Duct Size Calculator: Round and Rectangular Air Ducts

Enter airflow or choose a preset to compare round duct diameters, air velocity, calculated straight-duct friction, and rectangular alternatives. The selected diameter is highlighted in the circle row, including sizes above 16 inches, so you can see how greater airflow changes the cross-section needed. The screening size uses limits of 900 FPM and 0.08 in. w.g. per 100 ft. For long runs and flex duct, check the friction rate against the available pressure budget and use the flex manufacturer resistance data.

Airflow, diameter and material determine straight-duct velocity and friction. This tool selects the smallest listed round size meeting both assumed limits: friction no greater than 0.08 in. w.g./100 ft and velocity no greater than 900 FPM. It does not solve a complete network or let you set the allowable friction rate. Rectangular candidates are screened separately for equal-resistance diameter, actual-area velocity and an aspect ratio no greater than 4:1.

These are selected results at specific airflows, not maximum capacities for each diameter. Every row is computed with the same shared function as the tool. The rectangular option is its first candidate, not a guarantee that it fits your chase or meets a quieter branch velocity target.

• Using an assumed friction target without calculating available static pressure and total effective length
• Ignoring fittings — use the resistance or equivalent length for the actual elbow, takeoff and transition geometry
• Applying straight-metal results to flex — use product data and account for compression, sag and bends
• Sizing only the supply — include the return ducts, filters, grilles and closed-door transfer paths in the pressure budget
• Leaving no balancing strategy — provide accessible adjustment where needed and verify room delivery after installation

Duct sizing follows the load and equipment decisions. ACCA Manual J establishes room heating and cooling loads; Manual S selects equipment using those loads and manufacturer performance; Manual D designs the supply and return distribution. Equal-friction sizing is one duct-design method, but matching straight-duct friction rates does not automatically balance paths with different lengths and fittings. Branch flows still need allocation and commissioning. This tool estimates straight galvanized-duct loss and a rectangular equal-resistance diameter; it is a preliminary dimension check inside that larger workflow. The ASHRAE Handbook duct-design chapter explains the pressure-loss relationships and equivalent-diameter approach used for these checks.

The allowable friction rate comes from a pressure budget, not a preferred round number. Establish the blower's available external static pressure at the required airflow, then account for the components inside and outside the manufacturer's rating boundary. Do not subtract an internal coil twice if its resistance is already included in the rated blower data. Available static pressure for the ducts divided by the critical path's total effective length, multiplied by 100, gives a planning friction rate per 100 ft. Fittings add resistance through equivalent length or loss coefficients. The 0.08 assumption here may be above or below your actual budget, so a result can satisfy this calculator and still fail the project design.

A trunk and a branch carry different airflows. The main supply begins with blower flow, then its airflow drops as branches take air off; each branch should be sized for its assigned room demand. Residential planning ranges often place trunks around 700–900 FPM and quiet branches nearer 600 or below, but acceptable speed depends on fittings, duct construction and terminal sound criteria. The 900-FPM screen here does not guarantee quiet branches. Sheet metal and flex also need separate resistance assumptions. Fully extended flex performs differently from compressed or sagging flex, and a one-size increase cannot reliably offset arbitrary installation defects. Plan routing, supports and bends before choosing its diameter.

Return paths deserve the same pressure analysis as supplies. Restrictive returns, loaded filters and small grilles can limit delivery even when the supply trunk is large enough. Check whether air can leave each supplied room when its door is closed, then size return grilles and transfer paths for airflow and sound. Multiple returns may help distribution, but they are not an automatic requirement or substitute for calculation. Use the static pressure calculator to organize a pressure check and the CFM calculator to establish a reference airflow. Final delivery depends on the blower curve and all system resistances, not one duct segment.

Resistance rises sharply when diameter is reduced at a fixed airflow, but a single area-change multiplier does not describe every duct. Diameter, velocity, hydraulic behavior and roughness interact, and fittings can dominate a short run. A constant-airflow blower may consume more power while trying to maintain flow; other blower types may simply deliver less. Rectangular ducts with equal area can have different equivalent diameters, so compare both resistance and actual-area velocity. High aspect ratios can be harder to route, reinforce and seal, but shape alone does not establish leakage or noise. See the guide to sizing ductwork, then verify airflow and static pressure after installation.

This tool selects the smallest listed round diameter meeting both 0.08 in. w.g. per 100 ft straight-duct friction and 900 FPM velocity. It computes Darcy-Weisbach pressure loss with the Haaland friction approximation, standard-air density 0.075 lb/ft^3, kinematic viscosity 0.000163 ft^2/s, and galvanized-duct roughness 0.0003 ft. Friction displayed is calculated for the selected size, not the target. Large diameters require fabrication and a project-specific duct design.

At the default 400 CFM, the selected round diameter is 12 inches. Area = π × (12 / 24)² = 0.7854 sq ft, so velocity = 400 / area, approximately 509 FPM. Calculated straight-duct friction is 0.035 in. w.g./100 ft, below the assumed 0.08 target. At 1,200 CFM, the same checks select 16 inches, 859 FPM and 0.063 in. w.g./100 ft.

The selected stock step passes both limits; a velocity-only calculation can suggest a smaller size that fails friction. Conversely, the displayed friction is often well below 0.08 because the next listed diameter is larger than the mathematical minimum. Run length is not an input: multiply straight friction by length / 100, then add fitting and component losses before comparing with available fan pressure.

Rectangular options use De = 1.30 × (a × b)^0.625 / (a + b)^0.25, with sides in inches. They must have equivalent diameter at least as large as the selected round diameter, aspect ratio no greater than 4:1, and actual-area velocity no greater than 900 FPM. The first 400-CFM candidate is 16 × 8 inches, with actual-area velocity about 450 FPM. Equal-resistance diameter is different from equal area.

Measure clear inside dimensions rather than outside dimensions of insulated duct. Check whether lining, transitions and structural constraints change the usable flow area. Under Manual D, determine allowable friction from available static pressure and total effective length after accounting for filters, coils, grilles and fittings within the proper equipment rating boundary; 0.08 is not universal.

Find the airflow to size a duct.

The ASHRAE Handbook describes duct pressure loss and equal-resistance diameter conversion. ACCA Manual D addresses the residential system pressure budget.

ASHRAE Handbook — Fundamentals: duct design. For the load-calculation workflow, read how to calculate HVAC load.