| Size | Neck Area (ft²) | CFM at 750 FPM | CFM at 900 FPM | CFM at 1000 FPM | CFM at 1200 FPM | Status |
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This VAV box sizing calculator turns zone cooling and heating loads into the airflow values you need to select a variable-air-volume terminal unit. Enter the cooling and heating loads in BTU/h along with the room setpoint and the cooling and heating supply-air temperatures, and the tool returns the cooling maximum CFM, the minimum CFM, the heating CFM, and the controlling minimum airflow used to set the box.
It then recommends a round neck size by checking each standard inlet against a 900–1200 FPM velocity band at the maximum airflow. Sizing the neck inside that band keeps inlet pressure drop, controllability, and discharge noise in balance — an oversized box controls poorly at low flow, while an undersized box runs noisy and high in pressure.
| Cooling max CFM | Qmax = Loadclg ÷ (1.08 · ΔTc), ΔTc = Troom − Tcool SAT |
| Heating CFM | Qhtg = Loadhtg ÷ (1.08 · ΔTh), ΔTh = Theat SAT − Troom |
| Minimum CFM | Qmin = max(Qmax × min%, OAmin) |
| Controlling min | Qctrl = max(Qmin, Qhtg) |
| Neck velocity | V (FPM) = Q ÷ A, A = π · (d ÷ 24)² (d in inches) |
The constant 1.08 is the standard sea-level sensible-heat factor for air: 60 min/h × 0.075 lb/ft³ air density × 0.24 BTU/lb·°F specific heat ≈ 1.08. The default minimum-flow fraction of 30% follows ASHRAE 90.1 turndown guidance, and the 900–1200 FPM neck-velocity band follows standard ASHRAE and manufacturer VAV-selection practice. A round neck is flagged Recommended when the maximum CFM lands inside that band, Undersized above it, and Oversized below it.