Pipe Pressure Drop Calculator

It depends on the pipe size, the flow rate, and the fluid. For water in clean steel or plastic pipe at normal speeds, most systems lose about 1 to 4 psi per 100 feet of straight pipe. That is roughly 2 to 9 feet of head.

Loss climbs fast if the pipe is too small. Going from 5 ft/s to 10 ft/s in the same pipe makes the loss about four times bigger.

Common design practice for water lines is 1 to 4 psi per 100 feet, about 2 to 9 feet of head per 100 feet. Long transfer lines often use less, near 1 psi per 100 feet, to save pump power.

The other check is speed. Keep water near 3 to 10 ft/s (1 to 3 m/s). Pump suction lines should stay slower, around 2 to 5 ft/s, so the pump does not cavitate.

They measure the same energy loss in two ways.

• Head loss (h_f) is given as a height of the fluid, like feet or meters.
• Pressure drop (ΔP) is given as pressure, like psi, kPa, or bar.

Convert with ΔP = ρ × g × hf. Head loss does not change with fluid density, but pressure drop does. The same 10 feet of head is more psi in heavy brine than in light gasoline.

For water, 1 psi = 2.31 feet of head, and 1 foot of head = 0.433 psi. So 20 psi is about 46 feet.

In metric, 1 bar ≈ 10.2 meters of water and 1 meter of water ≈ 9.81 kPa.

For other fluids, divide 2.31 by the specific gravity. Oil with a specific gravity of 0.85 gives about 2.72 feet of head per psi.

In order of impact:

• Use a bigger pipe. Loss falls roughly by the fifth power of diameter. Going up one pipe size can cut the drop by more than half.
• Lower the flow rate if the job allows it.
• Cut out fittings. Fewer elbows, tees, and sharp turns mean less loss.
• Swap globe valves for gate or ball valves. A globe valve can cost 30 times more loss than a gate valve.
• Use smoother pipe like PVC or drawn copper instead of old cast iron.
• Shorten the run with a more direct route.

A bigger pipe does not raise the pressure at the source, but it does deliver more pressure at the far end because less is lost to friction along the way.

Friction loss drops very fast as the pipe widens. Doubling the diameter can cut the loss by about 30 times at the same flow rate. That is why a long, undersized line feels weak even when the supply pressure is fine.

A standard 90° elbow acts like about 30 pipe diameters of extra straight pipe. A long-radius elbow is gentler, near 20 diameters. A sharp mitered knee can reach 60.

In a 2-inch water line at 5 ft/s, one elbow costs roughly 0.05 psi. That sounds tiny, but 30 elbows in a plant loop adds up to a real loss.

Major losses come from friction along the straight pipe wall. Minor losses come from fittings, valves, entrances, exits, and size changes.

The word minor is misleading. In a short, fitting-heavy run like a pump skid, minor losses can be larger than the straight-pipe loss. In a long pipeline, straight-pipe friction dominates.

No. Friction loss and elevation are separate. Friction always takes pressure away. Elevation gives it back if the pipe goes downhill.

Add elevation with ΔP = ρ × g × Δz. For water, every 10 feet of rise costs about 4.33 psi, and every 10 feet of fall gives back the same amount. A pump must beat friction plus the lift.

A Moody chart is a graph that gives the Darcy friction factor. You read across with the Reynolds number and pick the curve that matches your relative roughness (ε/D). Where they cross is your friction factor.

It is a hand-drawn version of the Colebrook-White equation. The chart also shows the laminar line (f = 64/Re) on the left and the flat fully-rough zone on the right, where friction stops changing with Reynolds number.

Yes, but only a little. Hot water is thinner. At 20 °C water has a viscosity near 1.0 mPa·s; at 60 °C it is about 0.47 mPa·s.

In turbulent flow this usually cuts friction loss by roughly 5 to 10 percent. For thick fluids like oil the effect is huge, because oil viscosity can drop several times over with a modest temperature rise.

Nominal size (NPS or DN) is just a label, not a measurement. The real bore depends on the wall thickness, which is set by the schedule.

• 2" Schedule 40 steel: 2.067" inside (52.5 mm)
• 2" Schedule 80 steel: 1.939" inside (49.3 mm)
• 4" Schedule 40 steel: 4.026" inside (102.3 mm)

Always use the real inside diameter in pressure drop math. Using the nominal number can throw the answer off by 20 percent or more.

Start with velocity. Pick a target speed, then size the pipe with D = √(4Q / πV).

Common targets: water 3 to 10 ft/s (1 to 3 m/s), pump suction 2 to 5 ft/s, compressed air 30 to 100 ft/s (10 to 30 m/s).

Quick water guide at about 7 ft/s: 1" carries roughly 20 gpm, 2" about 75 gpm, 3" about 170 gpm, 4" about 300 gpm. Then check that the pressure drop over the full run is still acceptable.

For liquids, no. Water is nearly incompressible, so the same flow through the same pipe gives the same drop whether the line runs at 30 psi or 300 psi.

For gases, yes. Higher inlet pressure squeezes the gas, so density goes up and velocity goes down for the same mass flow. That means less pressure drop. This is why compressed air lines at 100 psi lose far less than the same flow at 10 psi.

Skip it for anything except water in turbulent flow at normal temperatures. It fails for:

• Oil, glycol, slurry, or any thick fluid
• Air, steam, or other gases
• Laminar flow (low Reynolds number)
• Very hot or very cold water
• Very high velocities

Hazen-Williams has no viscosity term at all, which is why it breaks outside water. Darcy-Weisbach with Colebrook-White works for every fluid and every flow regime.

Relative roughness is the wall bump height divided by the pipe diameter (ε/D). It has no units.

The same 0.046 mm bumps matter much more in a 15 mm tube (ε/D = 0.003) than in a 400 mm main (ε/D = 0.0001). That is why small pipes feel rougher and lose more pressure per foot.

In laminar flow roughness is ignored entirely. In fully turbulent flow it controls the friction factor almost by itself.