The Moody Diagram

An Important Chart in Fluid Mechanics and LEV Design

LEVCentral Expert Commentary

If Fletcher’s Nomogram was the traditional graphical calculator for duct pressure losses, then the Moody Diagram is the chart that makes those calculations possible.

The Moody Diagram is one of the best-known engineering charts in fluid mechanics. Developed by Lewis Ferry Moody in 1944, it provides a graphical method of determining the Darcy friction factor for fluid flow in pipes and ducts. The friction factor is then used within the Darcy-Weisbach equation to calculate pressure losses due to friction.

Although originally developed for general fluid mechanics, the Moody Diagram has become an essential tool in ventilation engineering. It allows LEV designers to estimate the friction losses that occur as air travels through ductwork, enabling accurate fan selection and pressure loss calculations.

Today, engineers rarely use printed Moody charts in day-to-day work because the calculations are performed automatically within software, spreadsheets and mobile apps. Nevertheless, understanding how the Moody Diagram works remains one of the best ways of appreciating the engineering principles that underpin modern LEV design.

Moody Diagram

Download Moody Diagram

What is the Moody Diagram?

The Moody Diagram is a graphical representation of the relationship between:

Reynolds Number (Re)
Relative Pipe Roughness (ε/D)
Darcy Friction Factor (f)

Once the friction factor has been obtained from the chart, it can be substituted into the Darcy-Weisbach equation to calculate friction pressure losses within pipes or ventilation ductwork.

Key Learning Points

The Moody Diagram helps engineers determine:

Darcy friction factor.
Pressure losses in ductwork.
Pressure losses in pipework.
The influence of duct roughness.
The effect of Reynolds Number.
Laminar flow behaviour.
Transitional flow.
Turbulent flow.
Fully rough turbulent flow.
Relative roughness effects.
The relationship between friction factor and pressure loss.
Fan pressure requirements.

Understanding the Three Flow Regimes

One of the strengths of the Moody Diagram is that it illustrates how friction behaves differently depending upon the nature of the airflow.

Flow Regime	Reynolds Number	Characteristics
Laminar Flow	Below approximately 2,300	Smooth flow with little mixing. Friction depends almost entirely on Reynolds Number.
Transition Zone	Approximately 2,300–4,000	Unstable flow where behaviour becomes unpredictable. Pressure loss calculations are less reliable.
Turbulent Flow	Above approximately 4,000	The regime encountered in almost all industrial LEV duct systems. Friction depends upon both Reynolds Number and duct roughness.

Industrial LEV systems are almost always designed to operate within the turbulent flow region, where transport velocities are sufficient to keep contaminants suspended within the ductwork.

Why is it Important in LEV?

Every duct system creates resistance to airflow.

As air passes through straight ductwork, friction develops between the moving air and the duct wall. The greater the friction, the greater the pressure loss and the harder the fan must work.

The Moody Diagram enables engineers to estimate this resistance by taking account of:

duct diameter;
air velocity;
airflow regime;
internal surface roughness; and
fluid properties through the Reynolds Number.

These friction losses form a major component of the system resistance used during LEV design and fan selection.

Source Information

Developer: Lewis Ferry Moody

First Published: 1944

Resource Type: Engineering Design Chart

Primary Topics: Darcy Friction Factor, Darcy-Weisbach Equation, Pressure Loss, Duct Design, Fluid Mechanics, LEV Design, Fan Selection.

LEVCentral Perspective

The Moody Diagram is one of those engineering tools that every LEV designer should understand—even if they never actually use the printed chart.

Modern software performs these calculations instantly, but unless engineers understand what the program is calculating, it becomes difficult to recognise when results appear unreasonable.

The Moody Diagram also demonstrates an important engineering principle:

Pressure loss is influenced by much more than airflow alone.

Internal duct roughness, duct diameter and airflow regime all affect friction losses. Two ducts carrying exactly the same airflow may therefore require significantly different fan pressures simply because one is rougher or smaller than the other.

Understanding these relationships leads to better fan selection, lower energy consumption and more efficient LEV systems.

Further Resources

Fletcher’s Nomogram – Traditional graphical method for estimating duct friction losses.
OXYL8 LEV App – Includes a growing range of practical LEV engineering calculators, replacing many of the traditional nomograms and charts used by earlier generations of engineers.
Basic LEV Maths – Fan Laws (OXYL8 / LEVCentral)
ACGIH – Industrial Ventilation: A Manual of Recommended Practice for Design
HSG258 – Controlling Airborne Contaminants at Work

Recommended Learning

Thought Leadership

Before computers, engineers relied on engineering charts such as the Moody Diagram and Fletcher’s Nomogram because solving the governing equations by hand was both time-consuming and impractical. Those graphical methods allowed complex calculations to be performed quickly using nothing more than a ruler and a good understanding of fluid mechanics.

Today, software performs those same calculations almost instantly, yet every spreadsheet, duct calculator and smartphone app is still based on the same relationships between Reynolds Number, surface roughness, friction factor and pressure loss first brought together in Moody’s chart.

From a LEVCentral perspective, understanding the Moody Diagram is less about learning to read a chart and more about understanding how ventilation systems really behave. Engineers who understand the underlying physics are far better equipped to diagnose poorly performing systems, optimise duct designs and select fans that deliver both effective contaminant control and energy-efficient operation. That knowledge remains just as valuable today as it was when Lewis Moody first published his famous diagram over eighty years ago.

Useful diagram for estimating the relative duct roughness factor in calculating pressure losses in ducts.

Additional Notes

Reynolds Number (Re) a dimensionless number which is obtained from

where is the density of the air at temp and pressures prevailing and is the Viscosity of the air. V is the velocity in the duct, d is the diameter of the duct.