HVAC System Sizing Calculator (Tonnage)
Estimate the required HVAC system size in tons of cooling/heating capacity using a simplified Manual J-style load calculation based on square footage, climate zone, insulation quality, occupancy, and window area.
Formulas Used
Total Cooling Load (BTU/hr) = Envelope Load + Window Load + Occupant Load + Internal Load + Infiltration Load
- Envelope Load = Area × 25 BTU/hr·ft² × (Ceiling Height / 8) × Climate Factor × Insulation Factor × Stories Factor
- Window Load = Window Area × 200 BTU/hr·ft² × Orientation Factor × Climate Factor
- Occupant Load = Occupants × 250 BTU/hr per person (ASHRAE sensible heat, light activity)
- Internal Load = Area × 10.24 BTU/hr·ft² (≈ 3 W/ft² lighting & appliances × 3.412 BTU/W)
- Infiltration Load = Volume × 0.35 ACH / 60 × 1.1 BTU·min/(ft³·°F) × 20°F ΔT
- Tonnage = Total BTU/hr ÷ 12,000 BTU/hr per ton, rounded up to nearest 0.5-ton increment