In short: friction loss is the head a length of pipe consumes as water pushes against its walls, driven mainly by velocity, diameter, length and pipe roughness. For example, 10 GPM through 100 ft of 1-inch PVC pipe loses about 6.9 ft of head (3.0 psi). Diameter matters most — undersizing a pipe by even one size can multiply the loss several times over. Use the calculator above for your exact flow, pipe and material.
These figures are planning aids based on published hydraulic formulas. For code-required pipe sizing or pump selection, confirm with a licensed engineer.
How to calculate pipe friction loss
This calculator uses the Hazen-Williams equation by default, the standard method for water flowing through pipes at ordinary temperatures. It needs just four inputs — flow rate, inside diameter, pipe length and a roughness coefficient (C) for the pipe material:
Worked example. Take 10 US GPM through 100 ft of 1-inch (ID) PVC pipe, C = 150. Converted to SI: Q = 6.309×10⁻⁴ m³/s, D = 0.0254 m, L = 30.48 m. Plugging into the equation gives hf ≈ 2.112 m, which converts to 6.93 ft (3.00 psi) of head loss — matching the calculator's default result.
For flows outside water's normal temperature range, non-water fluids, or laminar flow, switch to the Darcy-Weisbach method in the calculator. It computes a friction factor from the Reynolds number and the pipe's absolute roughness (via the Reynolds number and the Swamee-Jain approximation), then applies hf = f × (L/D) × V²/(2g). It is more work to set up but valid for any flow regime. For a closer look at each method on its own, see the dedicated Hazen-Williams calculator and Darcy-Weisbach calculator.
Friction loss vs. total head. This tool reports friction loss only. A pump or supply line's total dynamic head also includes elevation (static) change and any pressure the system needs to deliver at the far end — friction loss is just one component you add into that total.
Why diameter, velocity and material change the answer
Diameter dominates. Head loss scales with diameter to about the power of −4.87 in Hazen-Williams, so a pipe one size smaller can lose several times more head at the same flow. This is why "just use the pipe you have on hand" is a common and costly mistake in long runs.
Velocity compounds it. Because velocity rises as the square of the diameter reduction for a fixed flow, and head loss rises with velocity roughly squared to the 1.85 power, small pipes at high flow can produce surprisingly large losses — see the reference table below.
Roughness (material and age) sets the coefficient. Smooth, new pipe — PVC, copper — carries more flow for the same head loss than old, corroded or tuberculated pipe. As metal pipe ages and scales internally, its effective C value (or roughness ε) worsens, and friction loss for the same flow increases even though nothing else about the system changed.
Reference: friction loss per 100 ft (PVC, C = 150)
Head loss in feet per 100 ft of straight PVC pipe, by inside diameter and flow rate. Every value matches what the calculator returns for the same inputs.
| ID | 5 GPM | 10 GPM | 20 GPM | 30 GPM | 50 GPM |
|---|---|---|---|---|---|
| 0.75 in | 7.79 | 28.13 | 101.55 | 215.18 | 554.20 |
| 1 in | 1.92 | 6.93 | 25.01 | 53.00 | 136.51 |
| 1.25 in | 0.65 | 2.34 | 8.44 | 17.88 | 46.04 |
| 1.5 in | 0.27 | 0.96 | 3.47 | 7.36 | 18.95 |
| 2 in | 0.07 | 0.24 | 0.86 | 1.81 | 4.67 |
Worked examples
Undersized supply line
25.0 ft
20 GPM through 100 ft of 1-inch PVC (C = 150) loses about 25.0 ft of head (10.8 psi) — velocity is a high 8.17 ft/s. Stepping up to 1.25-inch pipe cuts that to about 8.4 ft for the same flow.
200 ft of 2-inch steel main
12.2 ft
50 GPM through 200 ft of 2-inch new steel pipe (C = 130) loses about 12.2 ft of head (5.3 psi), at a comfortable velocity of 5.1 ft/s.
Well-sized 1-inch PVC run
6.9 ft
10 GPM through 100 ft of 1-inch PVC (C = 150) — the calculator's default — loses about 6.9 ft of head (3.0 psi) at a comfortable 4.1 ft/s.
What this calculator doesn't include
Fittings and valves. Elbows, tees, reducers and valves each add their own "minor loss," typically estimated with a K-factor (hL = K·V²/2g) or converted to an equivalent length of straight pipe and added to the pipe length you enter here. Use the minor loss calculator to estimate those separately.
Elevation change. This tool reports friction (dynamic) head loss only, not the static head from lifting water vertically. Combine both, along with any target discharge pressure, when sizing a pump — see the total dynamic head calculator.
Non-water fluids. The Hazen-Williams coefficients here assume water at ordinary temperatures. For other fluids, or water well outside 0–100 °C, use the Darcy-Weisbach method with a roughness and viscosity appropriate to your fluid.
Frequently asked questions
What is friction loss in a pipe?
Friction loss is the head, or pressure, that water loses as it rubs against the inside wall of a pipe on its way from one point to another. It grows with flow velocity, pipe length and pipe roughness, and shrinks sharply as pipe diameter increases. It is separate from elevation (static) head and from minor losses at fittings and valves.
Hazen-Williams or Darcy-Weisbach — which should I use?
Hazen-Williams is the industry default for ordinary water pipe design: it is simple, needs only a pipe material coefficient (C), and is accurate for turbulent flow of water at normal temperatures. Darcy-Weisbach is the more rigorous physics-based method, valid for any fluid, temperature or flow regime, and is preferred for laminar flow, non-water fluids, or when you need to account for exact pipe roughness and viscosity.
How does pipe diameter affect friction loss?
Diameter dominates the result. In the Hazen-Williams equation, head loss varies with diameter to the power of about 4.87, so doubling the pipe diameter for the same flow cuts friction loss by roughly a factor of 29. Undersizing a pipe by even one nominal size can multiply friction loss several times over.
Does water temperature affect friction loss?
Only slightly for the Hazen-Williams method, which is calibrated for ordinary water temperatures and does not take temperature as an input. Temperature matters more for Darcy-Weisbach, because it changes water's viscosity and therefore the Reynolds number and friction factor. This calculator uses your entered temperature to compute the Reynolds number and, when Darcy-Weisbach is selected, the friction factor.
Does this calculator include losses from fittings and valves?
No. This tool calculates straight-pipe friction loss only. Elbows, tees, valves and other fittings add separate minor losses, which are usually estimated with a K-factor or converted to an equivalent length of straight pipe and added on top of this result.
What C value should I use for my pipe material?
Typical Hazen-Williams C values are about 150 for new PVC or CPVC, 140 for copper or cement-lined ductile iron, 130 for new steel or concrete, and 100 for old, tuberculated cast iron. Rougher, older pipe carries a lower C and produces more friction loss for the same flow. Use the pipe material selector to apply a typical value, or enter your own.
Sources & references
- Williams, G. S., & Hazen, A. (1933). Hydraulic Tables (3rd ed.). John Wiley & Sons.
- American Water Works Association (AWWA). Manual M11: Steel Pipe — A Guide for Design and Installation.
- Swamee, P. K., & Jain, A. K. (1976). Explicit equations for pipe-flow problems. Journal of the Hydraulics Division, ASCE, 102(5), 657–664.
- Moody, L. F. (1944). Friction factors for pipe flow. Transactions of the ASME, 66(8), 671–684.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)