Duct Size Calculator

Use this duct size calculator to estimate the required rectangular and round duct size based on airflow and either target friction rate or target air velocity. You can toggle between IP or SI unit. Hybrid unit practitioners can use the MY mode.

Enter the airflow, target friction rate or velocity, and duct height to determine the duct size. If you use round duct, treat the duct height as the available height clearance. Either way, the calculator will show you the recommended rectangular and round duct size.

How to Use the Duct Size Calculator

The calculator can size a rectangular or round duct using either the friction-rate method or the velocity method.

1. Select the Unit Mode

Choose the unit system that matches your project or design practice.

IP Mode

Use IP mode when working with:

• Airflow in CFM
• Friction rate in in.wg/100 ft
• Velocity in fpm
• Duct dimensions in inches

SI Mode

Use SI mode when working with:

• Airflow in L/s
• Friction rate in Pa/m
• Velocity in m/s
• Duct dimensions in millimetres

MY Mode

Use MY mode when working with:

• Airflow in CFM
• Friction rate in in.wg/100 ft
• Velocity in fpm
• Duct dimensions in millimetres

MY mode uses the same airflow and pressure-loss basis as IP mode while presenting duct dimensions in metric sizes. Dedicated for hybrid practitioners (actually is for my own use…).

2. Select the Sizing Method

Choose either:

• Friction Rate
• Velocity

The selected method determines which design criterion controls the duct size.

3. Enter the Airflow

Enter the airflow passing through the duct section.

The airflow should represent the design airflow for that specific section of ductwork, not necessarily the total system airflow.

For example:

• Main duct airflow may include several downstream branches.
• A branch duct should use only the airflow supplied through that branch.
• A terminal run should use the airflow required by the connected grille or diffuser.

4. Enter the Design Criterion

When using the friction-rate method, enter the target friction rate.

When using the velocity method, enter the target air velocity.

5. Enter the Duct Height

Enter the available rectangular duct height.

The calculator will determine the required width based on the selected height.

A very shallow duct may require a much larger width. This can result in a high aspect ratio, which may be impractical for fabrication or installation. Try to stay within 4:1 ratio.

6. Calculate the Duct Size

Select Calculate Duct Size.

The calculator will show:

• Recommended rectangular duct size
• Required round duct diameter
• Resulting friction rate
• Equivalent diameter
• Equivalent-round velocity
• Actual rectangular velocity
• Reynolds number
• Darcy friction factor
• Velocity pressure
• Calculated width before rounding

Friction Rate or Velocity: Which Method Should You Use?

Both methods are useful, but they serve different purposes.

Size by Friction Rate

The friction-rate method determines the duct size based on an allowable pressure loss per unit length.

It is commonly used as part of the equal-friction duct design method.

This method is generally more suitable when:

• Designing a complete duct system
• Estimating fan static-pressure requirements
• Sizing multiple duct sections consistently
• Controlling pressure loss throughout a duct network
• Comparing alternative duct sizes

A common starting range for low-pressure HVAC ductwork is approximately:

• 0.08–0.10 in.wg/100 ft
• 0.65–0.82 Pa/m

Higher friction rates may reduce duct size but increase system resistance, fan pressure, energy consumption, and noise risk.

Lower friction rates normally require larger ducts but reduce pressure loss.

Size by Velocity

The velocity method calculates the required duct area based on airflow and target air velocity.

It is useful for:

• Quick preliminary duct sizing
• Space-planning exercises
• Checking whether a duct velocity is reasonable
• Noise-sensitive applications
• Estimating duct size before a full pressure-loss calculation

The velocity method is simpler, but it does not replace a complete duct-system pressure-loss calculation.

Two ducts with the same velocity can have different friction rates because friction also depends on:

• Duct diameter
• Duct shape
• Surface roughness
• Air density
• Reynolds number

Recommended Approach

For full duct-system design, use friction rate as the main sizing basis and review the resulting velocity.

For quick preliminary sizing, velocity is often sufficient.

A good design should satisfy both:

• Reasonable friction rate
• Reasonable air velocity

Typical Duct Velocity Ranges

The following values are general starting ranges only.

Approximate SI equivalents:

These ranges should not be treated as mandatory limits.

The correct velocity depends on:

• Building type
• Duct location
• Noise criteria
• Available ceiling space
• Duct lining
• Distance from occupied spaces
• Fan pressure
• Airflow-control requirements

Typical Duct Friction Rates

The table below shows common starting values for low-pressure duct systems.

A friction rate of 0.10 in.wg/100 ft is approximately 0.82 Pa/m.

The final value should be selected based on the complete duct system, not only one section.

How the Calculator Works

Method 1: By Friction-Rate

When friction rate is selected, the calculator follows this general process:

1. Calculates the round duct diameter required to meet the target friction rate.
2. Converts the required round diameter into an equivalent rectangular duct.
3. Uses the entered duct height to solve for the required width.
4. Rounds the width to a practical duct increment.
5. Checks the resulting friction rate after rounding.
6. Increases the width where necessary to keep the friction rate within the allowed tolerance.

The practical width increments used are:

• IP: 2 in
• SI: 50 mm
• MY: 50 mm

The width increment is used because sheet-metal ducts are normally fabricated in standard dimensional steps rather than to any exact calculated width. After the theoretical width is determined, the calculator rounds it to the nearest practical fabrication size, making the result easier to manufacture, install, and coordinate on site.

Method 2: By Velocity

When velocity is selected, the calculator:

1. Calculates the duct area required at the selected airflow and velocity.
2. Calculates the rectangular width using the entered duct height.
3. Rounds the width upward to the next practical increment.
4. Calculates the resulting friction rate and actual velocity.
5. Calculates the equivalent round diameter.

The width is rounded upward in velocity mode so that the actual rectangular velocity does not exceed the target.

Duct Sizing Formulas

Airflow, Area, and Velocity

The relationship between airflow, duct area, and air velocity is:

$Q=AV$

Therefore:

$A = \frac{Q}{V}$

Where:

• $Q$ = airflow
• $A$ = duct cross-sectional area
• $V$ = air velocity

For a rectangular duct:

$A=WH$

Therefore:

$W = \frac{Q}{V H}$

Where:

• $W$ = duct width
• $H$ = duct height

Unit consistency is important when applying these equations.

Rectangular Equivalent Diameter

The calculator uses the following equal-friction equivalent-diameter relationship:

$D_e = 1.3 \frac{(WH)^{0.625}} {(W+H)^{0.25}}$

Where:

• $D_e$ = equivalent round diameter
• $W$ = rectangular duct width
• $H$ = rectangular duct height

This equivalent diameter allows the rectangular duct to be evaluated using a round-duct friction calculation.

Friction Loss

The calculator evaluates friction using the Darcy–Weisbach relationship:

$\Delta P = f \frac{L}{D} \frac{\rho V^2}{2}$

Where:

• $\Delta P$ = pressure loss
• $f$ = Darcy friction factor
• $L$ = duct length
• $D$ = hydraulic or equivalent diameter
• $\rho$ = air density
• $V$ = air velocity

The Darcy friction factor is calculated using the Colebrook–White relationship for turbulent flow.

Example 1: Size by Friction Rate

Assume the following design values:

• Airflow: 700 CFM
• Target friction rate: 0.10 in.wg/100 ft
• Duct height: 10 in
• Unit mode: IP

The calculator first determines the required round duct diameter corresponding to the target friction rate.

It then converts the round-duct requirement into a rectangular duct with a fixed 10 in height.

The theoretical width is rounded to the nearest practical 2 in increment.

The result section will show:

• Recommended rectangular duct size
• Required round diameter
• Resulting friction rate
• Actual rectangular velocity
• Equivalent diameter

The resulting friction rate may not be exactly 0.10 in.wg/100 ft because the rectangular width must be rounded to a practical size.

Example 2: Size by Velocity

Assume:

• Airflow: 700 CFM
• Target velocity: 900 fpm
• Duct height: 10 in
• Unit mode: IP

The required duct area is:

$A = \frac{700}{900}$

$A = 0.778 \text{ ft}^2$

The required width is:

$W = \frac{0.778 \times 144}{10}$

$W = 11.2 \text{ in}$

The width is then rounded upward to the next 2 in increment.

The recommended rectangular size becomes approximately:

$12 \times 10 \text{ in}$

The actual velocity will be slightly lower than 900 fpm because the width was rounded upward.

Understanding the Results

Recommended Duct Size

This is the practical rectangular duct size after applying the relevant width increment.

The first dimension is the calculated width.

The second dimension is the entered duct height.

For example:

400 × 250 mm

means:

• Width: 400 mm
• Height: 250 mm

Required Round Diameter

This is the theoretical round duct diameter required by the selected sizing method.

For friction-rate sizing, it represents the round diameter needed to meet the target friction rate.

For velocity sizing, it represents the round duct area needed to meet the target velocity.

The calculated diameter may not correspond to a standard manufactured duct size. Select the next appropriate standard round size where necessary.

Resulting Friction Rate

This is the estimated friction rate of the selected rectangular duct size.

It is calculated after the width has been rounded.

A significant increase above the design target may indicate that the selected duct is too small.

Equivalent Diameter

This is the round diameter that gives approximately the same friction characteristics as the selected rectangular duct.

It is not always the same as the required round diameter because the rectangular width is rounded to a practical size.

Equivalent-Round Velocity

This is the velocity calculated using the equivalent round diameter.

It is used internally for the round-duct friction calculation.

Actual Rectangular Velocity

This is the actual air velocity through the rectangular duct area.

For noise and air-distribution checks, this is normally the more relevant velocity to review.

Velocity Pressure

Velocity pressure is the pressure associated with the movement of air.

In IP units, it is shown in inches of water gauge.

In SI units, it is shown in pascals.

Reynolds Number

Reynolds number indicates whether the airflow is laminar or turbulent.

HVAC duct airflow is normally turbulent.

Darcy Friction Factor

The Darcy friction factor represents resistance caused by duct-wall friction.

It depends on:

• Reynolds number
• Duct roughness
• Equivalent diameter

Calculated Width Before Rounding

This is the theoretical rectangular width before a practical duct increment is applied.

Comparing this value with the recommended size helps show the effect of rounding.

Why Duct Size Should Not Be Selected by Velocity Alone

Velocity is important, but it is only one part of duct design.

A duct may have an acceptable velocity but still create excessive pressure loss if:

• The duct is too small
• The duct run is long
• There are many elbows or fittings
• The duct has a rough internal surface
• The system includes restrictive dampers or equipment

Similarly, a duct may meet the friction-rate target but still have a velocity that is too high for a noise-sensitive space.

A complete duct design should review:

• Friction rate
• Actual velocity
• Total duct length
• Fitting losses
• Fan static pressure
• Noise
• Duct aspect ratio
• Available installation space

Duct Aspect Ratio

The aspect ratio is the ratio between the longer and shorter sides of a rectangular duct.

For example:

• 400 × 200 mm has an aspect ratio of 2:1
• 600 × 150 mm has an aspect ratio of 4:1

Very high aspect ratios may:

• Increase sheet-metal surface area
• Increase fabrication cost
• Increase heat transfer
• Increase leakage potential
• Make reinforcement more difficult
• Create less efficient airflow distribution
• Require more installation space

Where possible, avoid unnecessarily wide and shallow ducts.

A more balanced rectangular duct is generally preferable.

Common Duct-Sizing Mistakes

1. Using the Total System Airflow for Every Section

Airflow reduces after each branch take-off.

Each duct section should be sized using the airflow that actually passes through that section.

2. Ignoring Fitting Losses

Elbows, transitions, branches, dampers, grilles, diffusers, filters, and coils all create pressure loss.

Straight-duct friction is only one part of total system resistance.

3. Selecting a Very High Velocity to Reduce Duct Size

High velocity may reduce the required duct area, but it can increase:

• Friction loss
• Noise
• Fan pressure
• Energy use
• Air-balance difficulty

4. Using a Very Low Duct Height

A low height may force the duct to become excessively wide.

Check the resulting aspect ratio and installation practicality.

5. Treating the Calculated Round Diameter as a Standard Size

The required round diameter is theoretical.

The selected duct should normally be adjusted to an available standard diameter.

6. Ignoring the Resulting Friction Rate

When sizing by velocity, always review the calculated friction rate.

A duct sized only by velocity may create excessive pressure loss.

7. Ignoring the Actual Velocity

When sizing by friction rate, always review the actual rectangular velocity.

A duct may meet the friction target but still be unsuitable for a noise-sensitive application.

Calculator Assumptions

This calculator assumes:

• Standard air conditions
• Galvanized sheet-metal duct roughness
• Fully developed airflow
• Straight-duct friction
• A rectangular equivalent-diameter method
• Practical duct-width increments
• No additional liner roughness
• No fitting losses

The result should be used as a design estimate rather than a complete duct-system analysis.

Calculator Limitations

This calculator does not:

• Calculate total external static pressure
• Calculate elbow or fitting losses
• Size grilles or diffusers
• Calculate air-balancing damper losses
• Select a fan
• Check room noise criteria
• Check duct breakout noise
• Check duct leakage
• Check insulation requirements
• Evaluate condensation risk
• Calculate duct reinforcement
• Check local code requirements
• Design a full duct network automatically

Complete duct design requires evaluation of the entire airflow path from the fan to the air terminal and back through the return-air system.

Frequently Asked Questions

Related HVAC Tools

Continue with these related calculators:

• Duct Friction Loss Calculator
• HVAC CFM Calculator
• Duct Equivalent Diameter Calculator
• Grille Size Calculator
• Airflow Conversion Calculator
• Fan Static Pressure Calculator
• Fresh Air Calculator

Related Duct Design Guides

Recommended related articles:

• How to Calculate Duct Size
• Equal Friction Method Explained
• Duct Velocity Guidelines
• How to Calculate Duct Friction Loss
• Duct Static Pressure Explained
• How to Calculate Duct Fitting Loss
• How to Size a Return Air Duct
• How to Size HVAC Grilles and Diffusers
• Common Duct Design Mistakes

Final Technical Note

The calculator provides a practical starting point for duct sizing.

The final duct size should be reviewed against:

• Complete system pressure loss
• Air velocity
• Noise requirements
• Fan performance
• Duct routing
• Fitting losses
• Installation space
• Project specifications

For critical, complex, or high-performance systems, complete duct calculations and engineering review are recommended.