Moody Diagram Calculator

What the Moody diagram calculator does

The friction factor sets how much pressure a pipe loses to wall friction. The Moody diagram plots it against Reynolds number for different relative roughness values, but reading the chart by eye is slow and imprecise. The Moody diagram calculator solves the Colebrook equation directly, the formula the chart is drawn from, so you get the friction factor to four or five figures.

Friction factor formula

For laminar flow, Re below 2,300:

For turbulent flow the calculator iterates the Colebrook equation:

It also shows the explicit Swamee-Jain equation as a check, f = 0.25 ÷ [log₁₀(ε/D ÷ 3.7 + 5.74 ÷ Re^0.9)]², which agrees within about 1% over its range. The Fanning factor is f ÷ 4.

Moody diagram calculator example

Water in a 50 mm commercial steel pipe, roughness 0.045 mm, at Re = 100,000:

The Swamee-Jain check gives 0.02199, and the Fanning factor is 0.00546. On a Moody chart this point sits just above the 0.02 line.

How to use the friction factor calculator

Enter the Reynolds number of the flow and the inside diameter of the pipe in millimeters. Choose the pipe material to set the roughness, or type your own roughness in millimeters. Press Calculate to read the Darcy friction factor, ready to use in the Darcy-Weisbach equation.

How to read a Moody diagram result

Typical turbulent values run from about 0.008 for smooth pipe at very high Reynolds numbers to 0.05 or more for rough pipe. At high Re the friction factor flattens out and depends only on roughness: this is the fully rough zone. Between Re 2,300 and 4,000 the flow can switch between laminar and turbulent, so treat any value there as uncertain.

When to use a Darcy friction factor calculator

• Pressure drop and head loss in water or oil lines
• Pump sizing for a piping system
• Comparing pipe materials
• Checking hand calculations against the Moody chart
• Coursework in fluid mechanics

Common Moody diagram calculator mistakes

• Mixing Darcy and Fanning. Darcy is four times Fanning.
• Using outside diameter. Use the inside diameter.
• Mismatched units for ε and D. Both must be in mm here.
• Trusting values at Re 2,300 to 4,000. Flow there is unstable.

Tips for using a friction factor calculator

• Use a higher roughness for old pipe.
• Check Re before choosing laminar or turbulent methods.
• Keep the Darcy value for Darcy-Weisbach.

Related fluid tools

The friction factor sits between two other steps. The Reynolds number calculator gives the Re to enter, the pressure loss calculator turns f into a pressure drop, and the fluid velocity calculator finds the flow speed.

Darcy Weisbach friction factor in a full pressure drop

A pipe calculation usually runs in three steps. First find the velocity from the flow rate and diameter with the fluid velocity calculator, then the Reynolds number with the Reynolds number calculator. With Re and ε/D, this calculator gives f. Finally, Δp = f × (L ÷ D) × ρv² ÷ 2, which the pressure loss calculator applies when you enter the friction factor.

Colebrook equation vs reading the Moody chart

The Moody chart is a plot of the Colebrook equation, so solving the equation gives the same answer without reading errors on log scales. Explicit forms such as Swamee-Jain or Haaland avoid iteration and stay within a few percent, which is close enough for most designs. Roughness itself is the biggest unknown: pipes scale and corrode with age, and an old steel line can have several times its new roughness.

Moody chart calculator values: Darcy f by Re and ε/D

Pipe roughness table

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