HVAC · Heating · Cooling
BTU
Calculator
Find the right AC or heater BTU for any room. Accounts for sunlight, occupants, kitchen heat, insulation, and climate zone. Includes BTU converter and energy cost estimator.
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❄️ AC / Cooling BTU Calculator
Find the right BTU rating for an air conditioner based on room size and conditions.
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Room Factors (add BTU)
☀️ Very Sunny
🍳 Kitchen
👤👤 2+ People
🏠 Top Floor
💧 High Humidity
🔥 Heating BTU Calculator
Find the required BTU output for a heater or furnace.
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ft
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🔄 BTU Unit Converter
Convert between BTU/hr and all equivalent energy/power units.
💰 AC / Heater Running Cost
Estimate monthly and yearly electricity cost for your HVAC unit.
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BTU Analysis
Understanding BTUs for Heating & Cooling
A BTU (British Thermal Unit) is the amount of energy needed to raise 1 pound of water by 1°F. In HVAC, BTU/hr measures the heating or cooling capacity of a unit. One ton of refrigeration = 12,000 BTU/hr.
BTU Quick Reference
Room sizing (cooling): ~20 BTU/hr per sq ft (basic)
Small room (150 sq ft): 5,000–6,000 BTU
Medium room (350 sq ft): 8,000–10,000 BTU
Large room (550 sq ft): 12,000–14,000 BTU
Open plan (1,000 sq ft): 18,000–21,000 BTU
Conversions:
1 ton = 12,000 BTU/hr = 3.517 kW
1 kW = 3,412 BTU/hr
1 BTU/hr = 0.2931 Watts
Energy cost formula:
Watts = BTU/hr ÷ EER
Daily kWh = Watts × hours ÷ 1000
Monthly cost = daily kWh × 30 × $/kWh
What BTU do I need for a 12×15 room?
A 12×15 = 180 sq ft room. Base BTU: 180 × 20 = 3,600. Adjust up for high ceilings (above 8ft add 10%), sunny rooms (+10%), kitchens (+4,000 BTU), extra occupants (+600 BTU/person). A standard 5,000 BTU window unit handles 100–150 sq ft; 8,000 BTU handles ~350 sq ft. For a 180 sq ft bedroom, a 5,000–6,000 BTU unit is typically right for a moderate climate.
What does EER and SEER mean?
EER (Energy Efficiency Ratio) = BTU/hr of cooling ÷ Watts consumed. A 12,000 BTU unit with EER 12 uses 1,000 Watts. Higher EER = more efficient. SEER (Seasonal EER) averages over a cooling season and is typically 10–15% higher than EER. Federal minimum: 14 SEER. Energy Star: 15+ SEER. Best units: 20–25 SEER. A high-SEER unit costs more upfront but saves on electricity — payback period is typically 3–7 years in a hot climate.
Can an AC be too big for a room?
Yes — oversized AC is a common mistake. An oversized unit cools too quickly (short-cycling) without running long enough to dehumidify the air. Short-cycling also reduces efficiency and lifespan. The room feels cold but clammy. Signs of oversized AC: unit turns on/off frequently (every few minutes), high humidity indoors despite the AC running, uneven cooling. Proper sizing by BTU ensures the unit runs for 15–20 minute cycles to fully dehumidify and condition the air.
How much does it cost to run an AC per month?
Example: 12,000 BTU window AC with EER 12, running 8 hrs/day at $0.13/kWh. Power = 12,000/12 = 1,000W = 1 kW. Daily: 1 kW × 8h = 8 kWh. Monthly: 8 × 30 = 240 kWh. Cost: 240 × $0.13 = $31.20/month. A central 3-ton (36,000 BTU) HVAC system at SEER 16 running 12 hrs/day would cost: 36,000/16 = 2,250W. Daily: 2.25 × 12 = 27 kWh. Monthly: 27 × 30 × $0.13 = $105/month.