Pressure Drop Calculator
Calculate friction loss through pipes using the Hazen-Williams equation. Account for fittings and elevation changes.
📉 The Hazen-Williams Equation
Allen Hazen and Gardner S. Williams published their empirical flow formula in Hydraulic Tables in the early 1900s, and it remains the working tool for water at ordinary temperatures in plumbing and fire protection. It is empirical rather than derived: a roughness coefficient C stands in for everything about the pipe wall, so it is only valid for water, and only in the range of velocities and diameters the original data covered. For compressible fluids, hot oils or anything viscous, Darcy-Weisbach is the correct equation and this one is not.
Formula Reference
The friction expression is the psi-per-foot arrangement published by the Copper Development Association for its own pressure-loss tables, where it is stated with C = 150 for copper. This calculator instead uses C = 140 for copper, PEX and CPVC, which predicts about 14% more loss for the same flow — the conservative direction, but a real difference: at 10 GPM in 3/4-inch Type L the CDA table gives 0.098 psi per foot where this tool gives 0.111.
The fitting allowances are L/D ratios this calculator assumes, and they are not the copper industry's published equivalent lengths. They agree closely for 90° elbows and drift elsewhere. Where a run has many valves, work the fittings from the manufacturer's or the CDA's own figures instead.
🔩 How the Fitting Allowances Compare
Equivalent length is the standard dodge for fitting losses: express each fitting as the length of straight pipe that would waste the same pressure, add it to the run, and use one friction calculation for the lot. The published allowances for copper are in the Copper Development Association's tables, and this is how the tool's L/D assumptions land against them:
| Fitting (Type L bore) | This calculator | CDA published |
|---|---|---|
| 3/4" 90° elbow | 1.96 ft | 2 ft |
| 2" 90° elbow | 4.96 ft | 5.5 ft |
| 3/4" 45° elbow | 1.05 ft | 0.5 ft |
| 2" tee, through branch | 9.93 ft | 9 ft |
| 2" gate valve | 1.32 ft | 0.5 ft |
The 90° elbows and the tee are close. The 3/4 inch 45° elbow is not — the tool allows 1.05 ft against CDA's half a foot, twice the real figure. Gate valves are the outlier, but in the forgiving direction. A fully open gate valve is one of the lowest-loss fittings there is — Crane's Technical Paper 410 puts it at an L/D of 8, which is exactly what this tool uses, and CDA Table 14.7 allows just half a foot at 2 inch. The tool's 1.32 ft is about two and a half times that, so it predicts slightly more loss than you will meet. Do not confuse it with a butterfly valve, which CDA puts at 7.5 ft in the same row at the same size — a genuinely restrictive fitting. The CDA figures also assume streamlined soldered or recessed threaded fittings, and its tables note that ordinary threaded fittings double the allowance.
⛰️ Elevation and the 0.433 Figure
A column of water exerts its own weight as pressure, and nothing about the pipe changes that. Cold water weighs about 62.4 pounds per cubic foot, and a cubic foot has 144 square inches of base, so every foot of vertical rise costs 62.4 ÷ 144 = 0.433 psi. Twenty feet of lift is 8.7 psi gone before friction is counted, which is why the top-floor shower is the one that complains. Running downhill gives it back.
🧮 Worked Example
10 GPM through 100 ft of 3/4-inch Type L copper with six 90° elbows, rising 10 ft, from a 60 psi supply:
- Friction over the straight run: 11.14 psi
- Six elbows = 11.8 ft of equivalent length: 1.31 psi
- 10 ft of rise: 4.33 psi
- Total drop 16.78 psi, leaving 43.2 psi at the outlet
Note where the loss actually is. The friction term dominates because 10 GPM in a 0.785-inch bore runs at 6.63 ft/s — fast. Step up to 1-inch tube and the same flow slows to 3.89 ft/s while the friction loss falls from 11.14 psi to 3.04 psi, about three quarters of it gone. Pressure problems are usually pipe-size problems.
❓ Questions People Actually Ask
How much pressure drop per 100 ft of pipe?
It depends almost entirely on flow and bore, so there is no single figure. Two points from the Copper Development Association's published loss tables for Type L copper, computed at C = 150: 10 GPM through 3/4-inch tube loses 0.098 psi per foot, or about 9.8 psi per 100 feet, while 20 GPM through 1-inch tube loses 0.096 psi per foot. Halve the flow and the loss falls by roughly three quarters; step 10 GPM up from 3/4-inch to 1-inch tube and it falls by about the same. If you are seeing large numbers per 100 feet, the pipe is undersized rather than the length being excessive.
What is the rule of thumb for pressure drop in pipe?
The honest version is a budget rather than a rule: take the static pressure at the meter, subtract 0.433 psi for every foot the water has to climb, subtract friction over the developed length, subtract the fitting and valve allowances, subtract the loss through the meter, softener and any backflow preventer, and check what is left against the flowing pressure the furthest fixture is specified to need. Fixture flow rates are themselves measured at set pressures — federal standards test showerheads at 80 psi and faucets at 60 psi — so a fixture starved of pressure will not deliver its rated flow. Any thumb rule that skips the elevation or the meter will overstate what arrives.
What causes a pressure drop in a pipe?
Three separable things, and this calculator reports them separately for that reason. Friction against the pipe wall, which rises with roughly the 1.85 power of flow and falls with roughly the 4.87 power of bore — so it is dominated by pipe size. Fittings and valves, each of which behaves like an extra length of straight pipe. And elevation, at 0.433 psi per foot of rise regardless of flow. Ageing adds a fourth: scale and corrosion lower the roughness coefficient, and at the values this calculator uses, aged galvanised pipe at C = 80 loses about 2.8 times what copper at C = 140 does at the same flow and bore.
An estimate for water at ordinary temperatures only. It does not include the loss through your meter, pressure-reducing valve, softener, filter or backflow preventer — often several psi each — and it does not check that the fixture at the end gets the flowing pressure it needs.
Hazen-Williams is empirical and its coefficients are approximations of real pipe condition. This is not an engineered design and does not replace a licensed plumber.
The friction formula in the form used here, with its 4.52 constant, its units and its C = 150 value for copper, is published in the Copper Development Association's Copper Tube Handbook (CDA publication A4015-26/24) as the basis of its pressure-loss tables; the per-foot loss figures and the fitting equivalent lengths quoted above are from the same tables, as is the note that threaded fittings double the allowance. The method itself is Allen Hazen and Gardner S. Williams', from their Hydraulic Tables. The 0.433 psi per foot follows from the density of cold water, which the USGS Water Science School gives as 62.408 lb/ft³ at 50°F. The 80 psi and 60 psi test pressures for showerheads and faucets are set in federal law at 10 CFR 430.32(o) and (p). If the drop looks unacceptable, the fix is usually a larger bore — our pipe sizing calculator covers that. The questions above came from Google's People Also Ask panel for "water pressure drop in pipe" on 31 July 2026.