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The required airflow for this duct run.
This typically comes from a room-by-room load calculation or the equipment's rated CFM divided across its duct runs.
The design airflow speed through the duct.
Lower velocities (500-700 fpm) reduce noise but need larger ducts; higher velocities (800-900+ fpm) allow smaller ducts but can be noisier — main trunk lines and branch runs are often designed differently.
Recommended round duct diameter
10.2 in (round duct diameter)
This sizes for velocity only — a full duct design also accounts for static pressure and total system friction loss (duct length, fittings, filters) using manufacturer duct-sizing charts.
- Required cross-sectional area
- 0.57 sq ft
They open the calculator with your figures already in it
HVAC Duct Size Calculator: 10.24 in (round duct diameter) — 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)
- Standard duct sizing formula: cross-sectional area (sq ft) = CFM / velocity (ft/min); round duct diameter = sqrt(4 x area / pi)
- Common design velocity for residential supply ducts is roughly 700-900 ft/min; return ducts are often designed slightly lower
Inputs used
- Airflow
- 400 CFM
- Target air velocity
- 700 ft/min
Intermediate steps
- Required cross-sectional area
- 0.57 sq ft
Confidence note: This sizes for velocity only — a full duct design also accounts for static pressure and total system friction loss (duct length, fittings, filters) using manufacturer duct-sizing charts.
What this calculation does not cover
- Velocity is the entire model. The diameter comes from airflow divided by the speed you type, with no friction rate, no duct length, no elbows or takeoffs and no filter or coil resistance in it, so a run sized here can still be one the blower cannot push its rated air through.
- This sizes one duct run; it is not a duct system design and not an ACCA Manual D. It produces no total effective length, no available static pressure and no friction rate, so where a permit, an energy-code compliance path or an equipment warranty calls for a documented duct design, this does not stand in for one.
- The answer is a theoretical diameter, not a size you can buy. Round pipe and flex come in fixed increments and nothing here steps the result onto one — take the next size up, because dropping to the size below raises velocity, noise and pressure loss at the same time.
- Duct material and condition are not part of the calculation. Flexible duct is rougher than smooth galvanised pipe at the same nominal diameter, and rougher again where it is left compressed, sagging or coiled between supports, so the same figure on this page does not deliver the same air in every material.
- Nothing checks the velocity you entered against a noise limit. The number is taken as a design choice, and whether the run will be audible in the room depends on the fittings, the register and how close the duct passes to occupied space — none of which this calculation sees.
Add the equipment this sizes
This result is a specification — 10.2 in (round duct diameter) — 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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations2
- Standard duct sizing formula: cross-sectional area (sq ft) = CFM / velocity (ft/min); round duct diameter = sqrt(4 x area / pi)
- Common design velocity for residential supply ducts is roughly 700-900 ft/min; return ducts are often designed slightly lower
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