How to Calculate Duct Size from CFM

The friction rate is the pressure the duct is allowed to lose per 100 ft. Pick one rate for the whole system and the duct sizes come out consistent. For supply trunks and branches 0.1 is the usual starting point, and DuctCalc fills it in for you.

How do you size a duct from CFM in the app?

Open the Ductulator tab and make sure Size a duct is selected.
Pick the Material under the inputs. Galv. steel is the default.
Type the airflow in Airflow, for example 400.
Leave Friction rate at 0.1 in.wg/100ft, or type your own design rate.
Read Required round duct, then the Use stock duct size line below it.

What duct size do 400 and 1,200 CFM need at 0.1 in.wg/100ft?

Computed with the same Colebrook friction model the app uses
Airflow	Exact round	Velocity at exact	Stock size	Velocity at stock	Friction at stock
400 CFM	9.7 in	781 fpm	10 in	733 fpm	0.086
1,200 CFM	14.6 in	1,028 fpm	16 in	859 fpm	0.064

The stock line always rounds up, never down, so the friction rate you end up with is lower than the one you asked for. That is why 10 in at 400 CFM runs at 0.086 and not 0.100.

What if you know the velocity you want?

Tap Velocity above the second field and type a number in Target velocity. At 400 CFM and 700 fpm the exact duct is 10.2 in, which lands on a 12 in stock size at 509 fpm and 0.035 in.wg/100ft. The jump is big because the next stock step after 10 is 12.

Common mistakes

Sizing to the exact diameter. Nobody sells 9.7 in duct; the number to order is the stock one.
Leaving the material on Galv. steel when the run is flex. See the flex guide for how much that moves the answer.
Using a supply rate for a return. Returns are designed lower, 0.05 to 0.08 in the Reference tab, so change Friction rate before you size one.

Fittings, filters and coils are outside this calculation. DuctCalc sizes straight duct at a friction rate and nothing else.

Questions and answers

What size duct do I need for 400 CFM?

At 0.1 in.wg/100ft in galvanized steel, 9.7 in exact, so 10 in round. With flex it is 11.0 in exact and 12 in stock, because flex is rougher.

What friction rate should I use?

The Reference tab lists 0.08 to 0.10 in.wg/100ft for supply ducts and 0.05 to 0.08 for returns. 0.1 is the default the app starts on.

Why does the app show two diameters?

One is the exact solved diameter and one is the next catalogue size up. The stock line also shows the velocity and friction you will really get at that size.

Does it work in metric?

Yes. Switch Settings > Units to Metric and enter airflow in L/s or m³/h with the friction rate in Pa/m.

How to calculate duct size from CFM