Duct Size Calculator

Duct Size Calculator helps you calculate required round or rectangular duct size for a given CFM. Use this free browser-based calculator from XTRAW Tools to compare inputs, estimate results, and plan your next step.

How to Use the Duct Size Calculator

1. Enter the required calculator inputs shown in the form.
2. Review the roofing, gutter, drainage, ramp, tile, grout, BTU, HVAC, room-size, airflow, duct, or ventilation assumptions.
3. Run the calculator, then adjust one input at a time to compare scenarios or check your estimate.

What This Calculator Helps With

Calculate required round or rectangular duct size for a given CFM. It is designed for contractors, homeowners, builders, remodelers, roofers, HVAC users, tile installers, accessibility planners, DIY users, and construction planners who want a fast browser-based estimate before relying on supplier charts, contractor estimates, engineering plans, HVAC design software, or professional sources.

• Airflow (CFM): 400 CFM
• Air Velocity: 700 FPM
• Duct Shape: round

Common Uses

People use this calculator when they need a quick way to estimate ACH, CFM, duct size, airflow, ventilation rate, and indoor-air-quality assumptions.

• Measurement: Use dimensions, surface area, slopes, grade, pitch, room volume, BTU capacity, airflow, and project measurements to calculate a practical result.
• Project Planning: Plan job scope, material quantities, comfort assumptions, airflow needs, slope requirements, and construction details before starting a project.
• Material Estimating: Estimate materials, waste factors, bundles, downspouts, tiles, grout, ventilation equipment, duct sizes, and project quantities before ordering supplies.

Formula or Method

Area (ft²) = CFM / velocity (FPM); Round: D = √(4A/π)

Example

Example inputs: Airflow (CFM): 400 CFM; Air Velocity: 700 FPM; Duct Shape: round

Example result: Using the default inputs, required duct area is 0.571 ft²; round duct diameter is about 10.24 in, or equal-side rectangular duct about 9.07 in per side.

Related Tools

• Air Changes Per Hour Calculator
• CFM Calculator
• Ventilation Calculator
• HVAC Tonnage Calculator
• Room Size Calculator

Duct Size Calculator FAQ

Is the Duct Size Calculator free?

Yes. This calculator is free to use on XTRAW Tools.

Do I need to create an account?

No. XTRAW tools are designed for browser-based use without requiring an account.

Are the results exact?

Results may be estimates depending on the inputs, formula, rounding, project assumptions, waste factor, coverage rate, airflow method, sizing rule, supplier specifications, and purpose of the calculator. Always verify important results before relying on them.

Disclaimer: This calculator is provided for general informational, educational, and convenience purposes only. It is not construction, engineering, HVAC design, accessibility compliance, safety, building-code, structural, roofing, drainage, ventilation, contractor, supplier, legal, financial, or professional advice. Always verify important results with a qualified professional, contractor, engineer, HVAC professional, supplier, manufacturer documentation, local code authority, or authoritative source before making decisions.