Duct Size Calculator
Duct size by the velocity method: round and rectangular duct from airflow (cfm or L/s) and your velocity, standard sizes, equivalent round, or check a duct.
Cross-section area, the round diameter it needs, the standard sizes below and above with their velocities, the rectangular duct with one side fixed and its Huebscher equivalent round, or the velocity an existing duct carries.
Example: 1 200 cfm at 900 fpm needs 192 in² — a 15.6 in round; 14 in runs at 1 123 fpm, 16 in at 859; with one side at 10 in the other is 19.2 in, equivalent round 15.0 in; 24 × 6 in carries it at 1 200 fpm.
Area from airflow
and the velocity you choose.
What the velocity method is, how rectangles and equivalent rounds are handled, and what the page leaves to you.
The velocity method
The simplest duct-sizing method fixes an air velocity and takes the area from it: A = Q ÷ V. A round duct then needs a diameter D = √(4A/π); a rectangular duct with one side fixed needs the other side = A ÷ a. The nearest standard round sizes either side (even inches, or the ISO sequence in millimetres) are listed with the velocity each would actually carry at your airflow, since you buy a catalogue size, not the exact one.
Equivalent round
A rectangular duct with the same area as a round one has more wall per unit of airflow and therefore more friction. The Huebscher relation in the ASHRAE Handbook — De = 1.30·(a·b)^0.625 ÷ (a + b)^0.25 — gives the round diameter with the same friction per unit length at the same flow; it is smaller than the area-equivalent circle, and it is the number to use on a friction chart. Aspect ratios above about 4 : 1 are flagged: they cost metal, friction and noise.
What is yours
The design velocity — this page holds no default and no recommended range. It depends on the system (residential, commercial, industrial), whether the run is a main or a branch, the noise the space tolerates and the code that applies; take it from your design guide. Friction loss, fittings, static pressure and fan selection are not computed here — see the fan-laws and pipe-flow tools for the pressure side. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- A = Q ÷ V, D = √(4A/π); equivalent round of a rectangle De = 1.30·(a·b)^0.625 ÷ (a + b)^0.25 (Huebscher, ASHRAE Handbook — Fundamentals, equal friction); no design velocity or code limit is assumed
Last reviewed 22 September 2026. How results are checked: How we verify.