Queendom Tools
Industrial hydraulics · pipe flow engineering

Darcy-Weisbach Pressure Drop Calculator

Darcy-Weisbach head loss and pressure drop with the friction factor computed for you: 64/Re in laminar flow and Swamee-Jain from Re and pipe roughness in turbulent flow.

Pipe pressure drop

Fluid motion input

Switching between velocity and flow converts the typed value through the diameter, V = 4Q/(πD²), and never resets it.

Reynolds number Re
—
—
Darcy friction factor f
—
—
Head loss hf
— m
— ft
Pressure drop ΔP
— kPa
— bar · — psi

—

Formulas: Re = ρVD/μ · laminar (Re < 2300): f = 64/Re · transitional (2300 to below 4000): reported as "transitional, uncertain", f from the turbulent expression · turbulent (Re ≥ 4000): Swamee-Jain f = 0.25/[log10(e/(3.7D) + 5.74/Re^0.9)]² · hf = f·(L/D)·V²/(2g) · ΔP = ρ·g·hf, with g = 9.80665 m/s². Re is rounded to 6 decimals before it is compared with 2300 and 4000.

Sources: P. K. Swamee and A. K. Jain, Explicit equations for pipe-flow problems, Journal of the Hydraulics Division ASCE, vol. 102, no. 5, May 1976, pp. 657-664 (doi 10.1061/JYCEAJ.0004542). 64/Re is the Hagen-Poiseuille laminar friction factor. The roughness 0.045 mm (0.00015 ft) for commercial steel is the Moody chart value (L. F. Moody, 1944, Trans. ASME).

For preliminary estimation only, not a substitute for review by a qualified/licensed engineer.

A friction factor without the Moody chart

Sizing a pipe run or checking pump head comes down to one stubborn number: the Darcy friction factor. Reading it off a Moody chart is slow and hard to defend in a calculation file, and iterating the Colebrook equation by hand is worse. Here f is resolved for you from the Reynolds number and the relative roughness e/D.

In laminar flow the page uses the Hagen-Poiseuille result, f = 64/Re. In turbulent flow it uses the Swamee-Jain explicit equation, which needs only Re and the pipe roughness. Between Re 2300 and 4000 the flow is labelled "transitional, uncertain" and the turbulent expression is still evaluated, so you get a number while being warned that it is not reliable.

It is written for fluid systems engineers who need head loss in metres and feet and pressure drop in kPa, bar and psi from the same inputs. The default roughness of 0.045 mm is the value usually quoted for commercial steel pipe; replace it for your own pipe material.

How to use it

  1. Enter the pipe inside diameter in mm and the length in m.
  2. Set the absolute roughness in mm. Leave 0.045 for commercial steel, change it for other pipe material.
  3. Choose velocity or flow, then type V in m/s or Q in m³/h. The toggle converts the value using the diameter instead of resetting it.
  4. Enter the fluid density in kg/m³ and dynamic viscosity in cP.
  5. Read Re with its regime, f, head loss in m and ft, and pressure drop in kPa, bar and psi.

FAQ

What friction factor does this Darcy-Weisbach calculator use?

64/Re in laminar flow (below Re 2300) and the Swamee-Jain explicit equation, f = 0.25/[log10(e/(3.7D) + 5.74/Re^0.9)]², for Re 4000 and above, so no Moody chart iteration is needed.

Why does a Reynolds number of exactly 2300 show as transitional?

Re is rounded to 6 decimals before it is compared with the 2300 and 4000 limits, so an exact 2300 is not misread as 2299.9999999999995; between 2300 and 4000 the tool prints "transitional, uncertain" and still evaluates the turbulent Swamee-Jain expression for f.

What absolute roughness should I use for commercial steel pipe?

The default 0.045 mm (0.00015 ft) is the Moody chart value for commercial steel; it is an input, so replace it with your pipe's roughness and keep it in the same unit as the diameter.

How do I convert between velocity and flow rate in the pipe?

V = 4Q/(πD²), so 2 m/s in a 100 mm pipe is 56.55 m³/h; when you switch the toggle the typed value is converted through the current diameter, never reset to a default.