Enter your flow in L/min, the pipe material and its inside diameter, and the run length, and get the water velocity, the friction loss in kPa over the run and per 100 m, a pass/fail check against the 1.5 m/s velocity limit, and the smallest pipe that keeps the flow under it. Works in L/min and millimetres or GPM and inches.
Use the flow each zone carries - from the sprinkler zones guide. Sizing help
PVC vs poly pipe - which to use where
Pick a stock size or set a measured inside diameter below.
Friction loss grows with the length, so use the longest run from the source to the furthest outlet.
Velocity vs the 1.5 m/s limit
1.19 m/s - OK
At or below the recommended 1.5 m/s.
Friction loss (30 m)
18.3 kPa
Loss per 100 m
61.1 kPa
Min. recommended ID
23.2 mm
Velocity is the flow divided by the pipe's bore; friction loss is the Hazen-Williams pressure drop over the run. Keep velocity at or below 1.5 m/s to avoid water hammer and wear, and add the loss above to your source pressure budget.
A pipe can be too small in two ways, and this calculator checks both. Push a given flow through it and the water travels at a velocity set by the bore - too fast and it erodes fittings and hammers the line when a valve shuts. Along the run the water also gives up pressure to friction against the pipe walls - too much and the far end runs weak. Both come straight from the flow, the inside diameter, the pipe material and the run length:
velocity = flow ÷ bore area, bore area = (π ÷ 4) × inside diameter²
Hazen-Williams friction (head loss in metres of water over the run):
hf = 10.67 × length × flow^1.852 ÷ ( C^1.852 × inside diameter^4.8704 )
pressure drop: 1 m of head = 1.422 psi = 9.807 kPa
min. inside diameter to stay under the limit:
d_min = √( flow ÷ (velocity limit × π ÷ 4) )
The Hazen-Williams formula is the water-industry standard for water near room temperature. The two things to notice: friction loss rises a little faster than the flow (to the power 1.852) and grows with the length, but it falls steeply as the bore grows (to the power 4.87) - which is why stepping up one pipe size cuts the loss so sharply. The C value is the pipe’s roughness, higher for smoother pipe; picking the material sets it. Everything runs in exact metric (cubic metres per second, metres, metres of head) internally, so switching between GPM / inches and L/min / millimetres never changes the answer.
This example follows the unit system you pick in the calculator above.
30 L/min through a 25 mm class PVC line (inside diameter ≈ 20 mm), C = 150, over a 40 m run:
The friction loss depends on the inside diameter (the bore), not the nominal (marked) size, which is just a label. Schedule 40 PVC is sold by nominal inch size; metric markets sell class-rated PVC and poly by outside diameter. These are typical figures - confirm the bore against the spec of your exact pipe, since a thicker wall (higher schedule or class) means a smaller inside diameter:
| Nominal size | Typical inside diameter (Sch 40 PVC) | Metric class PVC (nominal → ID) |
|---|---|---|
| 1/2” | ≈ 0.622” (15.8 mm) | 20 mm → ≈ 16 mm ID |
| 3/4” | ≈ 0.824” (20.9 mm) | 25 mm → ≈ 20 mm ID |
| 1” | ≈ 1.049” (26.6 mm) | 32 mm → ≈ 26 mm ID |
| 1-1/4” | ≈ 1.380” (35.1 mm) | 40 mm → ≈ 33 mm ID |
| 1-1/2” | ≈ 1.610” (40.9 mm) | 50 mm → ≈ 42 mm ID |
| 2” | ≈ 2.067” (52.5 mm) | 63 mm → ≈ 53 mm ID |
Poly (PE) and MDPE tubing vary more by manufacturer, so read the inside diameter off the roll’s spec. When in doubt, enter a measured bore using the Custom option in the size list.
The C value is the pipe’s smoothness - higher means less friction loss. The calculator sets it from the material you pick:
| Material | Hazen-Williams C | Notes |
|---|---|---|
| PVC / CPVC (new) | 150 | Smoothest common supply pipe |
| Polyethylene (PE / MDPE / poly tube) | 150 | Same as PVC when new |
| Copper | 135 | Slightly rougher than plastic |
| Galvanised / old steel | 100 | Roughens further with age and scale |
Old, scaled or corroded pipe runs a lower C than new, so a real-world line loses a little more than the clean figure - leave some headroom in your pressure budget.
For the full method see what size pipe for a sprinkler system, and to choose the pipe itself, PVC vs poly pipe for irrigation.
Sizing an irrigation system? Work out the flow each zone carries first with the sprinkler zone calculator, then bring that flow here to confirm the pipe to each zone is big enough.
In the United Kingdom, flow is rated in litres per minute (L/min) and pipe is sized as metric pipe in millimetres (for example 25 mm or 32 mm class-rated PVC). Enter your flow and the pipe's inside diameter in those units in the calculator above and it sizes the run to match.
Buried supply pipe is typically blue MDPE (polyethylene) in metric sizes, with class-rated PVC for above-ground runs. Pipe is specified by its outside diameter, so check the wall thickness (class/SDR) to get the inside bore the calculator needs.
Big enough that the water neither runs too fast nor loses too much pressure to friction over the run. Size on two checks: keep velocity at or below about 1.5 m/s, and keep the friction loss within your pressure budget. Enter your flow, the pipe inside diameter and the run length and the calculator gives both - and the smallest inside diameter that keeps the flow under the velocity limit if your pipe is too small.
This calculator uses the Hazen-Williams formula, the water-industry standard for water near room temperature: head loss rises with the flow (to the power 1.852) and the run length, and falls steeply as the inside diameter grows (to the power 4.87). It is computed in metres of head internally and shown as kPa over your run and per 100 m. Friction loss is roughly proportional to length, so a run twice as long loses about twice the pressure.
Irrigation and water-supply practice keeps velocity at or below about 1.5 m/s (5 ft/s). Faster water erodes fittings over time and slams the line with water hammer when a valve shuts. The calculator flags pass or fail against the 1.5 m/s limit and, when a pipe runs too fast, shows the minimum inside diameter that would bring it back under.
No. The nominal (marked) size is a label, not the bore. Metric class-rated PVC and MDPE are sold by outside diameter, so a 25 mm pipe has an inside diameter nearer 20 mm depending on the wall thickness (class/SDR). The friction loss depends on the actual inside diameter, so enter the bore from the spec sheet, not the nominal number. The size list in the calculator uses typical inside diameters - confirm yours.
C is a roughness coefficient: higher means a smoother pipe with less loss. New PVC and polyethylene are about 150, copper about 135, and galvanised or old steel about 100. Pick the material in the calculator to set C. Old, scaled or corroded pipe runs lower than new, so a real-world line loses a bit more than the clean figure.
Yes - friction loss is roughly proportional to the run length, so doubling the distance roughly doubles the loss. Use the longest run from your source to the furthest outlet when sizing, because that is the worst case the system has to deliver against. If the loss eats too much of your pressure, step up a pipe size: a larger bore cuts the loss sharply.
Static pressure is what your supply reads with no water moving; friction loss is the pressure the moving water gives up to the pipe walls along the run. The pressure left at the outlet is the static pressure minus the friction loss (and any height the water has to climb). Size the pipe so the friction loss leaves enough pressure for whatever the line feeds - sprinkler heads, taps or an appliance.
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