BTU for AC Calculator | Free Construction Calculators
Calculate the precise air conditioning capacity needed for your room or space in BTU (British Thermal Units) and tonnage. Our professional AC BTU calculator helps homeowners, HVAC contractors, and property managers accurately size air conditioning units based on square footage, ceiling height, sun exposure, room type, and occupancy. Get instant recommendations for the right AC capacity to ensure efficient cooling, lower energy bills, and optimal comfort. Perfect for residential rooms, offices, commercial spaces, and whole-home HVAC planning.
Accurate AC Sizing
Industry-standard calculations account for all factors affecting cooling requirements
Energy Efficiency
Properly sized units save money on electricity and extend equipment lifespan
Professional Results
Get BTU and tonnage recommendations used by HVAC professionals
How to Use This Calculator
- Measure your room's square footage (length × width)
- Enter the ceiling height in feet (standard is 8 feet)
- Select sun exposure level based on window orientation and shade
- Choose room type (kitchens require additional cooling capacity)
- Enter the typical number of occupants in the space
- Click Calculate to see BTU requirements and AC tonnage recommendation
Calculation Formula
Base BTU: Square footage × 20 BTU/sq ft
Ceiling adjustment: +10% per foot above 8ft ceiling
Sun exposure: -10% (shaded) to +20% (very sunny)
Occupants: +600 BTU per person beyond first two
Kitchen: +4,000 BTU for heat-generating appliances
About BTU and AC Sizing
BTU (British Thermal Unit) is the standard measurement for air conditioning cooling capacity. One BTU represents the energy needed to raise one pound of water by one degree Fahrenheit. In air conditioning, BTU measures how much heat an AC unit can remove from a space per hour. Proper AC sizing based on accurate BTU calculations is critical for comfort, efficiency, and equipment longevity.
The industry standard for residential cooling is approximately 20 BTU per square foot, but this baseline must be adjusted for ceiling height, sun exposure, insulation quality, climate zone, and room use. An undersized AC unit will run continuously without adequately cooling the space, leading to high energy bills, poor humidity control, and premature equipment failure. An oversized unit will short-cycle (turn on and off frequently), failing to dehumidify properly and wasting energy.
Professional HVAC contractors use Manual J load calculations (a comprehensive industry standard) for whole-home systems, but this calculator provides accurate quick estimates for individual rooms, window units, mini-splits, and portable AC units. For critical applications or whole-home systems, consult a licensed HVAC professional who can account for insulation R-values, ductwork efficiency, and local climate factors.
BTU to AC Tonnage Conversion
0.5 Ton: 6,000 BTU - Small bedroom, office (up to 250 sq ft)
1.0 Ton: 12,000 BTU - Bedroom, small living room (300-450 sq ft)
1.5 Ton: 18,000 BTU - Large bedroom, medium room (450-650 sq ft)
2.0 Ton: 24,000 BTU - Large living room, small apartment (650-1,000 sq ft)
2.5 Ton: 30,000 BTU - Large apartment, small house (1,000-1,300 sq ft)
3.0 Ton: 36,000 BTU - Medium house (1,300-1,600 sq ft)
4.0 Ton: 48,000 BTU - Large house (1,800-2,300 sq ft)
5.0 Ton: 60,000 BTU - Very large house (2,300-3,000 sq ft)
Frequently Asked Questions
How many BTU do I need per square foot for air conditioning?
The general rule is 20 BTU per square foot for residential cooling in moderate climates. However, this varies significantly: shaded north-facing rooms may need only 15-18 BTU per square foot, while sunny south-facing rooms with large windows may require 25-30 BTU per square foot. Rooms with high ceilings, poor insulation, or many heat-generating appliances need additional capacity beyond the baseline.
What happens if my AC unit is too big or too small?
An undersized AC runs constantly, fails to cool adequately, creates high humidity, increases energy costs, and wears out quickly. An oversized AC short-cycles (rapid on/off), doesn't remove humidity, creates temperature swings, wastes energy, and costs more upfront. Proper sizing typically means the unit runs 15-20 minutes per cycle on hot days. If your AC runs less than 10 minutes or more than 30 minutes per cycle, it's likely incorrectly sized.
Should I use a window unit, portable AC, or mini-split system?
Window units (5,000-25,000 BTU) are most cost-effective for single rooms, install easily, but are visible and block windows. Portable ACs (8,000-14,000 BTU) are moveable and require no installation, but are less efficient and noisy. Mini-splits (9,000-36,000 BTU) are quiet, efficient, and aesthetic, but cost more upfront and require professional installation. Choose based on your budget, installation capability, and permanent vs temporary needs.
How does ceiling height affect AC requirements?
Standard calculations assume 8-foot ceilings. For every foot above 8 feet, add 10% more BTU capacity. A room with 10-foot ceilings needs 20% more cooling capacity than the same room with 8-foot ceilings because there's more air volume to cool. For vaulted or cathedral ceilings (12+ feet), increase capacity by 40-50% or use ceiling fans to improve air circulation and reduce AC requirements.
How many people can one BTU cool?
Each person generates approximately 400-600 BTU of body heat per hour. The baseline calculation includes two occupants; add 600 BTU for each additional person who regularly occupies the space. For crowded spaces like offices, classrooms, or workout rooms with 5+ people, this addition becomes significant. A gym with 10 people needs 4,800 additional BTU (8 extra people × 600 BTU) beyond the base square footage calculation.
Related BTU Calculators
Professional AC Sizing Tips
- Never oversize: Bigger is NOT better with AC units - proper sizing is critical
- Consider climate zone: Hot, humid climates need 25-30% more capacity
- Insulation matters: Poor insulation requires 20-30% more cooling capacity
- Window treatment: Blackout curtains can reduce cooling needs by 10-15%
- SEER rating: Higher SEER (16+) units are more efficient but cost more upfront
- Professional assessment: For whole-home systems, get a Manual J calculation from an HVAC pro
- Regular maintenance: Clean filters monthly and service annually for peak efficiency