What this calculator does
Ducts come in standard sizes, so the design question is the velocity you are willing to accept. This calculator turns airflow and a velocity budget into a required area, the next standard round duct and the rectangular equivalent that fits the ceiling space.
How to use it
- Enter the airflow the branch must carry.
- Set the design velocity — 400–600 fpm for branches in low-velocity residential work, 800–1,000 fpm for main runs.
- Set the rectangular aspect ratio, 2:1 being the usual default.
- Check the actual velocity in the nominal size — it is always lower than the target because of the step-up in size.
Governing formula
A = Q / v ; D = √(4A/π) Where:
-
Q= Airflow [ft³/min] -
v= Design velocity [ft/min]
Worked example — 2,000 CFM at 800 fpm
The figures below were produced by running this page's own engine with the example inputs (Airflow (CFM), Design velocity (fpm), Rectangular aspect ratio (width:height) (: 1)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Nominal round duct | 22 in |
|---|---|
| Actual velocity | 757.63 fpm |
| Required free area | 2.5 ft² |
| Rectangular equivalent width | 26.83 in |
| Rectangular equivalent height | 13.42 in |
| Assessment | Round duct 22" (758 fpm actual). Rectangular equivalent ≈ 26.8" × 13.4". |
- Required area is 2,000 / 800 = 2.5 ft², equivalent to a 19.4 in round duct.
- The next standard size is 20 in, where the actual velocity falls to about 765 fpm.
- A rectangular equivalent at 2:1 is roughly 28 in × 14 in — check it fits the available ceiling void.
Assumptions & limitations
Engineering calculation. Sizes a duct from airflow and a chosen velocity (equal-friction method) and reports the next nominal round size, the resulting velocity and a rectangular equivalent at your chosen aspect ratio.
- No equal-friction or pressure-drop iteration — the velocity is entered, not solved for.
- Noise criteria (NC), pressure class and fitting loss allowances must be checked separately.
- Rectangular equivalents assume a sharp-cornered duct; take the published dimensions of the actual duct series for ordering.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ASHRAE Handbook — Fundamentals, duct design chapter.
- SMACNA — duct construction standards and pressure classes.