BTU Calculator — Room and Home AC & Heating Size Estimator
Calculate the BTU per hour needed to heat or cool a room or whole house. Accounts for room size, insulation quality, ceiling height, sun exposure, and occupancy for accurate HVAC sizing.
How to Use This Calculator
- Enter the square footage of the room or space to be heated/cooled
- Select the ceiling height (higher ceilings require more BTU)
- Choose insulation quality — better insulation reduces required BTU
- Select sun exposure and climate zone for your location
- Enter the typical number of occupants
- Click "Calculate BTU Required" for sizing recommendations
How the Calculation Works
// Base calculation (ACCA Manual J simplified)
base_btu = sqft × 20
ceiling_adj = base_btu × (ceiling_height - 8) / 8 × 0.25 // adjustment per extra foot above 8ft
insulation_multiplier = { poor: 1.25, average: 1.0, good: 0.85, excellent: 0.70 }
sun_adj = { high: +10%, moderate: 0%, low: -5% }
occupant_adj = max(0, (occupants - 2)) × 600 BTU
total_btu = (base_btu + ceiling_adj) × insulation_multiplier × sun_adj + occupant_adj
tonnage = total_btu / 12000 The base BTU/h is estimated at 20 BTU per square foot — the standard starting point for moderate climates with 8-foot ceilings. Adjustments are made for ceiling height above 8 feet, insulation quality (ranging from 70% for excellent to 125% for poor), sun exposure (south/west-facing rooms gain heat), and occupancy (each person generates approximately 600 BTU/h of body heat above 2 people). The final result is converted to tons (1 ton = 12,000 BTU/h) for equipment selection.
Variables:
- base_btu: Starting estimate: square footage × 20 BTU/h
- ceiling_adj: Additional BTU for ceilings above 8 feet
- insulation_multiplier: Scaling factor based on insulation quality (0.70–1.25)
- sun_adj: Adjustment for solar heat gain (±5–10%)
- occupant_adj: Additional 600 BTU/h per occupant above 2 people
- tonnage: Equipment size in tons (1 ton = 12,000 BTU/h)
Pro Tips
- Bigger is not always better — oversized AC units short-cycle, causing humidity problems and uneven temperatures
- 1 ton of cooling = 12,000 BTU/h — a 2,000 sq ft home typically needs 2.5–3.5 tons depending on climate
- Manual J calculation is the industry standard for precise sizing — this calculator provides a reasonable estimate
- Add 10% BTU capacity for kitchens due to heat from cooking appliances
- High ceilings (>8 ft) significantly increase the volume of air to be conditioned — always account for ceiling height
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Frequently Asked Questions
How many BTU do I need for a 1,000 square foot house?
A rough estimate for 1,000 sq ft is 20,000 BTU/h (1.67 tons) with average insulation and an 8-foot ceiling in a moderate climate. In hot climates or poorly insulated homes, you may need 25,000–30,000 BTU/h. In mild climates with good insulation, 16,000–18,000 BTU/h may suffice.
What is a "ton" in air conditioning?
One ton of air conditioning capacity equals 12,000 BTU per hour — a measure that originally referred to the cooling effect of melting one ton of ice over 24 hours. Residential AC units range from 1.5 to 5 tons. Most homes need 1 ton per 400–600 sq ft, depending on climate and insulation.
Should I oversize my AC or heat pump?
No — oversized equipment short-cycles (turns on and off frequently) without running long enough to dehumidify the air. This causes clammy, uncomfortable conditions even at the right temperature. It also increases wear on the compressor and reduces efficiency. Proper sizing runs in longer, more efficient cycles.
What is Manual J calculation?
Manual J is the ACCA (Air Conditioning Contractors of America) standard method for calculating residential heat gain and heat loss. It accounts for local climate data, construction materials, window size and orientation, infiltration, and internal heat gains. It is required by many building codes for new construction HVAC installations.
How do I convert BTU to kilowatts for heat pumps?
To convert BTU/h to kW: divide by 3,412. For example, 24,000 BTU/h ÷ 3,412 = 7.03 kW. Heat pump efficiency is expressed as COP (Coefficient of Performance) or SEER (for cooling). A heat pump with a SEER of 20 is significantly more efficient than one with SEER 14.