Duct Size Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Airflow (CFM): The volume of air moving through the duct, measured in cubic feet per minute.

Target air speed (FPM): The planned air speed in feet per minute. Lower FPM usually needs a larger duct.

Target friction rate (in. w.g. per 100 ft): The planned pressure loss for each 100 feet of straight duct, measured in inches of water gauge.

Recommended duct size: The practical duct size after the selected round-up rule is applied.

Exact math size before rounding: The duct size from the formula before rounding to a whole-inch or even-inch size.

Round equivalent diameter: For a rectangular duct, the round duct diameter that gives about the same friction loss.

Straight-duct pressure drop: The estimated pressure loss across the entered straight duct length, before adding losses from fittings or equipment.

Rectangular side ratio: The longer rectangular side divided by the shorter side. A higher ratio means a flatter duct.

Mistake: Entering room area or duct area in Airflow (CFM).
Fix: Use the actual airflow in cubic feet per minute for Airflow (CFM), such as the CFM from a load calculation, equipment table, or design plan.

Mistake: Choosing Size duct by Air speed but leaving Target air speed (FPM) blank or set to 0.
Fix: Enter a Target air speed (FPM) greater than 0, such as 700, or switch Size duct by back to Friction rate.

Mistake: Using Target friction rate (in. w.g. per 100 ft) as if it were the total pressure drop for the whole duct run.
Fix: Enter the friction rate per 100 feet, then use Straight-duct pressure drop to see the estimate for the Straight duct length (feet).

Mistake: Picking Rectangular for Duct shape but using the total width plus height in Rectangular duct fixed side (inches).
Fix: Enter only one fixed side, such as 10 inches high; the calculator finds the other side.

Mistake: Setting Round practical size to Do not round and treating the decimal Recommended duct size as a stocked duct size.
Fix: Use Nearest whole inch up or Nearest even inch up when you need a practical size to buy or build.

Mistake: Ignoring Important note when Air speed in the recommended size is above Warn above this air speed (FPM).
Fix: Review the duct size, target, and space limits before using a high-speed result.

• This calculator estimates straight sheet-metal duct behavior from airflow, size, and an empirical friction equation. A full duct design can include room loads, fittings, registers, balancing dampers, leakage, and equipment pressure limits.
• Straight-duct pressure drop excludes elbows, takeoffs, transitions, filters, coils, grilles, dampers, flex duct compression, and duct leakage.
• The Recommended duct size uses the selected round-up rule. It does not prove that the size is stocked locally or allowed by a specific code, plan, or manufacturer.
• For rectangular ducts, the calculator solves the missing side between 1 and 240 inches. If no solution is found, choose a larger fixed side, lower airflow, or a different duct shape.
• Warnings from Quick check and Important note are review aids. They do not replace ACCA Manual D or a professional HVAC design for a real building.
• Very flat rectangular ducts, unusual materials, rough interiors, or poorly sealed ducts can have pressure losses that differ from this simplified estimate.

How the size is found

The calculator first turns the chosen sizing target into an exact round diameter. If Size duct by is Friction rate, it uses this ductulator-style friction approximation:

exact_diameter_in = (0.109136 * airflow_cfm^1.9 / friction_rate)^(1 / 5.02)

If Size duct by is Air speed, it first finds the needed area, then converts that area to a round diameter:

area_sqft = airflow_cfm / target_speed_fpm

exact_diameter_in = 12 * sqrt(4 * area_sqft / pi)

How rectangular ducts are handled

For a rectangular duct, the exact round diameter becomes the target equivalent diameter. The fixed side comes from Rectangular duct fixed side (inches). The other side is found by binary search from 1 inch to 240 inches until this equal-friction equation matches the target:

equivalent_diameter_in = 1.30 * (width_in * height_in)^0.625 / (width_in + height_in)^0.25

solve 1.30 * (fixed_side_in * other_side_in)^0.625 / (fixed_side_in + other_side_in)^0.25 = target_equivalent_diameter_in

How the practical size is rounded

The exact size is then adjusted by Round practical size to. Do not round keeps the decimal size. Nearest whole inch up uses the next whole inch. Nearest even inch up uses the next even whole inch. The rule never rounds down.

if common_size_step = exact, use exact size

if common_size_step = whole_in, use ceil(exact size)

if common_size_step = even_in, use the next even whole inch greater than or equal to exact size

How the result is checked

After rounding, the calculator recomputes the air speed, friction rate, and straight-duct pressure drop using the recommended size, not the smaller exact size.

round_area_sqft = pi * diameter_in^2 / 4 / 144

rect_area_sqft = width_in * height_in / 144

actual_speed_fpm = airflow_cfm / area_sqft

actual_friction_rate = 0.109136 * airflow_cfm^1.9 / equivalent_diameter_in^5.02

total_straight_pressure_drop = actual_friction_rate * duct_length_ft / 100

For rectangular ducts, the side ratio check is:

rect_ratio = max(width_in, height_in) / min(width_in, height_in)

Mini example

With Airflow (CFM) = 400, Size duct by = Friction rate, Target friction rate (in. w.g. per 100 ft) = 0.10, Duct shape = Round, Straight duct length (feet) = 100, and Round practical size to = Nearest whole inch up, the exact diameter is about 9.83 inches. The recommended size is 10 inches. Rechecking the 10-inch duct gives about 733 FPM, about 0.0916 in. w.g. per 100 ft, and about 0.0916 in. w.g. straight-duct pressure drop for 100 feet.