About the Pipe Pressure Drop Calculator
This pipe pressure drop calculator estimates the friction loss when a liquid flows through a straight pipe, plus any fittings you add as a total K value. Choose the Darcy-Weisbach method, which works for any Newtonian liquid and solves the Colebrook-White equation for the friction factor, or the Hazen-Williams formula that water and fire-protection engineers use for cold water. Enter the flow rate, the pipe’s inside diameter and length, and pick a pipe material to set the roughness or C-factor.
Results include the pressure drop in psi and kPa, head loss in feet and metres, flow velocity, Reynolds number, flow regime and the friction factor. It is useful for sizing water mains, irrigation and process lines, checking a pump’s system curve, or seeing how much a smaller pipe would cost you in pressure. Work in gpm, inches and feet, or m³/h, millimetres and metres.
Defaults are for water at about 20 °C (specific gravity 0.998, kinematic viscosity 1.004 cSt). For other liquids, enter their specific gravity and viscosity and use Darcy-Weisbach — Hazen-Williams is only valid for water near room temperature in turbulent flow. Use the actual inside diameter for the pipe schedule, not the nominal size.
How to use the pipe pressure drop calculator
- 1Choose units and the calculation method.
- 2Enter the flow rate, the pipe’s actual inside diameter and its length.
- 3Pick the pipe material (or enter a custom roughness or C-factor).
- 4Add the total K of fittings and valves if you want minor losses included.
- 5For liquids other than water, enter specific gravity and viscosity and use Darcy-Weisbach.
Formula and method
Darcy-Weisbach gives friction head loss as the friction factor f times the length-to-diameter ratio times the velocity head v²/2g. The friction factor depends on the Reynolds number (Re = vD/ν) and the relative roughness ε/D: in laminar flow (Re < 2,300) f = 64/Re; otherwise the calculator solves the Colebrook-White equation iteratively, starting from the Swamee-Jain approximation. This method is valid for any Newtonian liquid if you supply its density and viscosity.
Hazen-Williams is an empirical formula for water at ordinary temperatures in turbulent flow; the C-factor (150 for plastic, about 100 for old cast iron) replaces roughness and viscosity. In both methods minor losses from fittings are added as K × v²/2g, and head loss is converted to pressure drop with Δp = ρ g h. Elevation change is not included — add ρ g Δz separately for vertical runs.
- hf
- Friction head loss
- f
- Darcy friction factor
- L, D
- Pipe length and inside diameter
- v
- Mean flow velocity (Q ÷ area)
- Re
- Reynolds number vD/ν
- ε
- Absolute pipe roughness
- C
- Hazen-Williams roughness coefficient
- K
- Total minor-loss coefficient of fittings
Worked examples
100 gpm through 100 ft of 2" Sch 40 steel (Darcy)
The 2.067-inch bore has 3.356 in² of area, so 100 gpm flows at 9.56 ft/s (2.91 m/s) and Re ≈ 152,000 (turbulent). With ε/D = 0.00086 the Colebrook friction factor is 0.0209, giving 17.2 ft of head loss, or about 7.45 psi per 100 ft.
Same pipe by Hazen-Williams (C = 120)
Hazen-Williams with C = 120 for new steel gives about 21.7 ft of head, or 9.4 psi — roughly a quarter more than Darcy-Weisbach with new-steel roughness, because a C of 120 reflects pipe that has been in service a while and the formula is empirical.
Metric: 20 m³/h, 80 mm PVC, 250 m, total K = 5
20 m³/h in an 80 mm bore is 1.105 m/s, Re ≈ 88,000. The smooth-pipe friction factor is about 0.0186, giving 3.61 m of pipe friction plus 5 × v²/2g = 0.31 m for fittings — about 3.93 m of head, or 38.4 kPa.
Frequently asked questions
How do you calculate pressure drop in a pipe?+
Find the velocity (flow ÷ area) and Reynolds number, get the friction factor from the Moody chart or Colebrook equation, then apply Darcy-Weisbach: head loss = f × (L/D) × v²/2g. Multiply head by density and gravity to get pressure drop. This calculator does all of those steps.
Darcy-Weisbach or Hazen-Williams — which should I use?+
Darcy-Weisbach is theoretically based and works for any liquid, temperature and flow regime. Hazen-Williams is simpler and widely used in water distribution and fire sprinkler design, but it is only valid for water at roughly 5–25 °C in turbulent flow.
What is a good velocity for water in a pipe?+
Many designers keep water velocity around 1.5–2.5 m/s (5–8 ft/s) in pressure lines, and lower in suction lines and residential plumbing to limit noise, erosion and water hammer. Higher velocities raise friction loss roughly with the square of speed.
Does doubling the flow double the pressure drop?+
No. In turbulent flow friction loss rises with roughly the square of velocity (the 1.852 power in Hazen-Williams), so doubling the flow increases pressure drop about 3.5 to 4 times. In laminar flow the loss is directly proportional to flow.
How do I include elbows and valves?+
Add up the loss coefficient K for each fitting from manufacturer data or a reference table and enter the total. Typical values are about 0.3–0.9 for a 90° elbow, 0.2 for an open gate valve and 2–10 for a globe valve, depending on size and type.
Should I use nominal or inside diameter?+
Always use the actual inside diameter. A 2-inch Schedule 40 steel pipe has a 2.067-inch bore, while Schedule 80 has 1.939 inches; that 6% difference raises friction loss by roughly 35–40% at the same flow, because loss varies with about the fifth power of diameter.