AC Unit Sizing Calculator for LA Heat
Size your air conditioner correctly for Los Angeles conditions. LA's hot, sunny climate requires more cooling capacity than national averages — this calculator uses Manual J-based adjustments specific to Southern California.
Formula & Methodology
This calculator uses a simplified ACCA Manual J approach adapted for Los Angeles conditions:
- Base Load: Area (sq ft) × 20 BTU/hr/sq ft (ASHRAE baseline)
- Volume Adjustment: × (Ceiling Height ÷ 8 ft) — accounts for air volume above standard 8 ft
- LA Climate Factor: × 1.10 — LA's 1% design temperature of 98°F (NOAA) is ~10°F above the 85°F national baseline used in standard tables
- Sun Exposure: −10% (heavy shade) / 0% (medium) / +15% (direct south/west sun)
- Insulation: +15% (poor) / 0% (average) / −10% (good)
- Floor Level: +5% (ground/slab) / 0% (middle) / +15% (top floor under roof)
- Occupants: +600 BTU/hr per person beyond 2 (ASHRAE 62.1 sensible heat value)
- Kitchen: +4,000 BTU/hr for cooking appliance heat gain
- Final: Rounded to nearest 500 BTU; matched to next standard unit size
Unit conversion: 1 ton of cooling = 12,000 BTU/hr
Assumptions & References
- Based on ACCA Manual J Residential Load Calculation, 8th Edition
- LA design conditions: 98°F dry bulb / 68°F wet bulb (NOAA Climate Design Data, Los Angeles Civic Center)
- Standard occupant sensible heat gain: 600 BTU/hr per person (ASHRAE Fundamentals Handbook)
- Kitchen appliance heat gain: 4,000 BTU/hr (ACCA Manual J Table 5A)
- California Title 24 requires minimum 15 SEER2 for new AC installations in Climate Zone 9 (Los Angeles Basin)
- Oversizing by more than 15% is discouraged — oversized units short-cycle, reducing dehumidification and efficiency
- For spaces over 3 tons, a full Manual J by a licensed California C-20 HVAC contractor is strongly recommended
- Does not account for duct losses, window area/type, or wall construction — these require full Manual J analysis
Formula & Methodology
This calculator uses a simplified ACCA Manual J approach adapted for Los Angeles conditions:
- Base Load: Area (sq ft) × 20 BTU/hr/sq ft (ASHRAE baseline)
- Volume Adjustment: × (Ceiling Height ÷ 8 ft) — accounts for air volume above standard 8 ft
- LA Climate Factor: × 1.10 — LA's 1% design temperature of 98°F (NOAA) is ~10°F above the 85°F national baseline used in standard tables
- Sun Exposure: −10% (heavy shade) / 0% (medium) / +15% (direct south/west sun)
- Insulation: +15% (poor) / 0% (average) / −10% (good)
- Floor Level: +5% (ground/slab) / 0% (middle) / +15% (top floor under roof)
- Occupants: +600 BTU/hr per person beyond 2 (ASHRAE 62.1 sensible heat value)
- Kitchen: +4,000 BTU/hr for cooking appliance heat gain
- Final: Rounded to nearest 500 BTU; matched to next standard unit size
Unit conversion: 1 ton of cooling = 12,000 BTU/hr