Duct Size Calculator (CFM & Velocity)
Round duct diameter for an airflow at your target velocity, with rectangular equivalent.
Round to the next standard size (100/125/150/200/250/315/400 mm). Oversizing costs sheet metal; undersizing costs fan energy forever.
Formula
Duct Size Calculator (CFM & Velocity) is a free duct size calculator for HVAC designers, technicians and energy auditors โ instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Duct Size Calculator (CFM & Velocity)
Round duct diameter for an airflow at your target velocity, with rectangular equivalent. The calculation implements A = Q/V; D = โ(4A/ฯ) (ASHRAE Handbook โ Fundamentals, ch. 21; SMACNA HVAC duct design). Round to the next standard size (100/125/150/200/250/315/400 mm). Oversizing costs sheet metal; undersizing costs fan energy forever.
How to use Duct Size Calculator (CFM & Velocity)
- 1Enter Airflow in L/s.
- 2Enter Target velocity in m/s (Mains 5โ7 ยท branches 3โ5 ยท low-noise 2.5 m/s).
- 3Read Round duct ร, Duct area instantly โ no submit button needed.
- 4Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Duct Size Calculator (CFM & Velocity)?
- โImplements the standard formula โ A = Q/V; D = โ(4A/ฯ)
- โReference cited on-page: ASHRAE Handbook โ Fundamentals, ch. 21; SMACNA HVAC duct design
- โOne-click SI โ Imperial toggle โ values convert in place, physics stays in SI
- โLive worked example: the substitution recomputes from your numbers
- โRuns entirely in your browser โ nothing uploaded, free forever
Frequently asked questions
What formula does the Duct Size Calculator (CFM & Velocity) use?+
It computes A = Q/V; D = โ(4A/ฯ), per ASHRAE Handbook โ Fundamentals, ch. 21; SMACNA HVAC duct design. The formula is displayed under the result along with a worked example substituted with your own inputs.
What should I keep in mind when using this calculator?+
Round to the next standard size (100/125/150/200/250/315/400 mm). Oversizing costs sheet metal; undersizing costs fan energy forever.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Duct Size Calculator (CFM & Velocity) free to use?+
Yes โ completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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