Duct sizing and geometry

Duct Size Calculator

Use airflow with a velocity or friction target to calculate round or rectangular duct dimensions.

Calculation inputs

This calculator runs in your browser and requires JavaScript.

An example is loaded automatically. Replace it with your project values; the result updates as you edit.

Sizing method Choose a velocity method or an equal-friction reference method. The available-size method compares only the supplied set; theoretical modes remain separate. These are preliminary straight-duct methods, not complete system design.

Design airflow (CFM) Use airflow for the section being sized.

Selected target velocity (FPM) This is an explicit design input, not a universal standard limit.

Loading example values…

Result

No result yet

Results appear automatically when inputs are valid.

Formula and source basis

Concepts behind this calculation

Size theoretical ducts or compare an actual supplied size list against velocity, equivalent-round friction and installation-space limits. Round, rectangular and flat-oval choices share one sizing workflow.

Velocity sizing starts with the required flow area. Friction sizing instead finds a circular reference that meets a straight-duct pressure gradient; converting that reference to a rectangle or flat oval does not preserve cross-sectional area.

A theoretical size can fall between available products. Candidate selection must consider clear air dimensions separately from the installation envelope, since insulation, joints and access consume additional space.

How to use this calculator

Choose velocity sizing with design airflow and selected target velocity, or friction sizing with the selected straight-duct friction rate, operating air density, kinematic viscosity and documented absolute roughness.
For a rectangle supply the selected width-to-height ratio or fixed short side; for a flat oval supply the fixed inside minor axis. These are clear air-passage dimensions.
To select available sizes, document the available-size and installation basis and confirm candidate applicability. Enter supplied candidate sizes with separate clear and envelope dimensions, then choose the active selection criteria and applicable space and rotation constraints.
Review all rejected candidates as well as the selected geometry before transferring dimensions to a project schedule.

Calculation method and applicability

Velocity sizing: A = Q/V. Friction sizing: solve ΔP/L = f_DρV²/(2D) with Colebrook for a circular reference; convert to rectangular or flat oval using ASHRAE Chapter 21 Equations 25–28. Actual-section velocity uses Q divided by actual area.

Q = design airflow (CFM); V = selected velocity (FPM); A = required clear area (ft²). In the friction equation, f_D is dimensionless Darcy friction factor, ρ is operating air density and D is circular-reference diameter; use consistent units for ΔP/L. The form accepts density in lbm/ft³, viscosity in ft²/s and roughness in ft.

Input definitions and units

Sizing method: Choose a velocity method or an equal-friction reference method. The available-size method compares only the supplied set; theoretical modes remain separate. These are preliminary straight-duct methods, not complete system design. Choices: Round: airflow and target velocity; Rectangular: airflow, velocity and aspect ratio; Rectangular: velocity with a fixed short side; Round: airflow and target friction rate; Rectangular: equal-friction reference and aspect ratio; Flat oval: equal-friction reference and fixed minor axis; Select from supplied available sizes and installation envelopes.
Design airflow (CFM): Use airflow for the section being sized.
Selected target velocity (FPM): This is an explicit design input, not a universal standard limit. Applies to Sizing method: Round: airflow and target velocity.
Selected target velocity (FPM): This is an explicit design input, not a universal standard limit. Applies to Sizing method: Rectangular: airflow, velocity and aspect ratio.
Selected target velocity (FPM): This is an explicit design input, not a universal standard limit. Applies to Sizing method: Rectangular: velocity with a fixed short side.
Width-to-height ratio (:1): Defines one rectangular geometry from the many shapes with the same required area. Applies to Sizing method: Rectangular: airflow, velocity and aspect ratio.
Fixed short side (in): For space-constrained sizing, enter the clear inside short dimension. Applies to Sizing method: Rectangular: velocity with a fixed short side.
Maximum aspect ratio (:1): Selected geometric constraint, not a universal code requirement. Applies to Sizing method: Rectangular: velocity with a fixed short side.
Selected straight-duct friction rate (in. w.g./100 ft): Enter the project-selected circular-reference friction rate. This is not total pressure or a universal design rate; use a compatible pressure budget. Applies to Sizing method: Round: airflow and target friction rate.
Air density at the operating condition (lbm/ft³): Use density matching the stated airflow and operating point. No sea-level or altitude correction is inferred. Applies to Sizing method: Round: airflow and target friction rate.
Air kinematic viscosity (ft²/s): Use the same air state as the density. Kinematic viscosity is not dynamic viscosity. Applies to Sizing method: Round: airflow and target friction rate.
Documented absolute internal roughness (ft): Supply a value for the actual installed surface; no material preset is inferred. Zero represents a smooth-wall mathematical case. Applies to Sizing method: Round: airflow and target friction rate.
Selected straight-duct friction rate (in. w.g./100 ft): Enter the project-selected circular-reference friction rate. This is not total pressure or a universal design rate; use a compatible pressure budget. Applies to Sizing method: Rectangular: equal-friction reference and aspect ratio.
Air density at the operating condition (lbm/ft³): Use density matching the stated airflow and operating point. No sea-level or altitude correction is inferred. Applies to Sizing method: Rectangular: equal-friction reference and aspect ratio.
Air kinematic viscosity (ft²/s): Use the same air state as the density. Kinematic viscosity is not dynamic viscosity. Applies to Sizing method: Rectangular: equal-friction reference and aspect ratio.
Documented absolute internal roughness (ft): Supply a value for the actual installed surface; no material preset is inferred. Zero represents a smooth-wall mathematical case. Applies to Sizing method: Rectangular: equal-friction reference and aspect ratio.
Selected straight-duct friction rate (in. w.g./100 ft): Enter the project-selected circular-reference friction rate. This is not total pressure or a universal design rate; use a compatible pressure budget. Applies to Sizing method: Flat oval: equal-friction reference and fixed minor axis.
Air density at the operating condition (lbm/ft³): Use density matching the stated airflow and operating point. No sea-level or altitude correction is inferred. Applies to Sizing method: Flat oval: equal-friction reference and fixed minor axis.
Air kinematic viscosity (ft²/s): Use the same air state as the density. Kinematic viscosity is not dynamic viscosity. Applies to Sizing method: Flat oval: equal-friction reference and fixed minor axis.
Documented absolute internal roughness (ft): Supply a value for the actual installed surface; no material preset is inferred. Zero represents a smooth-wall mathematical case. Applies to Sizing method: Flat oval: equal-friction reference and fixed minor axis.
Selected rectangular width-to-height ratio (:1): Applied to the equal-friction round diameter. The actual rectangular velocity is calculated from its own area. Applies to Sizing method: Rectangular: equal-friction reference and aspect ratio.
Fixed flat-oval inside minor axis (in): Diameter of the semicircular ends. If no major axis at least this large satisfies the reference diameter, a result is withheld. Applies to Sizing method: Flat oval: equal-friction reference and fixed minor axis.
Available-size and installation basis: Identify the actual supplier/fabrication schedule, clear inside dimensions, any liner effects, outside envelope including insulation/flanges/supports/project clearance, airflow and operating properties. A reference does not authenticate availability. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Candidate and project applicability: Confirm the supplied candidate set, actual clear dimensions, installed envelopes and project criteria. No universal stock series, aspect limit, roughness or design velocity is selected automatically. Choices: Not established; Documented candidates, installation envelopes and project criteria established. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Active selection criteria: Velocity-only makes no pressure claim. Friction uses an equivalent-round straight-duct estimate; both reference and actual section must be turbulent. Choices: Both maximum velocity and maximum friction rate; Maximum velocity only; Maximum equivalent-round friction rate only. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Maximum actual-section velocity (FPM): A positive project criterion, not a universal noise or code limit. Applies to Sizing method: Select from supplied available sizes and installation envelopes; Active selection criteria: Maximum velocity only or Both maximum velocity and maximum friction rate.
Maximum equivalent-round friction rate (in. w.g./100 ft): Positive project straight-duct criterion; fittings and components are excluded. Applies to Sizing method: Select from supplied available sizes and installation envelopes; Active selection criteria: Maximum equivalent-round friction rate only or Both maximum velocity and maximum friction rate.
Actual air density (lbm/ft³): Matches actual airflow and operating condition; no altitude correction is inferred. Applies to Sizing method: Select from supplied available sizes and installation envelopes; Active selection criteria: Maximum equivalent-round friction rate only or Both maximum velocity and maximum friction rate.
Actual air kinematic viscosity (ft²/s): Matches the same air state; not dynamic viscosity. Applies to Sizing method: Select from supplied available sizes and installation envelopes; Active selection criteria: Maximum equivalent-round friction rate only or Both maximum velocity and maximum friction rate.
Documented absolute internal roughness (ft): Actual surface input; zero is a mathematical smooth wall, not a material preset. Applies to Sizing method: Select from supplied available sizes and installation envelopes; Active selection criteria: Maximum equivalent-round friction rate only or Both maximum velocity and maximum friction rate.
Available installation width (in; 0=no width constraint): Compare the entered outside installation envelope, not clear inside flow width. Zero explicitly disables this constraint. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Available installation height (in; 0=no height constraint): Envelope must already include actual insulation, fittings/flanges, supports and project clearance. Zero explicitly disables this constraint. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Maximum clear-section aspect ratio (:1; 0=no aspect constraint): Largest clear dimension divided by smallest; round=1. Use an actual project limit or zero for no limit, not a presumed universal standard. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Installation orientation: Allow a 90-degree envelope rotation only if the actual product/connection/support arrangement permits it. Original orientation is retained when already feasible. Choices: Keep each supplied orientation; Allow a 90-degree rotation when needed to fit. Applies to Sizing method: Select from supplied available sizes and installation envelopes.
Supplied candidate sizes (ID, shape, clear width/diameter, clear height, envelope width, envelope height; in): No header. Shape 0=round (height=0), 1=rectangle, 2=flat oval (major then minor). Envelope dimensions are separate actual outside/clearance dimensions and must be at least the corresponding clear dimensions; round envelope must cover its diameter in both directions. Unique IDs 1–10000; at most 100 rows. Do not infer stock from these teaching rows. Applies to Sizing method: Select from supplied available sizes and installation envelopes.

How to interpret the result

Read theoretical dimensions as a sizing target. In candidate mode inspect each velocity, friction, envelope and aspect constraint; a fit does not establish fitting losses or acoustic suitability.

Worked examples

Round: airflow and target velocity

Inputs: 400 CFM at a selected 700 FPM.

Calculation: A=400/700 ft², then solve the equal-area circle.

Result: Theoretical diameter 10.2 in; recheck nominal size and pressure.

Rectangular: airflow, velocity and aspect ratio

Inputs: 400 CFM, 700 FPM and W:H=2:1.

Calculation: Required area 0.571 ft² split in a 2:1 ratio.

Result: About 12.8 × 6.4 in theoretical dimensions.

Rectangular: velocity with a fixed short side

Inputs: 400 CFM, 700 FPM, fixed short side 8 in, maximum 4:1.

Calculation: Required area determines a 10.3 in long side.

Result: About 10.3 × 8 in, ratio 1.29:1.

Round: airflow and target friction rate

Inputs: Illustrative case: 1,000 CFM; selected rate 0.10 in. w.g./100 ft; density 0.075 lbm/ft³; kinematic viscosity 0.00016 ft²/s; roughness 0.0003 ft.

Calculation: Solve Darcy-Weisbach and Colebrook together for the circular reference diameter, retaining the supplied air/surface properties.

Result: 13.646 in theoretical round diameter; about 984.609 FPM; Re≈116,631 and Darcy f≈0.01885. The inputs are example values, not defaults or design recommendations.

Rectangular: equal-friction reference and aspect ratio

Inputs: Same illustrative 1,000 CFM, 0.10 in. w.g./100 ft, density 0.075 lbm/ft³, kinematic viscosity 0.00016 ft²/s and roughness 0.0003 ft; selected rectangular ratio 2:1.

Calculation: Solve the 13.646 in circular reference, then invert ASHRAE Equation 25 at the selected ratio.

Result: 17.915 × 8.958 in theoretical rectangle. Actual rectangular velocity≈897.299 FPM; circular-reference velocity≈984.609 FPM. The reference friction rate is 0.10 in. w.g./100 ft.

Flat oval: equal-friction reference and fixed minor axis

Inputs: Same illustrative 1,000 CFM, 0.10 in. w.g./100 ft, density 0.075 lbm/ft³, kinematic viscosity 0.00016 ft²/s and roughness 0.0003 ft; fixed flat-oval minor axis 10 in.

Calculation: Solve the circular reference, then solve ASHRAE Equation 26 for a feasible major axis using the flat-oval area and perimeter.

Result: 17.117 × 10 in theoretical flat oval; actual-section velocity≈961.855 FPM. This is a straight-duct equivalent-round estimate, not a nominal fabrication selection.

Select from supplied available sizes and installation envelopes

Inputs: Constructed six-candidate list at 1,000 CFM, maximum 900 FPM and 0.10 in. w.g./100 ft. Density is 0.075 lbm/ft³, kinematic viscosity 0.00016 ft²/s and roughness 0.0003 ft. Available installation space is 26 × 20 in; aspect limit is 4:1 and rotation is disabled. Each outside envelope is explicitly supplied; this is not a stock catalog.

Calculation: Candidate 6 is a 20 × 10 in flat oval with 22 × 12 in installation envelope. Its clear area is 1.239860 ft², actual velocity 806.542781 FPM and equivalent-round friction rate 0.067406031 in. w.g./100 ft. It is the smallest eligible clear area in the supplied set.

Result: Candidate 5, a 20 × 8 in rectangle, reaches exactly 900 FPM but exceeds the friction criterion at 0.106156837 in. w.g./100 ft. Friction-only would select candidate 2 despite its velocity exceeding 900 FPM. Restricting installation width to 20 in changes the combined-criterion selection to candidate 4. These are project-criterion comparisons, not inventory, noise or whole-system approval.

Assumptions and limits

The original six methods return theoretical sizes. The supplied-size method selects only among explicitly entered candidates; it does not round to or authenticate a stock schedule. Installation fit uses the separately supplied outside envelope, not the clear flow dimensions.
Friction sizing solves a circular reference with explicit density, kinematic viscosity and absolute roughness. The reference and actual section must be turbulent (Re≥4,000); low-speed incompressible air is assumed. Actual-section Reynolds number uses its own area and hydraulic diameter.
Rectangular and flat-oval sizes use ASHRAE equivalent-round correlations at equal airflow and straight-duct friction. Their actual area and velocity differ from the circular reference. A direct hydraulic-diameter pressure calculation is a separate model and can produce a different value.
Available candidates are ranked by minimum eligible clear area, then lowest ID. This is an explicit site ranking within the entered set, not lowest cost, energy, noise or a global system optimum. Unsupported friction candidates retain geometry but receive no invented rate or pass. Velocity-only makes no friction claim.
The selected friction rate does not automatically limit velocity or noise. Review actual-section velocity using the project criteria, and include fitting/component losses separately in the pressure budget.
Flat oval means straight sides joined by semicircular ends. An infeasible fixed minor axis is rejected instead of producing a major axis smaller than the minor axis.
Flexible-duct compression/sag, product-specific corrections, system balancing, fan operating point, fittings and full ACCA Manual D or code compliance remain outside this calculation. Supply/return roles do not provide universal design limits.

FAQs

Why do velocity and friction methods produce different sizes?

They constrain different quantities. Velocity limits Q divided by actual area; friction also depends on roughness, viscosity, density and the selected straight-section model.

Can I round a theoretical diameter down?

A smaller clear section raises velocity and can exceed the selected friction criterion. Use supplied available candidates and review every active criterion before choosing a stock size.

Sources

ASHRAE 2025 Fundamentals, Chapter 21: Duct DesignEquations 18–19 Darcy/Colebrook and Equations 25–28 equivalent-round geometry were directly inspected. Friction sizing solves the circular reference, then converts shape; no restricted fitting or flex table is reproduced.
Vortia 3D: Duct pressure drop calculator (Colebrook)Inverse check of its published round-duct example: 1,000 m³/h, 250 mm, roughness 0.09 mm, 20°C, reported rate 1.54 Pa/m. This is a pressure-example inversion, not a directly observed sizing output.
aNOWysis: Duct SizingPeer controls expose design friction, air properties and roughness; used as task evidence, not formula authority.

Related calculators

Duct Airflow and Velocity Checker Enter duct dimensions and CFM or air speed to solve for airflow, velocity, or capacity. Duct Shape Converter Convert round, rectangular, and flat-oval dimensions using the selected area or friction basis. Available Static Pressure and Friction Budget Subtract component losses from available pressure and use route length to find the friction budget. Duct Pressure Drop Calculator Enter duct dimensions, airflow, and length to estimate friction and documented fitting pressure losses.

Find a tool.