Pipe friction loss calculator
Enter the flow rate, internal pipe diameter, pipe length and material. Pipe length starts at 100 metres, so change it to match the actual run.
Friction loss calculator
Enter a flow rate and diameter above to see the result.
What friction loss actually is
Water moving through a pipe rubs against the pipe wall. That friction converts some of the water's pressure energy into heat, so the pressure at the far end of the pipe is always lower than at the start, even on flat ground with no fittings. Engineers call this friction head loss, and it is one of the three things, along with elevation and minor losses through fittings, that make up the total dynamic head a pump has to overcome.
The Hazen Williams equation
Several methods estimate friction loss. The Darcy Weisbach equation is the most universal, correct for any fluid, but it needs a friction factor that has to be looked up from a Moody chart or solved iteratively from the Colebrook equation. For water specifically, the Hazen Williams equation gives a good result without that extra step, which is why it is the standard tool for sizing water supply and irrigation pipelines. This calculator uses it in SI units:
hf = 10.67 × L × Q1.852 ÷ (C1.852 × D4.8704)
Where hf is head loss in metres, L is pipe length in metres, Q is flow rate in cubic metres per second, C is the roughness coefficient for the pipe material, and D is the internal diameter in metres. Pressure drop follows directly from head loss, multiplying it by the specific weight of water, about 9,810 newtons per cubic metre.
Roughness coefficient by material
A higher C means a smoother pipe and less friction loss for the same flow and diameter.
| Material | Typical C |
|---|---|
| HDPE, PVC, fibreglass (new) | 150 |
| Ductile iron, asbestos cement | 140 |
| Copper | 135 |
| Cast iron, concrete (new) | 130 |
| Galvanised iron, steel (new) | 120 |
| Cast iron (old, riveted) | 100 |
A worked example
Take a solar water pumping scheme delivering 15 cubic metres an hour through 100 metres of 75 millimetre internal diameter HDPE pipe, C equal to 150. Head loss comes out to about 1.17 metres, a pressure drop of roughly 0.12 bar. Run the same flow through galvanised iron pipe of the same diameter, C equal to 120 instead, and head loss rises to about 1.77 metres, around 0.17 bar. That difference is why thesolar pump sizing guide treats pipe material and diameter as design choices to iterate on, not fixed inputs to accept as given.
Frequently asked questions
What formula does this calculator use?
The Hazen Williams equation, the standard method for calculating friction head loss in water pipes: head loss equals 10.67 times pipe length times flow rate to the power 1.852, divided by the roughness coefficient C to the power 1.852 and the internal diameter to the power 4.8704.
What is the Hazen Williams roughness coefficient C?
C describes how smooth or rough the inside of a pipe is. A higher C means a smoother pipe and less friction loss. Plastic pipe such as HDPE and PVC sits around 150, new steel or galvanised iron around 120, and old corroded cast iron as low as 100.
Is this the same as the Darcy Weisbach method?
No, though both estimate friction loss. Darcy Weisbach is more universal and works for any fluid, but needs a friction factor found through the Colebrook equation or a Moody chart. Hazen Williams is simpler and accurate enough for water at normal temperatures, which is why it is the standard choice for water supply and irrigation design.
Why does pipe material make such a difference?
A rougher internal surface disturbs the flow near the pipe wall more, which converts more of the flow's energy into heat instead of forward motion. Over a long pipeline that adds up. Switching a scheme from old cast iron to new HDPE can cut friction loss by more than half for the same flow and diameter.
Should I use internal diameter or outside diameter?
Internal diameter, always. This matters most for HDPE and PVC, which are sized by outside diameter, so the true internal diameter is smaller once wall thickness is subtracted. Using the outside diameter by mistake will understate the friction loss.
What is a reasonable friction loss target for a design?
There is no fixed rule, but many designers try to keep major friction loss under about 5 to 10 percent of the total dynamic head, and increase the pipe diameter if it runs higher than that. Going one pipe size up is almost always cheaper than the pump and array capacity needed to push through excess friction.
What velocity should the water be moving at?
Most water pipe designs stay in a range of about 0.1 to 3 metres per second. The calculator shows velocity alongside head loss and flags it if a combination of flow and diameter pushes outside that range, since the Hazen Williams equation itself is calibrated for that range and for internal diameters of 50 millimetres and above.