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Duct Size Calculator — SMACNA / ASHRAE Velocity Method
Turn an airflow into a duct size the way low-pressure duct design actually works: pick the velocity ceiling the space's allowed noise sets, then size a spiral round duct or even-dimension rectangular options to match.
Airflow input
Airflow
Space type (allowed noise sets max velocity)
Duct shape
How the velocity method works
Velocity is the noise driver in low-pressure duct design — higher feet-per-minute means more regenerated noise at fittings — so the space's allowed NC (Noise Criteria) level sets the velocity ceiling. This calculator uses main-supply-duct limits keyed to space type: quiet spaces like bedrooms, classrooms, and private offices (NC 25-30) cap at 700 FPM; standard spaces like open offices and retail (NC 35-40) at 1,000 FPM; warehouses and industrial space (NC 45+) at 1,600 FPM. Required area is simply CFM divided by that velocity.
Round output rounds UP to the next even inch — spiral duct comes in even sizes. Rectangular options are generated as even-dimension pairs (4" to 48" per side, max 4:1 aspect ratio) whose ASHRAE equal-friction equivalent diameter matches the required round size; the formula De = 1.3(ab)^0.625/(a+b)^0.25 deliberately penalizes flat, high-aspect ducts for their extra perimeter friction. An optional max-depth constraint handles tight plenums.
If you don't know the CFM, the room-estimate mode multiplies room volume by a rule-of-thumb air-change rate (5 ACH quiet, 8 standard, 4 warehouse) and divides by 60. That is a total supply-circulation estimate, not the ASHRAE 62.1 outdoor-air method — real CFM comes from the mechanical schedule.
Worked example
2,000 CFM in a standard space: 2,000 ÷ 1,000 FPM = 2 ft², which needs a 19.15" diameter — so a 20" spiral duct, running about 917 FPM actual.
Frequently asked questions
What velocity should supply ducts run at?
It depends on the noise the space can tolerate: about 700 FPM for quiet rooms (NC 25-30, bedrooms and classrooms), 1,000 FPM for standard commercial space (NC 35-40), and up to 1,600 FPM in warehouses (NC 45+). These are main-duct design ceilings, not hard code limits.
How do I convert a round duct size to rectangular?
Match equivalent diameter, not area. The ASHRAE equal-friction formula De = 1.3(ab)^0.625/(a+b)^0.25 accounts for the extra perimeter friction of rectangular duct — which is why a flat 48×12 duct carries less air than its cross-section suggests.
Can I estimate CFM from room size?
As a rough check only: room volume × air changes per hour ÷ 60. This calculator uses 5 ACH for quiet spaces, 8 for standard, and 4 for warehouses. It is a circulation rule of thumb — the mechanical schedule is the real source of design CFM.
Which standards govern duct sizing?
The sizing method here follows ASHRAE Fundamentals' velocity-based approach; SMACNA duct construction standards govern how the duct itself is built — gauge, reinforcement, hangers, and sealing.
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