Duct size calculator
Enter the airflow and a friction rate to get the round duct size, its velocity, rectangular sizes that carry the same air, and a CFM chart for every standard diameter.
Inputs
Result
| Size | Equiv. dia., in | Velocity, fpm |
|---|
Duct sizing chart at 0.08 in. w.c. per 100 ft
Airflow each standard round duct carries at the friction rate above. Change the friction rate to redraw the chart.
| Round duct | Airflow, cfm | Velocity, fpm |
|---|
How to use it
- Enter the airflow the section has to carry. For a branch that is the CFM of its register; for a trunk it is the sum of every branch downstream.
- Enter your design friction rate. If you have done a Manual D, use the friction rate it gave you.
- Read the round size, or pick a rectangular size from the list if the duct has to fit a joist bay.
How it works
The calculator solves the standard friction-loss equation for round galvanized steel duct carrying air at standard density, the same relationship printed on a ductulator:
It finds the exact diameter for your friction rate, rounds up to the next standard size, and steps up again if the velocity would exceed your limit. Rectangular sizes use the Huebscher equivalent-diameter formula from ASHRAE Fundamentals, De = 1.30 (ab)^0.625 ÷ (a+b)^0.25, and stay within a 4:1 aspect ratio.
Choosing a friction rate
The friction rate comes from the static pressure the blower can spend on duct friction and the longest run it has to push through: available static pressure × 100 ÷ total effective length. A system with 0.20 in. w.c. available and 250 ft of total effective length works out to 0.08. Without that calculation, 0.08 is a reasonable residential starting point and 0.10 is common for short, simple systems.
Questions
How many CFM does a 6 inch duct carry?
At a friction rate of 0.08 in. w.c. per 100 ft, a 6 in. round sheet-metal duct carries about 97 cfm; at 0.10 it carries about 109 cfm. An 8 in. duct carries about 207 cfm at 0.08 and a 10 in. about 372 cfm.
What friction rate should I use?
Use the friction rate from your Manual D if you have one. Otherwise 0.08 in. w.c. per 100 ft is a common residential design value, with 0.05 to 0.10 the usual range. A lower friction rate gives larger, quieter ducts.
What is equivalent diameter?
The diameter of a round duct with the same friction loss and airflow as a rectangular one. A 12 × 8 in. duct, for example, behaves like a 10.7 in. round duct.
Does this work for flex duct?
Not directly. The equation is for smooth galvanized steel. Flex duct has higher friction, so use the manufacturer's sizing chart or choose one size larger than the result here.
Is this a Manual D calculation?
No. Manual D designs the whole system, including fittings, equivalent lengths and the blower's available static pressure. This calculator sizes one section once you know its airflow and friction rate, the same job a ductulator does.
More HVAC calculators
Static pressure calculator
Total external static pressure from your readings, plus the friction rate for duct sizing.
AC tonnage calculator
Cooling capacity estimate by floor area and climate zone, with an honest check against real load calculations.