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Duct Area, Velocity and Pressure Budget Calculator

Calculate geometric duct area, check actual clear-dimension velocity and reconcile fan pressure, component losses and an equivalent duct path.

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Inputs that matter
Design airflow (CFM), Specified velocity ceiling (FPM), Proposed clear rectangular width (in), Sources and assumptions, and 9 more
Output to expect
Duct geometric area from airflow, Actual duct velocity check, Duct pressure and friction budget
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Duct geometric area from airflow

Calculate the free cross-sectional area corresponding to a design airflow and a separately chosen velocity ceiling. This geometric requirement is a starting point for a duct design; it does not include friction, fittings, noise, leakage or available static pressure.

Area in² = CFM ÷ FPM × 144; equal-area round diameter = √(4 × area ÷ π).

Actual duct velocity check

Calculate average air velocity in a documented clear round or rectangular section and compare it with your chosen design ceiling. Lining and other obstructions reduce the clear area; grille face dimensions are not automatically free duct area.

Round area = π × diameter² ÷ 4; rectangular area = width × height; FPM = CFM × 144 ÷ area in².

Duct pressure and friction budget

Subtract documented non-duct component losses from available fan pressure, then distribute the remaining allowance across the design path's total equivalent duct length. Component inclusions must match the fan-table pressure basis to avoid double counting.

Duct allowance = available fan pressure − sum(component losses); friction budget per 100 ft = positive duct allowance × 100 ÷ total equivalent length.

Updated: September 2026

Example Scenarios

The built-in example is an arithmetic illustration. Replace it with your own verified measurements or planning assumptions.

→ 64 in² minimum geometric area

The built-in example is an arithmetic illustration. Replace it with your own verified measurements or planning assumptions.

→ 733.386 FPM

The built-in example is an arithmetic illustration. Replace it with your own verified measurements or planning assumptions.

→ 0.35 in. w.c. duct allowance

FAQ

No. It returns a continuous geometry calculation without a capped stock-size table. Complete duct design also considers fittings, pressure, leakage, terminals, noise and installation constraints.

Use the actual source documents, units and operating conditions shown in the selected mode. Replace illustrative examples with your records, enter explicit zeros only where valid and retain unresolved assumptions.

Choose Save planning case for local history. Copy MD retains current inputs, notes and interpretation; Download CSV exports the result breakdown. Export a separate copy before changing devices or clearing browser storage.

About Duct Area, Velocity and Pressure Budget Calculator

Calculate the free cross-sectional area corresponding to a design airflow and a separately chosen velocity ceiling. This geometric requirement is a starting point for a duct design; it does not include friction, fittings, noise, leakage or available static pressure. Calculate average air velocity in a documented clear round or rectangular section and compare it with your chosen design ceiling. Lining and other obstructions reduce the clear area; grille face dimensions are not automatically free duct area. Subtract documented non-duct component losses from available fan pressure, then distribute the remaining allowance across the design path's total equivalent duct length. Component inclusions must match the fan-table pressure basis to avoid double counting.