Plenum Box Size Calculator
Sizes a supply or return plenum box (cross-section, width/height, length and depth) from airflow and target face velocity for AHU/FCU duct systems.
Inputs
Result
Recommended plenum size (W × H × L)
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Required area
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Actual velocity
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Aspect ratio W:H
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Recommended length
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Depth (fan check)
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Velocity range target
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Recommended next engineering action
Enter airflow and velocity, then press Calculate.
Formulas & Variables
A = Q / V
W × H = A (solve W from fixed H, or W = √(A × ratio), H = W / ratio)
L = multiplier × W (≥ minimum length past last takeoff); D ≈ 2.5 × d
| Symbol | Meaning | Unit basis (Imperial) | Unit basis (Metric) |
|---|---|---|---|
| Q | Design airflow | CFM (ft³/min) | m³/h → converted to m³/s |
| V | Target face velocity in plenum | FPM (ft/min) | m/s |
| A | Required cross-sectional area | ft² (×144 for in²) | m² |
| W, H | Plenum width and height | in | mm |
| L | Plenum length | in | mm |
| D | Plenum depth (entrance settling length) | in | mm |
| d | Blower wheel/outlet diameter | in | mm |
Step 1. Convert airflow and velocity to consistent units.
Step 2. Compute required cross-section A = Q / V.
Step 3. Solve W and H (fixed height or fixed W:H ratio, target 2–3:1).
Step 4. Set length L = 1–3×W (2×W preferred), ≥ minimum length past last takeoff.
Step 5. Optionally check depth D ≈ 2.5×d against available install space.
Assumptions & Limits
Design conditions / defaults
- Rectangular sheet-metal plenum, straight, no internal obstructions.
- Uniform velocity profile across the cross-section.
- Default velocities: residential supply 500–700 FPM, residential return 300–400 FPM, commercial/AHU supply 700–900 FPM, commercial return 500–800 FPM.
- Length: 1–3× width (2× preferred); minimum 18 in past the last takeoff (ACCA Manual D).
Included effects
- Airflow-velocity relationship for cross-section sizing.
- Aspect ratio guidance (target ≤ 4:1) for airflow stability.
- Optional blower-diameter based depth check.
Excluded effects
- Static pressure drop, fitting losses, and turning-vane effects.
- Acoustic/noise attenuation and insulation thickness.
- Number, spacing, or layout of branch takeoffs and diffusers.
- Sheet-metal gauge, reinforcement, and structural support.
Valid input range / intended use
- Preliminary sizing for residential and light-commercial AHU/FCU supply and return plenums.
- Airflow > 0 and velocity > 0; not intended for high-pressure ducted systems or cleanroom/lab exhaust plenums.
Engineering Notes
- Never neck the plenum down below the equipment opening — the plenum decelerates air, it should not restrict it.
- Keep the width 2–3× the connecting supply trunk width; use the pivot point sizing method for slot/linear diffusers if a manufacturer nomogram is available.
- An aspect ratio (W:H) beyond 4:1 tends to create dead zones and uneven takeoff distribution — consider splitting into two plenums instead.
- If the plenum feeds multiple branch takeoffs, keep 150–200 mm (6–8 in) spacing between collars and avoid placing outlets directly opposite the inlet.
Reference method
Based on ACCA Manual D box-plenum sizing guidance and standard AHU supply/return velocity practice (typical AHU design velocity 700–900 FPM).
Revision: v1.0 · August 2026.
Disclaimer: This tool provides preliminary, order-of-magnitude sizing for design development only. It does not account for static pressure losses, acoustics, structural support, or code-specific requirements. Final plenum dimensions must be verified and stamped by a licensed mechanical/HVAC engineer in accordance with applicable codes (e.g., ACCA Manual D, SMACNA, ASHRAE) and manufacturer data.
