PRESSURE LOSS
About this calculator
This pressure loss calculator computes friction loss in a water supply line using the Hazen-Williams equation. Pick the pipe material (different roughness coefficients), nominal size (which determines inside diameter), flow rate, and run length. The output is total head loss in feet, pressure drop in PSI, and water velocity in feet per second — exceeding 8 fps cold or 5 fps hot causes erosion, noise, and water hammer regardless of how the friction math looks. Use this for long service runs, multi-story buildings, irrigation mains, and any branch where the fixture at the dead end is losing pressure. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation. Hazen-Williams is the standard sizing equation in IPC Appendix E and ASHRAE Fundamentals.
How to use this calculator
Pick the pipe material — Hazen-Williams roughness coefficient (C) varies: copper Type L = 140, PEX-A = 150, CPVC = 150, new galvanized steel = 100 (drops to 60-80 with decades of age and corrosion).
Pick the nominal pipe size, expected peak flow in GPM (use the WSFU calculator to estimate this), and one-way run length in feet. The calculator returns head loss in feet, pressure drop in PSI, and velocity in feet per second. Velocity is the canary — exceeding 8 fps cold or 5 fps hot causes erosion, noise, and water hammer regardless of pressure-loss math.
Worked example
For a ¾" copper Type L line carrying 10 GPM over 100 ft:
ID 0.785", C = 140. Hazen-Williams head loss: ~3.4 ft over 100 ft. PSI drop: 3.4 × 0.433 = 1.5 PSI. Velocity: 10 GPM through 0.785" ID = 6.6 fps.
Within acceptable range — 1.5 PSI drop is small, velocity is below the 8 fps cold limit. Fixture at dead end gets 60 PSI - 1.5 = 58.5 PSI dynamic pressure (assuming 60 PSI static).
For the same flow on ½" copper: ID 0.545", velocity jumps to ~13.8 fps. Pressure loss jumps to ~17 PSI per 100 ft. Erosion and water hammer guaranteed within years.
For a long 200-ft irrigation main: 1" PEX (ID 0.875") at 12 GPM. Head loss ~6 ft per 100 ft × 2 = 12 ft = 5.2 PSI total. Velocity ~6.4 fps. Acceptable.
For old galvanized steel that's decades old: same ¾" line as the copper example. C drops to 70 (corroded). Head loss 12+ ft per 100 ft, pressure drop 5+ PSI on the same flow. This is why old galv-steel homes have weak fixture pressure — replace with copper or PEX.
Common mistakes & waste factors
Using nominal size as if it were ID. Hazen-Williams uses the inside diameter, not the nominal size. Wall thickness varies by material (PEX is thicker, type M copper is thinner). The calculator handles this — but doing it by hand needs the actual ID.
Ignoring velocity. Friction math gives a low PSI drop, but velocity over 8 fps cold (5 fps hot) erodes the inside of pipes. Plan for low velocity even if the friction math is fine.
Forgetting fittings. Each fitting adds equivalent length: 90° elbow ~2 ft, T-fitting ~3-5 ft. A 100-ft run with a dozen fittings is closer to 130 ft developed length.
Assuming new C-factors apply forever. Galvanized steel drops from 100 (new) to 60-80 (after 30+ years) due to mineral buildup and corrosion. Copper, PEX, and CPVC are stable for life.
Rules of thumb
Hazen-Williams C: copper Type L = 140, PEX-A = 150, CPVC = 150, new galvanized = 100, old galvanized = 60-80, PVC = 150.
Velocity limits: <8 fps cold, <5 fps hot water (hot is more erosive).
Pressure loss roughly: ½" pipe at 8 GPM ≈ 8 PSI per 100 ft; ¾" at 10 GPM ≈ 1.5 PSI per 100 ft; 1" at 15 GPM ≈ 1 PSI per 100 ft.
Long runs (>100 ft): always upsize. Service from street to house is typically 1" minimum on residential.
Multi-story homes: add 0.43 PSI per foot of vertical rise. A 30-ft head (3-story) costs 13 PSI before friction.
Acceptable total loss: <10 PSI from meter to worst-case fixture is the design target. Above 15 PSI = noticeable weak flow at fixtures.
Common questions
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