How to use the pipe pressure drop calculator (hazen-williams)
- Enter the flow rate and choose its unit.
- Enter the inside diameter, the length and the Hazen-Williams C value for the pipe.
- In More options change the water density if needed.
- Read the head loss, pressure drop and velocity, and compare with the US customary check.
Formula
Head loss hf = 10.67 x L x Q^1.852 / (C^1.852 x d^4.87) (meters, cubic meters per second)
Pressure drop = density x g x hf
Velocity = Q / (pi d^2 / 4)
US form: psi = 4.52 x Q^1.85 x L / (C^1.85 x d^4.87), Q in gpm, d in inches, L in feet
C is a roughness coefficient: higher means a smoother pipe. The two forms of the equation use slightly different rounding of the constants.
Three pipes
Water at 1 L/s through 100 m of 25 mm pipe with C = 150 loses 17.542 m of head, which is 171.72 kPa or 24.905 psi, and moves at 2.037 m/s. The US form gives 25.061 psi, about 0.62 percent different.
20 US gpm through 200 ft of pipe with an inside diameter of 1.049 inches and C = 140 loses 13.706 m of head, which is 19.459 psi. 30 L/s through 1 km of 200 mm pipe with C = 100 loses 8.087 m, which is 79.16 kPa, at a speed of 0.955 m/s.
Using the C coefficient
The roughness coefficient C comes from experience with the pipe material and its age. Smooth plastic pipe is usually listed near 150, and old, corroded iron can fall to 100 or lower, which makes the friction loss much larger. Use the number from your pipe maker or design standard, not the example. The equation is sensitive to it, because loss varies with C to the power minus 1.852.
To go with this, see the flow rate calculator, the pipe volume calculator, the irrigation zone flow calculator and the tank drain time calculator.
What the equation leaves out
Hazen-Williams applies to water at ordinary temperatures in full, turbulent flow in pipes of about 50 mm (2 inches) and larger. It does not cover other fluids, very slow flow or very small tubes. It also leaves out fittings, valves, bends and elevation change, which are added separately as equivalent lengths or additional head.
Assumptions and limits
- The pipe is full and the water is at an ordinary temperature. Density defaults to 998.2 kg/m3.
- The inside diameter is used, not the nominal pipe size.
- C must be between 40 and 160. The value you enter is your own choice for the pipe.
- Fittings, valves and height changes are not included. The reported drop is friction only.
Frequently asked questions
What is the Hazen-Williams equation used for?
It estimates friction head loss for water flowing in a full pipe. It is widely used for water supply, irrigation and fire sprinkler design because it is simple and needs only a roughness coefficient.
What C value should I use?
Use the value for your pipe material and age. New plastic pipe is listed near 150, and old iron much lower. The value you enter drives the answer strongly, so confirm it with a reference.
How do I convert head loss to pressure?
Multiply the head by the density of water and gravity: 1 m of head is about 9.79 kPa, and 1 ft of head is about 0.433 psi. The page does this for you.
Why do the two formulas differ slightly?
The SI form uses the exponent 1.852 and the constant 10.67, while the US form uses the rounded exponent 1.85 and the constant 4.52. The small difference is from rounding, not an error.