Air Flow | HVAC Lens App

Enter two values; the third updates live. Edit the highlighted output to cross-check.

BTU/hr

CFM

ΔT (°F)

Formula: BTU/hr = CFM × 1.08 × ΔT (°F)
Standard air density at sea level · Highlighted box is the live output

Linked calculators (VP ↔ FPM). Edit the highlighted output to cross-check. Zero is not valid.

SP / TP / VP

Static Pressure (in. w.c.)

Total Pressure (in. w.c.)

Velocity Pressure (in. w.c.)

Supply air: enter positive SP/TP

CFM / FPM / Area

Velocity (FPM)

Duct Area (sq.in.)

CFM

VP = TP − SP · FPM = 4005 × √|VP| · CFM = FPM × (Area sq.in. ÷ 144)
SA positive SP/TP · RA negative SP/TP · Area in sq.in. · Highlighted box is live output

What to calculate

Return Air Dry Bulb (°F)

Return Air Wet Bulb (°F)

Supply Air Dry Bulb (°F)

Manuf. Max Rise (°F)

Cooling ΔT = Return DB − Supply DB · Heating Rise = Supply DB − Return DB
Wet bulb used for RH & dew point in cooling modes · Heating defaults 35–75°F

What to calculate

Original RPM

New RPM

Original CFM

Original SP (in. w.c.)

Original BHP (optional)

Fan Law 1: CFM ∝ RPM · Law 2: SP ∝ RPM² · Law 3: BHP ∝ RPM³

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