Darcy–Weisbach pressure drop & head loss for water in a full circular pipe, friction factor from Colebrook–White (64/Re when laminar).
Indicative sizing only. Verify final design to project spec and CIBSE Guide B.
For design guidance only. Always verify with a qualified engineer.
About this pipe pressure drop calculator
This free pipe pressure drop (water) calculator estimates the pressure drop and head loss for incompressible water flowing through a full circular pipe. Enter the flow rate, internal bore, run length and pipe material, and it returns the mean velocity, Reynolds number, Darcy friction factor, pressure gradient (Pa/m), total pressure drop (kPa) and head loss (m). It is a quick sizing aid for heating, chilled-water and domestic services pipework at tender stage, with advisory bands flagging high velocity or an excessive pressure gradient.
How the pressure drop is calculated
The tool applies the Darcy–Weisbach equation, ΔP = f · (L/d) · (ρv²/2), with head loss h = ΔP / (ρg). The mean velocity comes from v = Q/A, and the Reynolds number Re = ρvd/μ sets the flow regime. For laminar flow (Re < 2300) the Darcy friction factor is f = 64/Re; for turbulent flow it is solved from the implicit Colebrook–White equation, 1/√f = −2·log₁₀(ε/3.7d + 2.51/Re√f), by fixed-point iteration seeded with the explicit Haaland estimate. Roughness presets follow standard published values; velocity (≤1.5 m/s typical, up to 3.0 m/s for steel mains) and the ≤200 Pa/m sizing criterion follow CIBSE Guide B guidance.
Frequently asked questions
Should I use the internal bore or the nominal pipe size?
Always use the actual internal diameter (bore). Nominal sizes and outside diameters overstate the cross-section, which understates velocity and pressure drop. Take the bore from the manufacturer's data for the wall thickness and material you are specifying.
What pipe roughness value should I choose?
Pick the preset that matches your material: copper or plastic 0.0015 mm, commercial steel 0.045 mm, galvanised steel 0.15 mm, cast iron 0.26 mm, or concrete 0.3 mm. For anything unusual, choose Custom and enter the absolute roughness ε in millimetres from a published table.
Does this include fittings and valves?
No. The result is the friction loss along the straight pipe run only. Add the equivalent length or a sum of K-factors for bends, valves and fittings separately, then re-run the calculator with the combined length for a full circuit pressure drop.