# Duct / CFM Sizing Calculator — Round & Rectangular > Free duct size calculator: turn required airflow (CFM or L/s) into a buyable round duct diameter and equivalent rectangular options, with a velocity cross-check and guard, editable friction rate and velocity caps, a tonnage-to-CFM helper, metric or imperial — all in your browser. Size supply and return ducts from required airflow — a buyable round diameter, equivalent rectangular options and a velocity check, in metric or imperial. A rule-of-thumb estimate, not a full ductulator or Manual-D design. - **HTML page:** https://siteometry.com/en/tools/duct-size-calculator - **Category:** [Decorating & Interiors](https://siteometry.com/en/tools/category/decorating-interiors) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## How to size a duct with this calculator 1. **Choose units and a sizing method** — Pick metric or imperial and equal-friction (the default) or the velocity method. 2. **Add your runs** — Enter each run's required airflow and its role — branch, trunk or return. Add a row per room for a whole-home schedule. 3. **Tune the assumptions** — Set the friction rate, velocity caps, aspect-ratio cap and a preferred rectangular height to match your own design targets and framing depth. 4. **Read the sizes** — Read each run's round diameter, equivalent rectangular options and velocity with its guard tier; the rail sizes the main trunk from the summed branch airflow. _On the page: https://siteometry.com/en/tools/duct-size-calculator#howto_ ## Duct / CFM sizing calculator Sizing a duct starts with airflow: how many cubic feet per minute (CFM), or litres per second, the run has to carry. Too small and it whistles, starves the room and runs the static pressure up; too large and it wastes sheet metal and space in the framing. This calculator turns a required airflow and a duct role — branch, trunk or return — into a buyable round diameter, a set of equivalent rectangular options for a fixed framing depth, and the resulting air velocity checked against a recommended cap. Enter several runs and it also sizes the main trunk from the summed branch airflow. Everything runs in your browser. _On the page: https://siteometry.com/en/tools/duct-size-calculator#duct-intro_ ## How to size a duct Pick your units and a sizing method, enter each run's airflow and role, and read the round diameter, rectangular options and velocity. The friction rate, velocity caps and every curve-fit constant are editable assumptions, so you can match your own design targets rather than a fixed rule of thumb. The default method is equal-friction: every duct is sized to the same pressure loss per unit length, the standard residential trade approach. The alternate method sizes straight from a target velocity instead. _On the page: https://siteometry.com/en/tools/duct-size-calculator#how-to-size_ ## The equal-friction round size The round diameter comes from the Huebscher equal-friction relation, which ties airflow and a design friction rate to a duct diameter. The default friction rate is 0.08 in.wc per 100 ft, a common residential design value. The raw diameter is then snapped up to the nearest buyable size on a manufacturer ladder — never down, because undersizing is the worse failure. Flex duct is rougher than smooth sheet metal, so when the material is set to flex the calculator upsizes the raw diameter before snapping. That multiplier is a labelled estimate you can adjust. _On the page: https://siteometry.com/en/tools/duct-size-calculator#round-sizing_ ## Equivalent rectangular ducts When a round duct won't fit the framing, the calculator offers rectangular ducts that move the same air. It uses the ASHRAE equivalent-diameter relation and, for each standard height, solves the width that matches the round target, rounding the width up to the nearest increment so the option is never undersized. Options whose aspect ratio exceeds the cap are dropped, because very flat ducts are inefficient and hard to seal. Set a preferred height to flag the row that matches your joist or soffit depth. _On the page: https://siteometry.com/en/tools/duct-size-calculator#rectangular_ ## The velocity cross-check Whatever size you land on, the air still has to move through it at a sensible speed. The calculator computes the velocity at the snapped diameter and compares it to the role's cap — branches quieter than trunks, returns in between — then flags it green when comfortable, amber near the cap and red when over. High velocity means noise and pressure loss; very low velocity can mean an oversized, wasteful duct. _On the page: https://siteometry.com/en/tools/duct-size-calculator#velocity-check_ ## Airflow from tonnage When you don't have a room-by-room airflow, you can estimate a total from the cooling capacity. The standard rule of thumb is about 400 CFM per ton of cooling. Enter the capacity and the airflow-per-capacity figure and the helper gives a total airflow you can drop straight onto a run to size the trunk. _On the page: https://siteometry.com/en/tools/duct-size-calculator#tonnage-to-cfm_ ## Metric and imperial Switch units at the top and every field, result and label follows. Imperial reads airflow in CFM, diameters in inches, friction in in.wc per 100 ft and velocity in feet per minute; metric reads litres per second, millimetres, pascals per metre and metres per second. Your entries convert once on the switch, so nothing drifts as you keep editing. _On the page: https://siteometry.com/en/tools/duct-size-calculator#metric-imperial_ ## Frequently asked questions ### What size duct do I need for a given CFM? Enter the airflow and role and the calculator returns a buyable round diameter sized to the design friction rate, plus equivalent rectangular options. As a rough guide at 0.08 in.wc per 100 ft, a 100 CFM branch is about a 6 in round and a 400 CFM run about a 10 in round — but the exact size depends on your friction rate and material, which are editable here. ### Should I size by friction rate or velocity? Equal-friction is the standard residential method and the default: every duct shares the same pressure loss per unit length. The velocity method sizes straight from a target speed and is handy when noise is the binding constraint. Either way the calculator always cross-checks the resulting velocity against the role cap. ### How do I convert a round duct to rectangular? The calculator uses the ASHRAE equivalent-diameter relation: for each standard height it finds the width that carries the same air as the round duct, rounds it up to a buyable increment and drops options that are too flat. Pick the row whose height matches your framing depth. ### Is this a Manual-D design? No. This is a rule-of-thumb sizing estimate, not a full static-pressure ductulator or an ACCA Manual-D duct design, which accounts for total effective length, fittings, registers and the equipment's available static pressure. Use this to sanity-check a size, then verify with an HVAC professional before fabrication. _On the page: https://siteometry.com/en/tools/duct-size-calculator#faq_ ## Related calculators - [BTU Calculator — AC Tonnage & Heater Sizing](https://siteometry.com/en/tools/btu-calculator) - [AC Unit Size Calculator — What Size Air Conditioner Do I Need?](https://siteometry.com/en/tools/ac-unit-size-calculator) - [Paint Coverage & Cost Calculator](https://siteometry.com/en/tools/paint) - [Tile & Flooring Calculator](https://siteometry.com/en/tools/tile)