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
Assumptions & References
- Based on ACCA Manual J residential load calculation methodology (simplified).
- Base envelope load of 25 BTU/hr·ft² assumes standard 8-ft ceilings and average U-values; scaled linearly with ceiling height (volume proxy).
- Window solar heat gain uses an average SHGC of ~0.40 and peak irradiance of ~500 BTU/hr·ft², yielding ~200 BTU/hr·ft² before orientation adjustment.
- Occupant sensible heat of 250 BTU/hr per person per ASHRAE 62.1 (seated/light activity).
- Internal gains of 3 W/ft² represent typical residential lighting and appliance loads (ASHRAE 90.2).
- Infiltration assumes 0.35 ACH (ASHRAE 62.2 minimum for residential) and a 20°F indoor/outdoor design temperature difference.
- Climate factors derived from ASHRAE climate zone design conditions (ASHRAE 169-2020).
- 1 ton of refrigeration = 12,000 BTU/hr (ASHRAE standard).
- Equipment is rounded up to the nearest 0.5-ton increment per industry practice to avoid undersizing.
- This calculator does not account for duct losses, humidity loads, or specific construction materials. A licensed HVAC professional should perform a full Manual J calculation.