping it fast converts that momentum into a pressure spike. It is heard as a bang and it stresses joints repeatedly.
Which makes velocity a design constraint rather than an outcome. Sizing pipework to keep velocity in range at the expected flow is what prevents both problems, and it is why the calculation runs from bore and velocity rather than solving for whatever fits.
Nominal size is not bore
A 15 mm copper pipe has about 13.6 mm of internal bore, and plastic pipe of the same nominal size differs again. Since flow depends on the square of the internal figure, using the nominal one overstates capacity noticeably.
Fittings compound it. Each elbow, tee and valve adds resistance equivalent to some length of straight pipe — often a metre or more — which is why short runs with many fittings behave like much longer ones.
The practical diagnosis for low delivery is to separate pressure from flow. Poor flow at good static pressure points to bore, blockage or fittings; genuinely low pressure is a supply problem and needs a different remedy entirely.
Where to go next
The Pipe Flow Rate question rarely arrives on its own. These are the ones that usually come with it:
- Pump Head Calculator — Choosing on static lift alone is why pumps stall.
- Water Heater Size Calculator — First-hour rating beats tank size.
- Sprinkler Coverage Calculator — Space heads at the radius, not the diameter.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate flow rate through a pipe?
Cross-sectional area times velocity. Because area goes with the square of the diameter, a 22 mm pipe carries about 2.15 times what a 15 mm one does at the same velocity.
What flow velocity is acceptable?
Domestic pipework is usually kept below about 2 m/s. Above that it becomes noisy, prone to water hammer, and erosive to copper elbows over time.
Will a bigger pump fix weak flow?
Rarely. Pushing more through a narrow pipe raises velocity, noise and friction loss rather than delivering much more water. Increasing the bore is almost always the better fix.
What is the Reynolds number?
A dimensionless ratio of inertial to viscous forces that identifies the flow regime. Below about 2,300 flow is laminar; above 4,000 it is turbulent, which is where almost all domestic plumbing sits.
Is nominal pipe size the same as bore?
No, and the difference matters. A 15 mm copper pipe has about 13.6 mm of internal bore, and plastic pipe of the same nominal size differs again. Use the internal figure.
Why does flow drop when another tap is opened?
Because the supply is shared and friction loss rises with flow. The pressure at each outlet falls, which is most noticeable on the outlet furthest from the supply.
What is water hammer?
A pressure surge when flow stops suddenly, heard as a bang. It is worse at high velocity, which is one reason domestic pipework is kept below about 2 m/s.
How do I improve low water pressure?
Establish first whether it is pressure or flow. Low flow at good pressure is usually a bore or blockage problem; genuinely low pressure needs a pump or a supply investigation.
Does pipe material change flow?
Through roughness and through internal bore. Plastic is smoother than old galvanised steel, and nominal sizes differ between materials — a 15 mm plastic pipe and 15 mm copper have different bores.
Why does the shower weaken when a tap opens?
Shared supply and rising friction loss. Increasing the bore of the shared section, rather than the branches, is usually what fixes it.
What is equivalent length?
A way of expressing the resistance of a fitting as a length of straight pipe. A single elbow can be worth a metre or more, which is why fittings dominate short runs.
Does limescale reduce flow?
Substantially over time in hard water areas. Because flow goes with the square of bore, even a thin deposit around the inside of a pipe cuts flow noticeably.
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