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The volumetric airflow moving through the duct.
This is the design airflow for the duct section being checked, in cubic feet per minute.
The internal cross-sectional area of the duct at the point being checked.
For a rectangular duct this is width x height; for a round duct it's π x (diameter/2)², both converted to square feet.
Duct air velocity
899 ft/min
Recommended velocity ranges vary by duct application (residential vs. commercial, trunk vs. branch) and are driven by noise, energy, and space constraints rather than a single code-mandated limit — compare your result against your project's design criteria or ASHRAE/SMACNA guidance for the specific duct type.
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Duct Air Velocity Calculator: 899 ft/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Velocity (ft/min) = Airflow (CFM) ÷ Duct Cross-Sectional Area (sq ft), the basic continuity relationship. Typical recommended duct velocity ranges (widely-cited ASHRAE/SMACNA design guidance, not a hard code limit): residential supply trunk ~700-900 fpm, residential branch ~500-700 fpm, commercial main duct ~1000-1500+ fpm — actual limits are set by noise, energy, and space constraints on each specific project.
Inputs used
- Airflow
- 800 CFM
- Duct cross-sectional area
- 0.89 sq ft
Confidence note: Recommended velocity ranges vary by duct application (residential vs. commercial, trunk vs. branch) and are driven by noise, energy, and space constraints rather than a single code-mandated limit — compare your result against your project's design criteria or ASHRAE/SMACNA guidance for the specific duct type.
What this calculation does not cover
- Airflow divided by area returns the average velocity across the whole opening, not the speed at any one point inside it — air runs faster down the centre of a duct than it does along the walls, so a single anemometer reading held mid-duct will sit above this figure.
- The relationship used is purely volumetric and carries no term for air density, so warm supply air, cold return air and the thinner air at altitude all report the same velocity while moving different masses of air past the same point.
- Whatever area you enter is treated as fully open. Internal insulation liner, duct board thickness and a balancing damper blade standing in that section all cut the free area, and because speed rises as area falls, the air squeezing past them is faster than the answer shown.
- Only the area reaches the arithmetic, never the proportions — a 20 by 8 duct and a 40 by 4 duct of identical square footage come back with exactly the same velocity. There is also no width-times-height helper on that field, so working the area out is yours to do and any slip in it carries straight into the velocity.
- The flow figure is entered once and held constant for the section being checked. Every takeoff upstream has already removed part of it, leakage removes more, and a blower on a lower speed tap or a loaded filter changes it again, so a long run has to be re-checked wherever the air it actually carries changes.
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This result is a specification — 899 ft/min — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Velocity (ft/min) = Airflow (CFM) ÷ Duct Cross-Sectional Area (sq ft), the basic continuity relationship. Typical recommended duct velocity ranges (widely-cited ASHRAE/SMACNA design guidance, not a hard code limit): residential supply trunk ~700-900 fpm, residential branch ~500-700 fpm, commercial main duct ~1000-1500+ fpm — actual limits are set by noise, energy, and space constraints on each specific project.
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