Duct CFM Calculator

Works both directions. Give it a CFM and it returns the duct size; give it a size and it returns the CFM that size will carry. Supply, return and flex are all three answered at once, because they are held to different limits and the same number is wrong for at least two of them. 400 CFM is 10 inch supply, 12 inch return, 12 inch flex, or 10 x 10 rectangular.

Duct CFM table

The same answers, worked out ahead of time. This is the part worth printing, and it is what the page shows if you have scripts turned off.

• Friction rate 0.1 in. w.g. per 100 ft, equal friction method
• Supply is galvanized steel, roughness 0.0003 ft
• Flex is fully extended, roughness 0.003 ft
• Return is galvanized and additionally capped at 700 fpm, whichever limit bites first
• Every figure is the most that size will carry, rounded down

Why one CFM gives three answers

The supply column is the one everybody means by a duct calculator. It is sized on friction alone: the largest flow that keeps the run at or under 0.1 in. w.g. per 100 ft.

The return column is smaller at every size, and it is smaller for a reason that has nothing to do with pressure. A return grille sits at head height in a hallway, so the limit that bites is noise, not friction. Held to 700 fpm, an 8 inch return carries 241 CFM where the same pipe on the supply side carries 241. Reading the supply column for a return is the single most common way a system ends up audible, and it does not show up until the house is quiet at night.

The flex column is smaller again, because the liner rides over a wire helix instead of sitting flat. Same diameter, more drag, less air. And it assumes the flex is pulled fully tight, which is the assumption most likely to be false on a real job.

What this does not do

It does not pick your friction rate. It uses 0.1 because that is the everyday residential convention, but the correct rate for a specific system is the static pressure the equipment has available divided by the total effective length of the worst run, fittings included. That calculation is Manual D, and on a long or complicated system the difference is not small.

It also does not size the whole system. One run at a time is the job a Ductulator does in your hand, which is exactly what this is. Room by room airflow comes from a load calculation, and the grilles at each end are sized on face velocity, which is the return grille CFM chart.

Working from CFM to a size across the full range, rather than one at a time? The duct CFM chart tabulates every step from 25 to 4000 CFM with rectangular equivalents.

These charts are free and always will be. If you also need to turn the job into a signable quote and an invoice, that is the paid tool this site pays for itself with. See what it costs.

Duct CFM calculator questions

How do you calculate CFM for a duct size?

Airflow through a round duct is the velocity times the cross sectional area, and the limit on velocity is the friction you are willing to spend. At the standard 0.1 in. w.g. per 100 ft, an 8 inch galvanized duct works out to about 241 CFM. The arithmetic is Darcy-Weisbach with the Haaland friction factor; the calculator above runs it for you.

What is the formula for duct CFM?

CFM equals face velocity times duct area, but velocity is not a free choice. Pick the friction rate first, usually 0.1 in. w.g. per 100 ft, then solve Darcy-Weisbach for the flow that produces exactly that pressure drop. That is why a duct calculator is not simple multiplication and why the answer differs between sheet metal and flex.

Why does the calculator give three different sizes for the same CFM?

Because supply, return and flex are held to different limits. For 400 CFM you get 10 inch supply in sheet metal, 12 inch for a return because returns are capped at 700 fpm for noise, and 12 inch in flex because flex is rougher. Using the supply number for all three is the most common way a system ends up noisy.

Does this replace Manual D?

No. This solves one duct run at a friction rate you supply. Manual D works out what that friction rate should be, from the static pressure the equipment actually has available divided by the total effective length of the worst run, fittings included. On a normal house 0.1 is close enough for a branch or a replacement. On a long or complicated system it can be well off.

Does the calculator work without an internet connection?

Once the page has loaded, yes. It runs entirely in your browser, nothing is sent anywhere, and there is nothing to sign in to. The printed table below it works with no browser at all, which is the version worth having in the truck.