Pipe Pressure Loss Calculator
Estimates pressure loss due to friction in plumbing pipes based on length, flow rate, and diameter. Use this when sizing pipes for water supply systems, irrigation, or fire protection layouts.
Last updated: September 2026
Formula below · 2 sources (osha.gov, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
Pressure loss in pipes due to friction is estimated here with the Hazen-Williams equation, the standard empirical formula for water piping. In US units the head loss per 100 feet of pipe is h = 0.2083 × (100/C)^1.852 × Q^1.852 / d^4.8655 feet of water, where Q is the flow in GPM, d is the inside diameter in inches and C is the roughness coefficient (120 is used here, a conservative value for copper or plastic service pipe). The calculator scales it to your length and converts feet of water to PSI (× 0.433): Pressure Loss (PSI) = h × (pipeLength / 100) × 0.433. Flow rate is raised to the power 1.852, so doubling the flow multiplies the loss by about 3.6. Diameter is raised to the power 4.8655, so even a small increase in pipe size dramatically reduces friction loss. The selected size is used as the inside diameter; actual inside diameters differ slightly by material (for example 1/2-inch PEX is about 0.475 in), so treat small-pipe results as estimates. Fittings add further loss not included here.
How to use
Suppose a pipe is 100 feet long, carries 10 GPM, and has a 1-inch inside diameter. Step 1 — (100/120)^1.852 ≈ 0.714. Step 2 — 10^1.852 ≈ 71.1 and 1^4.8655 = 1. Step 3 — head loss per 100 ft = 0.2083 × 0.714 × 71.1 ≈ 10.57 ft. Step 4 — scale to length: 10.57 × 100/100 = 10.57 ft. Step 5 — convert: 10.57 × 0.433 ≈ 4.58 PSI. The same flow in a 3/4-inch pipe loses about 18.6 PSI, and in a 1/2-inch pipe about 133 PSI, which shows why 10 GPM needs at least a 1-inch line over 100 feet.
Frequently asked questions
How does pipe diameter affect pressure loss in a plumbing system?
Pipe diameter has a dramatic effect on friction loss because it appears in the formula raised to the power of 4.87. Doubling the pipe diameter reduces pressure loss by a factor of roughly 2^4.87 ≈ 29 times. This is why upgrading from a 1-inch to a 2-inch pipe on a long run can almost eliminate pressure drop. Proper pipe sizing is the most cost-effective way to maintain adequate water pressure at fixtures.
What causes pressure loss in water supply pipes?
Pressure loss is primarily caused by friction between the moving water and the pipe wall, a phenomenon called head loss. It increases with pipe length, flow velocity, and surface roughness of the pipe interior. Fittings such as elbows, tees, and valves also contribute additional equivalent lengths of friction resistance. Corrosion or mineral buildup inside older pipes further increases roughness and worsens pressure loss over time.
How do I know if my pipe is too small for the required flow rate?
If calculated pressure loss leaves insufficient residual pressure at the end of the run, the pipe is undersized. For residential water supply, a minimum of 20–25 PSI at fixtures is generally required for proper operation. Run the calculator with progressively larger pipe diameters until the pressure loss is acceptable for your system's available supply pressure. Local codes and fixture manufacturers also publish minimum pressure requirements you should meet.