How to Read the Moody Chart: A Step-by-Step Guide

What the Moody Chart Shows

Lewis Ferry Moody published the chart in 1944 to give engineers a practical way to find the Darcy friction factor without solving implicit equations by hand. You can read the original paper in the ASME Digital Collection.

The chart answers one question: given a Reynolds number and a pipe roughness, what is the Darcy friction factor? That friction factor is what you plug into the Darcy-Weisbach equation to get pressure drop or head loss.

It is a graphical solution to the Colebrook-White equation. Before computers, this chart was the primary tool engineers used for pipe flow calculations. Today you can solve it numerically in milliseconds using the Moody Chart Calculator, but understanding the chart helps you catch errors, spot unusual results, and build physical intuition about pipe flow.

The Three Axes

The chart has three sets of values to orient yourself with before you try to find anything.

Left Vertical Axis: Darcy Friction Factor (f)

This is what you are solving for. It is dimensionless and typically ranges from about 0.008 to 0.08 in engineering applications. The axis uses a logarithmic scale, so the spacing between values is not uniform. Values at the top of the chart are higher friction factors, meaning more resistance to flow.

Important: the Moody Chart uses the Darcy friction factor, which is four times the Fanning friction factor used in some chemical engineering texts. See our Darcy vs Fanning guide to avoid mixing them up.

Bottom Horizontal Axis: Reynolds Number (Re)

Reynolds number runs along the bottom on a logarithmic scale, typically from about 600 to 100 million. Low values are on the left (laminar, slow or viscous flow), high values are on the right (turbulent, fast flow). You calculate Reynolds number with Re = ρVD/μ. If you need help with that calculation, the Reynolds number guide walks through it in detail.

Right Vertical Axis: Relative Roughness (ε/D)

The right axis labels the roughness curves. Each curve in the turbulent region corresponds to a specific relative roughness value, the ratio of absolute pipe roughness (ε) to pipe inner diameter (D). Higher values on this axis mean rougher pipes and higher friction factors. The curves are labeled on the right side of the chart, starting from smooth (bottom, near 0) to rough (top, around 0.05). You find your pipe material's roughness in the roughness table on the calculator page.

The Four Flow Zones

The chart divides into distinct zones from left to right. Each zone behaves differently, and the right calculation method depends on which zone you are in.

Zone 1: Laminar Flow (Re below 2,300)

This is the straight diagonal line on the left side of the chart. It follows the simple relationship f = 64/Re exactly. Friction factor is high at low Reynolds numbers and drops steeply as Re increases. Pipe roughness has no effect here because the viscous sublayer completely coats the pipe wall and prevents roughness elements from disturbing the flow.

In practice, laminar flow occurs in very slow flows, very small pipes, or very viscous fluids like heavy oil.

Zone 2: Transition Zone (Re between 2,300 and 4,000)

This is the blank, undefined region between the laminar line and the turbulent curves. Moody deliberately left it empty. Flow in this zone is unstable and friction factor can jump between laminar and turbulent values unpredictably. Engineers design systems to avoid this zone entirely, either staying below Re = 2,300 or pushing well above Re = 4,000.

Zone 3: Turbulent Smooth Pipe (left portion of turbulent region)

Just to the right of the transition zone, the roughness curves converge near the bottom of the turbulent region. Here, friction factor depends primarily on Reynolds number rather than roughness. The boundary layer is still thick enough to absorb the roughness elements. This is where drawn tubing and very smooth pipes operate at moderate Reynolds numbers.

Zone 4: Fully Rough Turbulent Flow (right portion of turbulent region)

As Re increases, each roughness curve eventually flattens out horizontally. When a curve goes flat, friction factor no longer changes with Reynolds number. You are in the fully rough zone. Here, the roughness elements fully protrude through the boundary layer and control the friction factor on their own. This is common in rough pipes like cast iron or concrete at high flow velocities.

In this zone, you can read friction factor directly from the right axis without knowing Reynolds number at all.

How to Read the Moody Chart: Step by Step

Follow these five steps every time.

Worked Example

Here is a complete example to put the steps into practice.

Common Mistakes When Reading the Moody Chart

Misreading the Log Scale

The most common error. On a logarithmic axis, the gridlines are not evenly spaced. Between 10,000 and 100,000, the value 50,000 is not halfway. It sits at 70% of the visual distance because log₁₀(50,000) = 4.7, which is 70% of the way from 4 to 5. Take your time reading the axis and count the gridlines.

Using the Wrong Friction Factor Definition

The Moody Chart gives you the Darcy friction factor. If you use this value in a formula that expects the Fanning friction factor, your pressure drop will be four times too high. See the Darcy vs Fanning guide to identify which definition your formula uses.

Ignoring the Transition Zone

If your Reynolds number falls between 2,300 and 4,000, the chart gives you no answer. Do not extrapolate between the laminar line and the turbulent curves. Either acknowledge the uncertainty or adjust your design to move out of this zone.

Using Absolute Roughness Instead of Relative Roughness

The chart uses ε/D, not ε alone. A 0.046 mm roughness on a 10 mm pipe gives ε/D = 0.0046, which is in the rough pipe region. The same roughness on a 1000 mm pipe gives ε/D = 0.000046, which is near smooth. The diameter matters as much as the surface condition.

Not Accounting for Pipe Age

Roughness values in reference tables are for new pipe. Older pipes corrode, scale, and foul. Cast iron mains that have been in service for decades can have roughness values three to five times the new pipe value. If you are analyzing an existing system with measured pressure drops that do not match your calculation, increased roughness from aging is often the cause.

Applying the Chart to Non-Circular Ducts

The Moody Chart applies to circular pipes. For rectangular ducts or non-circular cross-sections, use the hydraulic diameter (D_h = 4 × cross-sectional area / wetted perimeter) as an approximation. The result is less accurate as the shape departs further from circular, but it is the standard engineering approach for most duct work.

When to Use the Calculator Instead of the Chart

Reading the chart by hand is useful for building intuition and checking whether a result is in the right range. But for actual calculations, use the Moody Chart Calculator. Here is why.

Reading a log-log chart by eye introduces errors of 2 to 5% in the best case. For most preliminary engineering work that is fine. But when you are sizing a pump, specifying a pipe diameter, or checking pressure constraints in a safety-critical system, you want the Colebrook-White solution, not a graphical approximation.

The calculator is also faster for sensitivity analysis. If you want to see how friction factor changes as pipe diameter increases from 50 mm to 200 mm, running that through the chart four times is tedious. The calculator gives you all four results in seconds.

Use the chart to sanity-check your calculator results. If the calculator gives you f = 0.35 for a smooth turbulent pipe, you should immediately recognize that is far above the typical range (0.01 to 0.05) and investigate your inputs.

Frequently Asked Questions