01
Formula
Head feet = 4.52 × L × Q^1.852 ÷ (C^1.852 × d^4.87).
Psi = head ÷ 2.31. Pipe diameter in inches = square root of (GPM ÷ (2.448 × velocity)). The pipe size calculator is the velocity mode.
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Pressure drop calculator and pipe size calculator
This pressure drop calculator uses the Hazen-Williams formula for water in a round pipe. Head loss is in feet, and psi is that head divided by 2.31. Switch to the pipe size calculator to pick an inside diameter from flow and a target velocity. It is a planning estimate, not a engineered network model.
Find
Head loss
12.30ft
psi5.33
C factor140
Diameter1.00 in
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
Hazen-Williams head loss depends on flow, length, diameter, and a C factor.
Head loss in feet equals 4.52 times length times flow to the 1.852, divided by C to the 1.852 times diameter to the 4.87. Length is feet, flow is gallons per minute, and diameter is inside inches. A C factor of 140 is a common planning value for a smooth pipe. It is not a measured roughness. The pipe size calculator sets diameter from area: gallons per minute divided by 2.448 times velocity in feet per second, then the square root. Twenty gallons per minute at 5 feet per second needs about a 1.28 inch inside diameter. Use the pressure mode after you pick a real pipe size, because a stock pipe is not exactly 1.28 inches. Fittings are extra. Add an equivalent length to the pipe length if you want them in the loss.
02
Example
100 feet, 20 GPM, C of 140, 1 inch inside diameter.
Read the head loss and the psi on the card. They follow the formula exactly for those inputs. Switch to pipe size at 20 GPM and 5 feet per second and the inside diameter is about 1.28 inches.
03
Tips
C is not a constant of nature.
New plastic is often planned near 140 to 150. Older steel is lower. A wrong C moves the loss a lot because it is raised to 1.852. Diameter matters even more. Measure inside diameter, not the nominal trade size, when they differ.
04
Equipment
The pump has to overcome this head.
Dengaro arranges financing for a pump. Add this loss to static lift on the GPM calculator's head mode. The loss is not a loan amount.
Important questions
Know what to expect.
Is Hazen-Williams for any fluid?+
This pressure drop calculator is the water form. Slurries, steam, and air need a different method. Do not type oil into a water C factor and trust it.
Why is psi head divided by 2.31?+
One psi supports about 2.31 feet of water. The page reports both so you can read the unit your pump curve uses.
Does the pipe size calculator include fittings?+
No. It only hits a velocity. Friction still depends on the length and the fittings. Come back to the pressure mode with the stock diameter you will actually buy.
What velocity is reasonable?+
Five feet per second is a common planning target for many water lines, not a code. A suction line often wants to be slower. Type the velocity you were given.
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