Duct size calculator: round & rectangular
Enter airflow and design velocity to get the required area and round diameter. Fix a height for the ceiling space to get a rectangular size, rounded to sheet metal increments, with the equivalent round diameter and the actual velocity.
- Velocity method
- Round diameter
- Rectangular at fixed height
- Equivalent round
1 · Airflow
2 · Rectangular option
Velocity method: area = airflow ÷ velocity. Equivalent diameter of rectangular duct per the Huebscher equation (ASHRAE Fundamentals, Duct Design). Design velocity is an editable assumption; this is for estimating sizes, not a substitute for the engineer's duct design.
Live formula
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Copy these results, then add your approved sample files in the trial brief.
Design-build and checks
On plan-and-spec work, the engineer sizes the duct and the estimator takes it off. Estimators size duct themselves on design-build and design-assist projects, when pricing alternates, or when a drawing is missing sizes. The velocity method is the quick approach: required free area = airflow ÷ velocity. It is not a substitute for an engineered duct design, which also balances friction loss and fan pressure, but it gives a sound estimate of size, weight and cost.
- Airflow (CFM). From the equipment schedule or air distribution plan.
- Velocity (FPM). From the design criteria. Mains, branches and runouts typically use different velocities, and noise criteria may limit them.
- Fixed height. Ceiling space often limits duct depth; the width follows.
- Rounding. Sheet metal sizes usually go in 2 in increments.
Area to size
| CFM | At 800 FPM, round (in) | At 1,000 FPM, round (in) |
|---|---|---|
| 500 | 10.7 | 9.6 |
| 1,000 | 15.1 | 13.5 |
| 2,000 | 21.4 | 19.1 |
| 4,000 | 30.3 | 27.1 |
Equivalent diameter per the Huebscher equation (ASHRAE Fundamentals, Duct Design chapter). Design velocities are an engineering decision.
Size sets weight
Duct size drives sheet metal weight, gauge, insulation area and labor. Once sizes are set, the rectangular duct takeoff calculator turns them into surface area and pounds by gauge. Fittings, dampers, access doors and hangers follow the duct runs.
RenderDraw's AI takeoff proposes duct runs and sizes from mechanical plans with the sheet attached, and flags runs where it could not read a size.
Equipment counts and duct runs, checked before pricing.
RenderDraw reads mechanical plans with schedules and specs, proposes equipment counts, duct and pipe runs with the source attached, and puts uncertain items in front of your estimator first.
- Equipment from schedules and plans. Traceable to the source sheet.
- Duct and pipe runs. Proposed lengths your estimator accepts or corrects.
- Addenda through the same review. Send the changed sheets back through takeoff and estimator review.
- Your labor and pricing. Applied to reviewed rows only.
Send one mechanical set
Files stay private while RenderDraw assesses the request. No obligation.
RenderDraw does not publish accuracy percentages or time-saved claims. Test it on your own plan set.
Questions estimators ask.
How do you size a duct from CFM?
Divide the airflow by the design velocity to get the free area in square feet. For round duct, diameter = √(4 × area ÷ π) × 12 inches. 1,000 CFM at 1,000 FPM needs 1 sq ft, about 13.5 in round.
What velocity should I use?
Use the design criteria for the project. Mains, branches and runouts typically use different velocities, and acoustics may limit them.
How do I convert round duct to rectangular?
Use the same area at a fixed height for a quick estimate, then check the equivalent round diameter with the Huebscher formula.
Is this a duct design tool?
No. It is an estimating aid. Engineered duct design accounts for friction, fittings and fan pressure.
Can RenderDraw take off ductwork?
RenderDraw's AI takeoff proposes duct runs, sizes and fittings from drawings with sources and flags. Get a free sample takeoff.
Duct sizes from the drawings, not from a ruler.
Send a mechanical set. RenderDraw proposes duct runs and sizes with their sheets and flags runs it could not size.
- Upload the plans, specs and schedules, plus an addendum if you have one.
- RenderDraw reviews the set and the scope you want tested.
- We follow up with extracted quantities, flagged items and next steps.