HVAC & Refrigeration
Duct Sizing (Velocity Method)
Required duct area and round-duct diameter from airflow and a target design velocity. Target velocity is a design choice driven by noise and pressure-drop limits, not a fixed law — typical guidance is roughly 700–1200 fpm (3.5–6 m/s) for main trunk ducts and lower for branches near occupied, noise-sensitive spaces.
Standard method — stated scope limits apply
Units & Flow
Enter airflow and target velocity.
About This Calculation
Duct sizing by the velocity method is the simplest, most direct approach: pick a target air velocity appropriate for the duct’s location and noise sensitivity, then size the duct’s cross-section so the actual airflow achieves that velocity. It’s the starting point every more detailed method (equal-friction, static regain) builds on.
Frequently Asked Questions
What velocity should I actually design for?
It depends entirely on where the duct runs. Main trunk ducts in mechanical rooms commonly run 1000–1500+ fpm (5–7.5 m/s) where noise matters less; branch ducts near occupied, noise-sensitive spaces are usually kept lower, often 600–900 fpm (3–4.5 m/s), to avoid audible airflow noise. There is no single universal number — it’s a design trade-off between duct size (cost, space) and noise/pressure drop.
Why does a round duct diameter come out of an area calculation?
The velocity method only tells you the required cross-sectional AREA (Area = Flow ÷ Velocity) — converting that to a round duct diameter is just solving the circle-area formula backwards. If you need a rectangular duct instead, the required area is the same; you just choose a width and height whose product matches it.
Does a higher velocity always mean a smaller, cheaper duct?
A smaller duct, yes — cheaper isn’t guaranteed, since higher velocity also means more pressure drop (bigger fan, more energy) and more noise. Real duct design balances all three, not just minimizing duct size.
Is this the same as a full ASHRAE duct design (equal friction or static regain method)?
No — those methods size an entire duct SYSTEM (multiple branches) to a consistent friction rate or pressure profile across the whole network. This calculator sizes one duct section for one target velocity, which is the underlying building block those fuller methods use repeatedly.