Use the free Pipe Pressure Drop Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Water loses pressure as it travels through a pipe. Friction drags on the flow, and the longer the pipe, the narrower the bore, or the faster the flow, the more pressure is lost along the way. That loss is called pressure drop, and it explains why a tap at the far end of a house can run weaker than one near the main. The Pipe Pressure Drop Calculator estimates that loss using the Hazen-Williams formula, a standard simplified method used by plumbers and engineers for water pipe sizing.
Using the tool means entering four things: the flow in gallons per minute, the inside diameter in inches, the length of the run in feet, and the pipe material. Material matters because a smooth plastic pipe wastes less pressure than a rough old steel pipe, and the tool captures that with the Hazen-Williams C factor: 130 for PVC, 120 for copper, and 100 for old steel. Enter your numbers, and the tool shows the total PSI lost over the run, the loss per 100 feet, and the loss in bar.
The Hazen-Williams formula looks intimidating, but its message is simple. Pressure loss grows roughly with flow to the power of 1.85, so doubling the flow more than triples the loss. It falls sharply as diameter grows, to the power of about 4.87, so a small increase in pipe size cuts losses dramatically. And it grows in direct proportion to length, so a 200-foot run loses twice as much as a 100-foot run of the same pipe. That is why a long, narrow supply to a distant bathroom is the classic recipe for a weak shower.
A worked example shows the scale. Ten gallons per minute through 100 feet of one-inch PVC loses about 9.1 PSI. The same flow through a two-inch pipe loses less than 1 PSI, because the wider bore is so much more efficient. Push the flow to 20 gallons per minute in the one-inch pipe, and the loss jumps to roughly 33 PSI, which can turn a healthy supply into a disappointing one. These are estimates, but they show why pipe size is a design decision, not a detail to leave to chance.
This tool is an educational estimator only, not professional plumbing advice. The Hazen-Williams formula is a simplification: it assumes clean water at normal temperature, full-pipe flow, and a constant roughness factor, while real systems have fittings, valves, bends, and ageing that add extra loss. Treat the result as a planning estimate. Check your local plumbing codes, and consult a licensed plumber before any real installation or re-piping. Never design a fire supply, a medical supply, or any safety-critical system from an online estimator.
To make the estimate more honest, add a margin for fittings. A common rule is to add 10 to 30 percent to the straight-pipe length to allow for elbows, tees, and valves, depending on how many the run has. Measure the inside diameter, not the nominal size, and be realistic about flow: a household rarely draws its maximum flow through one pipe for long. If the estimated loss leaves you below the pressure your fixtures need, the fixes are a wider pipe, a shorter route, or a booster pump, in that order of preference.
The Pipe Pressure Drop Calculator is free, it works in your browser, and your numbers never leave your device. Use it before you run a long supply, size a pump, or diagnose a weak tap at the end of a line. Enter the flow, the bore, the length, and the material, and read the loss in PSI and bar. Pressure drop is invisible until you measure it, and this tool makes it visible before you bury the pipe or close the wall.
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