How do you calculate duct size from CFM?

Divide the required CFM by the target velocity in feet per minute to get the duct's cross-sectional area. Convert that area to a round diameter or a rectangular width and height.

What is equivalent diameter in duct sizing?

Equivalent diameter is the round duct size that carries the same airflow at the same friction loss as a given rectangular duct, calculated using the Huebscher equation.

Duct Size & CFM Calculator — Round, Rectangular, Equivalent Diameter

DuctMath · Airflow & Ducts

Duct Size & CFM Calculator

Size a round or rectangular duct from CFM and target velocity, or convert a rectangular duct to its equivalent round diameter. Reference CFM charts are built in below.

Airflow required (CFM)

Target velocity

Higher velocity = smaller, noisier duct. Lower velocity = larger, quieter duct. 600–900 fpm covers most residential work.

Airflow required (CFM)

Target velocity (fpm)

Known duct height (inches)

Set by your joist or ceiling space — the calculator solves for the width.

Rectangular side A (inches)

Rectangular side B (inches)

Useful when a rectangular trunk transitions into round pipe, or when comparing two duct shapes for the same airflow.

Straight answer

How duct size is calculated from CFM

Duct size comes from two numbers: how much air needs to move (CFM) and how fast it should travel (velocity, in feet per minute). Divide CFM by velocity to get the duct’s cross-sectional area, then convert that area to a diameter (round) or a width (rectangular). Faster velocity shrinks the duct but adds noise and pressure loss; slower velocity needs a bigger duct but runs quieter.

This is the velocity method, the same logic behind every duct sizing chart and ductulator. A full system design (ACCA Manual D) also balances total friction loss across the whole duct run — this calculator handles the sizing math for a single run or branch.

The formulas

Duct area (sq ft) = CFM ÷ Velocity (fpm)

Round diameter (in) = 12 × √(Area × 4 ÷ π)

Rectangular width (in) = (Area × 144) ÷ Height (in)

Equivalent round diameter (in) = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 — the standard Huebscher equation, where a and b are the rectangular duct’s two sides.

Worked example

Sizing a 400 CFM supply run

A branch needs to deliver 400 CFM at a typical branch velocity of 600 fpm.
Area = 400 ÷ 600 = 0.667 sq ft.
Round diameter = 12 × √(0.667 × 4 ÷ π) ≈ 11.1 inches — round up to the nearest stock size, 12″.
At the actual 12″ size, real velocity drops slightly below 600 fpm — normal, since stock sizes rarely land exactly on the target.

Reference

Round duct CFM chart

CFM capacity by standard duct diameter at common design velocities. Built from the same area × velocity formula the calculator uses above, so the numbers always match.

Common duct sizing terms

Term	Meaning
CFM	Cubic feet per minute — the volume of air moved
fpm	Feet per minute — how fast the air travels through the duct
Equivalent diameter	The round duct size that carries the same airflow as a given rectangular duct at the same friction loss
Trunk	The main duct run feeding branch ducts
Branch	A smaller duct running from the trunk to a single register

FAQ

Common questions

What velocity should I use for residential ducts?

Typical residential design targets roughly 700–900 fpm in main trunks and 500–700 fpm in branch ducts and returns. Lower velocities run quieter; commercial systems often run faster. This calculator lets you enter any target.

Does flex duct need a different size than sheet metal?

Often yes. Flex duct’s corrugated interior creates more friction than smooth sheet metal, so many installers size flex one size larger than the sheet-metal equivalent, or check the manufacturer’s own friction chart for that product. This calculator computes smooth-duct sizing; treat flex sizing as a starting point, not the final word.

Why round up to the nearest stock size instead of down?

Rounding down forces air through a smaller area, raising real velocity above your target and increasing noise and pressure loss. Rounding up keeps velocity at or below target — the safer direction when stock sizes don’t land exactly on the math.

What is equivalent diameter used for?

It lets you compare a rectangular duct and a round duct on equal footing, or size the round pipe needed where a rectangular trunk transitions to round. Two ducts with the same equivalent diameter carry the same airflow at the same friction loss per foot.

Does this replace a full duct design (Manual D)?

No. This tool sizes an individual run from CFM and velocity, which is the core math inside every duct sizing chart. A complete system design also balances total external static pressure and total friction loss across every run back to the air handler — that’s a full ACCA Manual D calculation.

Good to know: This tool uses the velocity method for individual duct runs. It does not calculate total system static pressure or balance multiple runs — for a full residential design, a Manual D calculation accounts for total equivalent length and fitting losses across the whole system.

Changelog: v1.0 — launched after passing its published test suite. Technical review by a licensed contractor: open — apply here.