Free Tool
Enter airflow in CFM and a target velocity and get the minimum round duct diameter, the next standard size, the resulting velocity, and an equal-area rectangular option. A tonnage helper fills CFM at 400 CFM/ton.
Velocity method: area = CFM ÷ velocity, rounded up to a standard size
Velocity method: area(in²) = CFM ÷ velocity × 144; round Ø = √(4·area ÷ π), rounded up to a standard size. Rectangular is an equal-area approximation — confirm against a friction chart or Manual D.
Get a feel for whether a duct run is undersized before you call for help — handy when airflow to a room is weak or noisy.
Turn a room's CFM into a duct size in seconds using the velocity method, with standard-size round and rectangular options. In ServicePros Hub the spec drops into a proposal line.
The velocity method sizes a duct straight from airflow: cross-sectional area = CFM ÷ target velocity.
Pick the duct's role — a supply trunk runs faster (around 800 fpm) than a branch or return (around 600 fpm) — and the calculator sets a typical target velocity you can override. It converts the required area to a minimum round diameter, rounds up to the next standard duct size, and reports the actual velocity at that size so you can see how much headroom you have.
Need rectangular? Enter a height and it returns an equal-area width. The tonnage helper fills CFM at the 400 CFM/ton rule of thumb. This is a quick field estimate — confirm final designs against a friction chart or ACCA Manual D.
Divide the airflow (CFM) by a target velocity (fpm) to get the required cross-sectional area, then pick the next standard duct size. This calculator does it with the velocity method and rounds up to a standard round diameter.
The common rule of thumb is about 400 CFM per ton of cooling. A 3-ton system is roughly 1,200 CFM. Use the tonnage helper to fill CFM automatically, then fine-tune per room.
Typical residential targets are around 700–900 fpm for supply trunks, ~600 fpm for branches, and ~500–700 fpm for returns. Lower velocities are quieter; higher velocities allow smaller ducts but add noise and static pressure.
Match the cross-sectional area: pick a height and solve for a width that gives the same area. This calculator returns an equal-area rectangular width; for tight designs use an equal-friction conversion chart.
Returns move the same air as the supply but are usually sized for a lower velocity, so they're larger. Enter the return CFM and a ~600 fpm target to size it — undersized returns are a common cause of noisy, starved systems.
This calculator is one of the trade tools inside ServicePros Hub — proposals, scheduling, invoicing, and card payments for home-service pros. One flat price. No tiers, no add-ons.
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