About the Duct Size Calculator
This duct size calculator finds the round duct diameter needed to carry a given airflow (CFM) at a chosen air velocity (feet per minute), rounds it up to the next standard round size, and converts it to an equivalent rectangular duct for any height you have room for. It also estimates the friction loss per 100 feet so you can check the design against your blower’s available static pressure.
It is meant for HVAC technicians, contractors and DIYers laying out supply trunks, branch runs and returns. Typical residential design velocities are about 700–900 FPM for supply trunks, 500–700 FPM for branches and 400–600 FPM for return ducts; higher velocities save space but increase noise and pressure drop.
Rectangular sizes use the Huebscher equal-friction equation, which is the method in ASHRAE tables, so a rectangular duct with the same equivalent diameter has the same friction loss per foot as the round one. Friction is an approximation for clean galvanized steel; flex duct has much higher losses. For whole-system design, a full Manual D calculation is still recommended.
With the default inputs, the required round duct diameter is 10.24 in. Change any value above to recalculate instantly.
How to use the duct size calculator
- 1Enter the airflow in CFM for the trunk or branch you are sizing.
- 2Choose a design velocity suited to the duct type and noise tolerance.
- 3Enter the rectangular height that fits your joist or ceiling space.
- 4Use the next standard round size or the suggested rectangular size.
- 5Check the friction rate against your system’s available static pressure.
Formula and method
The required free area is the airflow divided by the velocity: CFM ÷ FPM gives square feet, multiplied by 144 for square inches. The round diameter follows from the area of a circle. The calculator then picks the next standard round size and reports the actual velocity in it.
A rectangular duct with the same area as a round one has more wall surface and therefore more friction, so the rectangular width is solved from the Huebscher equivalent-diameter equation instead: it finds the width b that, with your height a, gives an equivalent diameter equal to the required round diameter. Friction loss per 100 ft uses a widely used curve fit for galvanized round duct (Q in CFM, D in inches) and is an estimate only.
- Q
- Airflow (CFM)
- V
- Air velocity (feet per minute)
- A
- Duct cross-section area
- D
- Round duct diameter (in)
- a, b
- Rectangular duct height and width (in)
- De
- Equivalent round diameter of a rectangular duct
- Δp
- Friction loss (in. w.c. per 100 ft)
Worked examples
400 CFM trunk at 700 FPM, 8" tall
400 ÷ 700 = 0.571 sq ft, or 82.3 sq in, which needs a 10.24" round duct — round up to 12". An 8"-tall rectangular duct needs to be about 11.0" wide for equal friction, so 8 × 12 is the practical size. The 12" round duct runs at about 509 FPM.
1,200 CFM (3-ton) supply at 900 FPM, 10" tall
1,200 ÷ 900 = 1.333 sq ft (192 sq in) → 15.6" round, rounded up to 16". With only 10" of height the equal-friction width is about 21.2", so a 10 × 22 duct would work.
100 CFM branch at 600 FPM
A 100 CFM bedroom branch needs 24 sq in, a 5.53" round duct, so a standard 6" run is typical and moves air at about 509 FPM.
Frequently asked questions
How do I calculate duct size from CFM?+
Divide CFM by the design velocity in FPM to get the area in square feet, multiply by 144 for square inches, then find the diameter with D = √(4A ÷ π). For 400 CFM at 700 FPM that is about 10.2 inches, so use 12".
What velocity should air ducts be designed for?+
Common residential targets are about 700–900 FPM for supply trunks, 500–700 FPM for branch runs and 400–600 FPM for returns. Commercial systems often run faster where noise is less of a concern.
How do I convert round duct to rectangular?+
Use the equal-friction (Huebscher) equivalent diameter, not equal area. A rectangular duct needs slightly more area than the round one to have the same friction; for example a 12" round duct is roughly equivalent to 8 × 16 or 10 × 12.
How much CFM does a 6 inch duct carry?+
At 600 FPM a 6" round duct carries about 118 CFM (0.196 sq ft × 600). In practice residential 6" rigid branches are usually designed for roughly 75–110 CFM depending on length and friction rate.
Does flex duct need to be larger?+
Usually yes. Flexible duct has higher friction than smooth metal, especially if compressed or sagging, so installers often go up one size for the same airflow and keep runs short and fully stretched.