HVAC Duct Size by CFM, Friction and Velocity Calculator

Calculator Input

Enter airflow, velocity goal, friction limit, and duct assumptions. The calculator then sizes a round or rectangular duct and checks the straight-run pressure drop.

Airflow (CFM)

Duct Shape

Target Velocity (FPM)

Max Friction (in.wg / 100 ft)

Straight Duct Length (ft)

Rectangular Aspect Ratio (W:H)

Rectangular Size Increment (in)

Air Density (kg/m³)

Dynamic Viscosity (Pa·s)

Internal Roughness (mm)

Use common round sizes

Formula Used

This calculator uses a practical straight-duct sizing sequence. It starts with airflow and target velocity, then checks friction rate and pressure drop.

Flow area: A = Q / V
Round duct diameter: D = √(4A / π)
Rectangular duct sizing: W × H = A with the selected aspect ratio
Hydraulic diameter: Dh = 4A / P
Reynolds number: Re = ρVDh / μ
Darcy friction factor: laminar or Swamee-Jain turbulent estimate
Pressure drop: ΔP = f × (L / Dh) × (ρV² / 2)

Friction is reported as inches of water gauge per 100 feet. Total straight-run drop is also calculated across the entered duct length.

How to Use This Calculator

Enter the design airflow in CFM.
Choose round or rectangular duct.
Set a target air velocity in FPM.
Enter the maximum allowable friction rate.
Provide the straight duct length to estimate pressure drop.
For rectangular ducts, enter the width-to-height ratio and increment.
Adjust air properties if your project differs from standard conditions.
Press calculate to see the recommended size, pressure drop, and graph.

Example Data Table

Case	Airflow (CFM)	Target Velocity (FPM)	Max Friction (in.wg / 100 ft)	Shape	Typical Output
Small branch	250	700	0.08	Round	8 in diameter
Office run	900	900	0.10	Round	14 in diameter
Main supply	1800	1000	0.08	Rectangular	24 in × 14 in
Large hall	3200	1100	0.09	Rectangular	32 in × 18 in

Frequently Asked Questions

1) What does this calculator actually size?

It sizes straight HVAC ducts from airflow, target velocity, and friction rate. It recommends a round diameter or rectangular width and height, then estimates straight-run pressure drop.

2) Why are velocity and friction both important?

Velocity affects noise, throw, and energy use. Friction affects pressure loss and fan demand. A good design usually balances both instead of chasing only one limit.

3) Is the pressure drop total for the whole air system?

No. It is the estimated straight-run pressure drop for the entered duct length only. Elbows, tees, dampers, transitions, terminals, and leakage add extra losses.

4) Why can rectangular ducts need more area?

Rectangular ducts often have more wetted perimeter for the same area. That can increase resistance, so the final rectangular size may grow beyond the first velocity-based estimate.

5) What is hydraulic diameter?

Hydraulic diameter converts non-round ducts into an equivalent flow diameter for friction calculations. It uses duct area and internal perimeter, not the outside dimensions.

6) Should I always pick the lowest friction result?

Not always. Very low friction can mean larger ducts, more material, and tighter installation conflicts. Designers usually balance noise, fan power, space, and cost together.

7) Can I use this for flexible duct?

Use caution. Flexible duct usually has much higher resistance than smooth sheet metal. Keep flex runs short and confirm losses with manufacturer data or project standards.

8) Are the defaults suitable for every job?

No. They are reasonable starting values only. Actual projects depend on system type, noise limits, air density, roughness, code requirements, and the selected design method.