This calculator does not model the pipe profile between the two ends.
It uses only the upstream and downstream elevations you enter. If the ground rises higher than either end somewhere in between, the pressure at that high point is lower than any pressure reported here. Run the calculator again for the length from the upstream end to the high point to check it.
Where the hydraulic grade line falls below the pipe, the water is under negative pressure. Air comes out of solution, a thin-walled pipe can collapse, and dirty groundwater can be drawn in through the joints. Keep the line under positive pressure everywhere, and consider an air valve at each high point.
The upstream pressure is a boundary condition you supply.
Read it from a gauge, from a tank water level (the height of water above the pipe), or from a pump curve. A pump delivers less pressure as flow rises, so use the point on the curve that matches the flow entered above.
Add up the minor (local) loss coefficients yourself.
Total the K values for every valve, bend, tee, meter, and entrance on the line, and enter that total. Follow the link on that input for typical values. On a long transmission main these losses are small next to friction, but in short station piping they can be most of the loss.
Hazen-Williams constants now match EPANET (August 2026)
In August 2026 the Hazen-Williams coefficient and exponent were changed to match EPANET. Head loss results differ from earlier versions of this page by up to 0.1 percent, which is far smaller than the uncertainty in the C value itself.