--- title: "Duct Size Calculator | Ventilation & Airflow Calculator | XTRAW Tools" description: "Calculate required round or rectangular duct size for a given CFM. Use this free browser-based calculator from XTRAW Tools for ACH, CFM, duct sizing, ventilation rate, and indoor airflow estimates." url: "https://xtraw.com/tools/everyday-utilities/home-household-tools/duct-size-calculator" date: "2026-05-21T23:56:42+00:00" language: "en-GB" --- [Wind Chill Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/wind-chill-calculator)[Air Changes Per Hour Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/air-changes-per-hour-calculator) [![Duct Size Calculator from XTRAW Tools](https://xtraw.com/images/virtuemart/product/xtraw-duct-size-calculator-360x360.jpg)](https://xtraw.com/images/virtuemart/product/xtraw-duct-size-calculator-360x360.jpg "Duct Size Calculator from XTRAW Tools") new [Back to: Home & Household Tools](https://xtraw.com/tools/everyday-utilities/home-household-tools "Home & Household Tools") # Duct Size Calculator Calculate required round or rectangular duct size for a given CFM. Use this free browser-based duct size calculator from XTRAW Tools. **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](https://xtraw.com/tools/air-changes-per-hour-calculator) - [CFM Calculator](https://xtraw.com/tools/cfm-calculator) - [Ventilation Calculator](https://xtraw.com/tools/ventilation-calculator) - [HVAC Tonnage Calculator](https://xtraw.com/tools/hvac-tonnage-calculator) - [Room Size Calculator](https://xtraw.com/tools/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. **Related Products** ![Tooltip]() [![Air Changes Per Hour Calculator from XTRAW Tools](https://xtraw.com/images/virtuemart/product/resized/xtraw-air-changes-per-hour-calculator-360x360_230x230.jpg) Air Changes Per Hour Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/air-changes-per-hour-calculator "Air Changes Per Hour Calculator") Calculate air changes per hour (ACH) for a room given CFM flow rate. Use this free browser-based air changes per hour calculator from XTRAW Tools. Custom prototype for related products [![CFM Calculator from XTRAW Tools](https://xtraw.com/images/virtuemart/product/resized/xtraw-cfm-calculator-360x360_230x230.jpg) CFM Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/cfm-calculator "CFM Calculator") Calculate required airflow (CFM) for a room based on ACH or HVAC tonnage. Use this free browser-based cfm calculator from XTRAW Tools. Custom prototype for related products [![Ventilation Calculator from XTRAW Tools](https://xtraw.com/images/virtuemart/product/resized/xtraw-ventilation-calculator-360x360_230x230.jpg) Ventilation Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/ventilation-calculator "Ventilation Calculator") Calculate minimum ventilation CFM for a space based on area and occupancy. Use this free browser-based ventilation calculator from XTRAW Tools. Custom prototype for related products [![HVAC Tonnage Calculator from XTRAW Tools](https://xtraw.com/images/virtuemart/product/resized/xtraw-hvac-tonnage-calculator-360x360_230x230.jpg) HVAC Tonnage Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/hvac-tonnage-calculator "HVAC Tonnage Calculator") Calculate required HVAC tonnage for a home or commercial space. Use this free browser-based hvac tonnage calculator from XTRAW Tools. Custom prototype for related products [![Room Size Calculator from XTRAW Tools](https://xtraw.com/images/virtuemart/product/resized/xtraw-room-size-calculator-360x360_230x230.jpg) Room Size Calculator](https://xtraw.com/tools/everyday-utilities/home-household-tools/room-size-calculator "Room Size Calculator") Calculate room area, volume, and perimeter from dimensions. Use this free browser-based room size calculator from XTRAW Tools. Custom prototype for related products **Related Categories** ![Tooltip]()[![Calculators category image](https://xtraw.com/images/virtuemart/category/2_calculators.jpg) Calculators](https://xtraw.com/tools/calculators "Calculators") Custom prototype for related categories [![Construction & Home Calculators category image](https://xtraw.com/images/virtuemart/category/19_construction_and_home_calculators.jpg) Construction & Home Calculators](https://xtraw.com/tools/calculators/construction-home-calculators "Construction & Home Calculators") Custom prototype for related categories [![Construction Calculators category image](https://xtraw.com/images/virtuemart/category/158_construction-calculators.jpg) Construction Calculators](https://xtraw.com/tools/calculators/construction-home-calculators/construction-calculators "Construction Calculators") Custom prototype for related categories [![Ventilation Calculators category image](https://xtraw.com/images/virtuemart/category/266_ventilation-calculators.jpg) Ventilation Calculators](https://xtraw.com/tools/calculators/construction-home-calculators/construction-calculators/hvac-calculators/ventilation-calculators "Ventilation Calculators") Custom prototype for related categories [![Home & Household Tools category image](https://xtraw.com/images/virtuemart/category/165_home-household-tools.jpg) Home & Household Tools](https://xtraw.com/tools/everyday-utilities/home-household-tools "Home & Household Tools") Custom prototype for related categories ## Schema ```json { "@context": "https://schema.org", "@graph": [ { "@type": "Organization", "@id": "https://xtraw.com#organization", "name": "XTRAW Tools", "alternateName": "XTRAW", "url": "https://xtraw.com", "logo": { "@type": "ImageObject", "@id": "https://xtraw.com#logo", "url": "https://xtraw.com/images/xtraw-logo.png#joomlaImage://local-images/xtraw-logo.png?width=1100&height=400", "contentUrl": "https://xtraw.com/images/xtraw-logo.png#joomlaImage://local-images/xtraw-logo.png?width=1100&height=400", "caption": "XTRAW Tools" }, "image": { "@id": "https://xtraw.com#logo" }, "email": "jason@vuumarketing.com", "description": "XTRAW Tools provides free browser-based calculators, converters, generators, developer tools, business tools, file tools, writing tools, and everyday utilities.", "knowsAbout": [ "calculators", "online calculators", "unit converters", "file tools", "developer tools", "business calculators", "finance calculators", "math calculators", "education calculators", "health calculators", "construction calculators", "real estate calculators", "writing tools", "random generators", "everyday utilities" ], "contactPoint": { "@type": "ContactPoint", "contactType": "customer support", "email": "jason@vuumarketing.com", "url": "https://xtraw.com/contact" } }, { "@type": "WebSite", "@id": "https://xtraw.com#website", "name": "XTRAW Tools", "alternateName": "XTRAW Tools", "url": "https://xtraw.com", "publisher": { "@id": "https://xtraw.com#organization" }, "inLanguage": "en-gb", "about": { "@id": "https://xtraw.com#organization" } }, { "@type": "ImageObject", "@id": "https://xtraw.com/tools/everyday-utilities/home-household-tools/duct-size-calculator#primaryimage", "url": "https://xtraw.com/https://xtraw.com/images/virtuemart/product/xtraw-duct-size-calculator-360x360.jpg", "contentUrl": "https://xtraw.com/https://xtraw.com/images/virtuemart/product/xtraw-duct-size-calculator-360x360.jpg", "caption": "Duct Size Calculator", "representativeOfPage": true }, { "@type": "WebPage", "@id": "https://xtraw.com/tools/everyday-utilities/home-household-tools/duct-size-calculator#webpage", "url": "https://xtraw.com/tools/everyday-utilities/home-household-tools/duct-size-calculator", "name": "Duct Size Calculator | Ventilation & Airflow Calculator | XTRAW Tools", "headline": "Duct Size Calculator", "description": "Calculate required round or rectangular duct size for a given CFM. 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