HVAC Duct Calculators

Duct component selection

Grille and Terminal Sizing Workbench

Use airflow, opening dimensions, and free-area data to check grille size and velocity.

Calculation inputs

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Choose a geometry check or check a curve copied from the exact product model, size and configuration. Missing curve data are never estimated.
Use the same intended duty point throughout the geometry comparison.
User-selected screening value, not a product rating.
Use the actual catalog free-area definition. Must be greater than 0 and no more than 1.

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Formula and source basis

Concepts behind this calculation

Estimate grille free/gross area, compare terminal opening geometry, or screen one exact product pressure curve against selected airflow limits.

A terminal has several meaningful areas: gross face, free opening, core and neck. Velocity must use the exact area definition attached to the product performance or selected criterion.

A geometric opening estimate cannot predict a product’s pressure curve. A curve check needs the exact model, size and configuration, with pressure points on a common rating basis.

How to use this calculator

  1. Choose free and gross grille area or required free area for preliminary sizing, the duct/terminal clear-area comparison for two clear areas, or the opening/duct diameter comparison for equivalent opening diameters.
  2. In the exact-product pressure-curve check enter design airflow, product source-defined reference area and its definition, plus the exact product reference and confirmed airflow/pressure points.
  3. Select the screening criteria and the applicable maximum pressure drop or reference-area velocity. Review only the criteria supported by that product data.

Calculation method and applicability

A_free = CFM / face velocity; A_gross = A_free/free-area ratio; V = CFM/A_clear; Δp(Q) is exact at supplied points and piecewise-linear only between adjacent points.

A_free and A_gross = free and gross face areas (ft²); V = velocity on the stated clear/reference area (FPM); Q = volume flow (CFM); Δp = positive pressure-drop magnitude (in. w.g.). The free-area ratio is a fraction, not a percentage.

Input definitions and units

Terminal check
Choose a geometry check or check a curve copied from the exact product model, size and configuration. Missing curve data are never estimated. Choices: Estimate free and gross grille area; Estimate required free area; Compare duct and terminal clear-area velocity; Compare opening and duct equivalent diameter; Check an exact product pressure curve.
Design airflow (CFM)
Use the same intended duty point throughout the geometry comparison. Applies to Terminal check: Estimate free and gross grille area or Estimate required free area or Compare duct and terminal clear-area velocity.
Selected face velocity (FPM)
User-selected screening value, not a product rating. Applies to Terminal check: Estimate free and gross grille area.
Product free-area ratio (fraction)
Use the actual catalog free-area definition. Must be greater than 0 and no more than 1. Applies to Terminal check: Estimate free and gross grille area.
Selected free-area velocity (FPM)
User-selected geometry input. Applies to Terminal check: Estimate required free area.
Duct clear area (ft²)
Clear internal area at the connection. Applies to Terminal check: Compare duct and terminal clear-area velocity.
Terminal clear area (ft²)
Use a documented clear/effective area for the actual terminal. Applies to Terminal check: Compare duct and terminal clear-area velocity.
Connecting duct equivalent diameter (in)
Geometric diameter only. Applies to Terminal check: Compare opening and duct equivalent diameter.
Terminal opening equivalent diameter (in)
Geometric comparison only; not effective area or pressure validation. Applies to Terminal check: Compare opening and duct equivalent diameter.
Design airflow (CFM)
Use an airflow within the exact product curve. No pressure is extrapolated beyond the entered endpoints. Applies to Terminal check: Check an exact product pressure curve.
Product source-defined reference area (ft²)
Copy the exact area basis used by the selected velocity criterion. Core, free, neck and gross areas are not interchangeable. Applies to Terminal check: Check an exact product pressure curve.
Reference area definition
Identify the matching manufacturer/test definition; the resulting FPM is named using this selected basis. Choices: Gross face area; Free area; Core area; Neck area; Other source-defined area. Applies to Terminal check: Check an exact product pressure curve.
Screen against
Optional limits are user-selected project criteria. Report-only mode does not imply pass/fail. Choices: Report source-supported point only; Maximum pressure drop; Maximum reference-area velocity; Both pressure and reference-area velocity. Applies to Terminal check: Check an exact product pressure curve.
Selected maximum pressure-drop magnitude (in. w.g.)
Enter a project/product comparison limit, not a universal criterion. Match the pressure definition used by the curve. Applies to Terminal check: Check an exact product pressure curve; Screen against: Maximum pressure drop or Both pressure and reference-area velocity.
Selected maximum reference-area velocity (FPM)
User-selected limit using the area basis selected above; it is not automatically a free-area, noise or comfort criterion. Applies to Terminal check: Check an exact product pressure curve; Screen against: Maximum reference-area velocity or Both pressure and reference-area velocity.
Exact product, size, configuration and curve source
Identify the manufacturer, model, nominal size, configuration, source page/revision and static-pressure basis. This entry is not authenticated. Applies to Terminal check: Check an exact product pressure curve.
Product curve confirmation
Confirm all points below belong to the same product, size and configuration, and their flow, area and pressure definitions match the labels above. Choices: Not confirmed; I confirmed the exact product curve basis. Applies to Terminal check: Check an exact product pressure curve.
Product airflow and static-pressure points (CFM, positive pressure-drop magnitude in. w.g.)
Enter 2–100 points from one exact product size/configuration. Convert source negative static pressure to positive loss magnitude; do not enter velocity pressure, total pressure, NC or a point from another size. Exact lookup or adjacent-point interpolation only. Applies to Terminal check: Check an exact product pressure curve.

How to interpret the result

Interpret area sizing as preliminary geometry. In product-curve mode retain the area basis, source bracket and separate pressure and velocity margins; a supported point does not establish throw or room sound.

Worked examples

Estimate free and gross grille area

Inputs: 400 CFM, selected 500 FPM and actual product free-area ratio 0.5.

Calculation: Free area 0.8 ft²; gross area 0.8/0.5.

Result: 0.8 ft² free and 1.6 ft² gross area.

Estimate required free area

Inputs: 400 CFM at selected 500 FPM.

Calculation: 400/500.

Result: 0.8 ft² required free area.

Compare duct and terminal clear-area velocity

Inputs: 400 CFM, duct area 0.667 ft² and terminal clear area 0.8 ft².

Calculation: Divide flow by each area.

Result: About 599.7 FPM duct and 500 FPM terminal.

Compare opening and duct equivalent diameter

Inputs: 8 in connecting duct and 7 in opening equivalent diameter.

Calculation: Compare only the entered geometry.

Result: Check opening size; this does not certify a product.

Check an exact product pressure curve

Inputs: Titus 350R/350F/350R-SS nominal 6×6 in source point: 114 CFM, 0.19 ft² core area, 0.073 in. w.g. negative static-pressure magnitude; user limits 0.08 in. w.g. and 650 FPM.

Calculation: Source table reports 600 FPM core velocity, 0.022 in. w.g. velocity pressure and 0.073 in. w.g. negative static pressure magnitude. CFM ÷ 0.19 ft² = 600 FPM; both entered limits are met at this exact point.

Result: Within entered limits: 0.073 in. w.g. at 114 CFM, 600 FPM at core area. Historic catalog point only; not a current product or installed-performance certification.

Assumptions and limits

  • Free area, gross face area, core area, Ak and neck area are not interchangeable. Use the exact product source definition selected for reference-area velocity.
  • Product curve points must match the same manufacturer model, nominal size, configuration, pressure definition, and test basis. The method uses exact points or disclosed piecewise-linear interpolation only between adjacent supplied points; no extrapolation or cross-size/accessory correction is performed.
  • Positive curve pressure inputs represent the magnitude of source negative static pressure, not velocity pressure or total pressure. Historical source results do not establish current product availability, recertification, installed pressure loss, sound, throw or certification.
  • Optional limits are supplied by the user. Meeting them does not establish a universal noise, comfort, code or engineering criterion.

FAQs

Can grille free-area ratio predict pressure drop?

It can convert free area to gross area for a preliminary opening estimate. Blade geometry, flow direction and product construction still require a matching pressure curve.

Why is product reference velocity different from duct velocity?

The product may rate gross face, core, free or neck area. The connecting duct uses its own clear area; equal CFM does not imply equal velocity at both planes.

Sources

Related calculators

Duct Size Calculator Use airflow with a velocity or friction target to calculate round or rectangular duct dimensions. Fan and System Curve Calculator Intersect entered fan data with a system resistance model to estimate the operating airflow and pressure.

Related reading

Select grilles, registers, and diffusers using free area, pressure, and throw Screen the opening geometry first, then use matching product data for pressure, noise, and throw before selecting a terminal. How to read a duct calculator result A useful result includes the number, its unit, its method, its assumptions, and the next check the number still needs. VAV terminal selection across operating states Check a terminal, controller, coil, pressure, and catalog-noise envelope across required states instead of selecting from airflow or inlet size alone.