How to Use This Calculator
Friction between water and pipe walls causes pressure to drop as water travels distance. On long supply runs—especially in multi-story buildings or to far-away fixtures—pressure loss becomes noticeable. Low pressure at the fixture means reduced flow, weak showers, and customer complaints. The Hazen-Williams formula calculates pressure drop (in psi per 100 feet) based on pipe diameter, flow rate, and the pipe material’s roughness.
Enter your flow rate (GPM), pipe diameter (inches), pipe material (which determines the roughness coefficient C), and the length of the run (feet). The calculator returns friction loss in psi for that length. Example: a 3/4-inch copper line carrying 5 GPM for 100 feet loses about 6–8 psi. That’s significant on a 40-psi incoming pressure (leaving only 32–34 psi at the fixture). Upsizing to 1 inch copper reduces loss to 1–2 psi.
The IPC expects incoming supply pressure of 20–80 psi and requires that pressure at fixtures not fall below 20 psi. This calculator helps you verify that oversized runs or high-demand scenarios still deliver adequate pressure.
Formula
Hazen-Williams Friction Loss: hf = 10.67 × Q^1.85 / (C^1.85 × d^4.87)
Where:
- hf = head loss (pressure drop) in psi per 100 feet
- Q = flow rate in GPM
- C = roughness coefficient (varies by material and age)
- d = pipe inside diameter in inches
Roughness Coefficients (C values):
- Copper (smooth, clean): C = 140–150
- PEX (plastic): C = 140–150
- PVC (plastic): C = 130–140
- Galvanized steel (new): C = 130–140
- Galvanized steel (aged/corroded): C = 80–120
- Cast iron (new): C = 130
- Cast iron (old/corroded): C = 60–100
Pressure Drop for Length: Total pressure drop (psi) = (hf × run length in feet) / 100
When to Use This
Long supply runs, multi-story buildings, and high-demand applications. A single-story house with a short run from the meter to the kitchen might lose only 1–2 psi across the entire system—no problem. A two-story house with 100+ feet of supply piping and multiple fixtures running simultaneously can easily lose 10–15 psi by the time water reaches the second-floor shower. That’s pressure to account for when designing.
Example scenario: You’re adding a bathroom to the far corner of a commercial building. The supply line is 200 feet of 3/4-inch copper, and you need 5 GPM at that fixture. Hazen-Williams shows about 12–15 psi loss for the run. If incoming pressure is 60 psi, the fixture pressure is 45–48 psi—still acceptable. If incoming pressure is only 40 psi, the fixture gets 25–28 psi—marginal but legal (IPC minimum is 20 psi).
Code References
- IPC Section 603.7: Design of water supply systems must account for pressure loss and friction
- IPC Section 604: Supply line pressure; system must maintain 20–80 psi at water service entrance, and minimum 20 psi at fixtures
- IPC Appendix B: Hazen-Williams and Manning formulas for sizing and loss calculations
- AWWA Standards: Engineering practice for water distribution design uses Hazen-Williams extensively
Frequently Asked Questions
Why does roughness coefficient C decrease with age?
As pipes age, interior surfaces corrode (especially galvanized steel) and accumulate mineral deposits (especially in hard-water areas). Rougher surfaces increase friction. A new galvanized steel pipe with C = 140 can drop to C = 80–100 over 20–30 years in hard water. Copper and PEX age more slowly and maintain higher C values longer.
What if I’m not sure whether my pipe is new or old?
Use a conservative (lower) C value. For galvanized steel, assume C = 120 if you don’t know age. For copper, assume C = 140. You’ll overestimate loss slightly, but that’s safer than undersizing and ending up with inadequate pressure.
How much pressure drop is acceptable?
The IPC allows any drop as long as minimum pressure at fixtures (20 psi) is maintained. In practice, most engineers aim for no more than 10% loss in branch lines. If inlet pressure is 60 psi, keep loss under 6 psi. On main supply lines serving many fixtures, 5% loss (3 psi on 60 psi) is ideal. Use this calculator to verify you’re in reasonable range.
If I upsize the entire system, how much do I save in pressure loss?
Pressure drop is proportional to the fourth power of diameter (d^4.87). Upsizing from 3/4” to 1” reduces loss significantly—not by 33% (the diameter increase), but by roughly 50–60% because of the exponential relationship. Upsizing is expensive but effective on long runs.
Can I use the Manning formula instead of Hazen-Williams?
Yes. Manning is often used for open-channel flow (sewers, storm drains); Hazen-Williams is standard for closed-pipe water supply. Both give similar results for water pipe applications. Unless your code specifies Manning, stick with Hazen-Williams for water supply.