Water Pressure Loss Calculator
Estimate how much pressure a water line gives up to friction at your design flow. Enter the flow in GPM, the run length, and the pipe material and size, and get the loss in psi and feet of head — per 100 ft and for the whole run — plus the flow velocity with a warning when it climbs past the 8 fps design ceiling. Use it to check whether the pressure left at the fixture clears its minimum, or to see what upsizing one trade size buys you. Free, no login.
Flow & Pipe
Friction Loss
Hazen-Williams: h (ft/100 ft) = 0.2083 · (100/C)1.852 · GPM1.852 ÷ d4.8655 — an estimate; add fitting equivalent lengths to the run for a fuller number. The ≤8 fps velocity target is design practice, not a code limit.
How this calculator works
Friction loss for water in pressure pipe uses the Hazen-Williams formula, the standard empirical method for plumbing design. With flow Q in GPM and inside diameter d in inches:
hf (ft per 100 ft) = 0.2083 × (100/C)1.852 × Q1.852 ÷ d4.8655 · psi = ft × 0.4333
C is the roughness coefficient — higher means smoother pipe and less loss (PEX and PVC at 150, copper at 140, new steel at 130, old steel down around 100). Two things dominate the result: diameter, at nearly the 5th power — one trade size up cuts the loss to roughly a third — and flow, at the 1.852 power. The calculator runs the formula on the same published average inside diameters as the other pipe tools, converts head to psi at 0.4333 psi per foot, and reports velocity (v = 0.4085 × Q ÷ d²) alongside, because a run that passes on pressure can still fail on erosion and noise above 8 fps. Fittings and valves are not itemized — add their equivalent lengths to the run for a fuller estimate.
Worked example
10 GPM through 100 ft of 3/4" copper Type L (ID 0.785", C = 140):
- The three factors: (100/140)1.852 = 0.5363, 101.852 = 71.12, 0.7854.8655 = 0.3080.
- Loss: 0.2083 × 0.5363 × 71.12 ÷ 0.3080 = 25.8 ft per 100 ft — the run is 100 ft, so total head loss is 25.8 ft.
- In pressure terms: 25.8 × 0.4333 = 11.18 psi.
- Velocity: 0.4085 × 10 ÷ 0.785² = 6.63 fps — under 8 fps, so the pipe passes on velocity but is eating a lot of pressure. One size up (1", ID 1.025) drops the loss to about 7.0 ft (3.1 psi).
These are the calculator’s default inputs — the panel above shows exactly these numbers.
Reference: Hazen-Williams C factors
Typical design values for the materials the calculator carries. C drops as pipe ages and scales — the “old steel” row is why replacing a galvanized run often fixes a pressure complaint without touching anything else.
| Material | C factor |
|---|---|
| Copper | 140 |
| PEX | 150 |
| PVC / CPVC | 150 |
| Steel (new) | 130 |
| Steel (old) | 100 |
Frequently asked questions
How do I calculate pressure loss in a water line?
This calculator uses the Hazen-Williams formula: head loss per 100 ft = 0.2083 × (100/C)^1.852 × Q^1.852 ÷ d^4.8655, where Q is flow in GPM, d is inside diameter in inches, and C is the roughness coefficient for the pipe. The result is scaled to your run length and converted to psi at 0.433 psi per foot of head. It also reports the flow velocity and warns when it exceeds 8 fps.
What C factor (roughness coefficient) should I use?
The calculator offers C = 140 for copper, 150 for PEX and PVC/CPVC, 130 for new steel, and 100 for older steel that has scaled up. Higher C means smoother pipe and less loss. If you are checking an existing galvanized system of unknown age, use the lower value — friction loss nearly doubles going from C 140 to C 100 at the same flow.
Does this include fittings and valves?
No — the calculation covers straight pipe only. The standard approach is to add the equivalent length of every fitting and valve to the actual run length before entering it: each elbow, branch tee, and valve behaves like extra feet of pipe. On a fitting-heavy run the fittings can contribute as much loss as the pipe itself, so skipping them understates the total.
How do I convert feet of head to psi?
Multiply by 0.433 — one foot of water column exerts 0.433 psi, so 10 ft of head loss is about 4.3 psi. Elevation works the same way and is separate from friction: lifting water 20 ft costs about 8.7 psi before any friction loss at all. This calculator reports friction loss in both units but does not add elevation change — account for risers yourself.
What is an acceptable water velocity in supply piping?
About 8 fps is the common ceiling for cold domestic supply, which is where this calculator's warning kicks in; above roughly 10 fps, erosion, noise, and water hammer risk climb fast. Hot-water lines are usually held lower — 5 fps is a widely used limit for copper on hot water because erosion-corrosion accelerates with temperature. Some code and manufacturer limits are stricter, so verify against the plumbing code adopted in your jurisdiction and the pipe manufacturer's data.
When does the Hazen-Williams method not apply?
Hazen-Williams is an empirical formula developed for water at ordinary temperatures in turbulent flow — the normal case in building supply piping, and the basis of standard plumbing and fire-protection sizing charts. It is not valid for gases, compressed air, glycol mixes, or other fluids, and it drifts at very high temperatures and unusually low velocities; those cases call for Darcy-Weisbach with real fluid properties. Treat the output as a solid design estimate, not a lab measurement.
Method: Hazen-Williams friction loss (hf per 100 ft = 0.2083 × (100/C)1.852 × Q1.852 ÷ d4.8655) on published average inside diameters, with head-to-pressure conversion at 0.4333 psi per foot. Hazen-Williams is an empirical estimate valid for water at ordinary temperatures in turbulent flow — not for glycol mixes, compressed air, or gas — and the C factors are typical design values, not guarantees. Fitting and valve losses are extra: add equivalent lengths to the run. The 8 fps velocity ceiling is design practice, not a code limit. Code-method supply sizing (meter, developed length, and WSFU tables) is a separate procedure — the adopted code and the AHJ govern.
Pair with the GPM Flow Rate Calculator and Pipe Sizing Calculator for the velocity side of the problem, or the water pipe sizing chart and pressure conversion chart for the code-table method and unit conversions.