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Calculate the heating or cooling BTU requirements for any room. Get accurate sizing for air conditioners, heaters, and HVAC systems.
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BTU (British Thermal Unit) is a unit of energy used to measure heating and cooling capacity. One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.
In HVAC applications, BTU/h (BTU per hour) indicates how much heat a system can add (heating) or remove (cooling) from a space in one hour. Higher BTU ratings mean more heating or cooling power.
Measures heat removed from a space. Air conditioners are rated in BTU/h or tons (1 ton = 12,000 BTU/h).
Measures heat output. Furnaces, heaters, and heat pumps are rated by BTU output.
The basic formula for calculating BTU requirements:
Cooling BTU = Square Feet × 20 × Adjustment Factors
Heating BTU = Square Feet × 30-45 × Adjustment Factors
| Factor | Poor/High | Average | Good/Low |
|---|---|---|---|
| Insulation | ×1.30 | ×1.00 | ×0.75 |
| Sun Exposure | ×1.20 | ×1.00 | ×0.90 |
| Climate (Cooling) | ×1.30 (Hot) | ×1.00 | ×0.90 |
| Climate (Heating) | ×1.40 (Cold) | ×1.00 | ×0.90 |
Quick reference for cooling BTU requirements by room size (standard conditions):
| Room Size (sq ft) | BTU Needed | Tonnage | Typical Unit |
|---|---|---|---|
| 100 - 200 | 5,000 - 6,000 | 0.5 | Small window unit |
| 200 - 350 | 7,000 - 8,000 | 0.6 | Medium window unit |
| 350 - 500 | 10,000 - 12,000 | 1.0 | Large window/mini-split |
| 500 - 750 | 14,000 - 18,000 | 1.5 | Mini-split |
| 750 - 1,000 | 18,000 - 24,000 | 2.0 | Central AC or multi-zone |
| 1,000 - 1,500 | 24,000 - 36,000 | 2.5 - 3.0 | Central AC |
| 1,500 - 2,000 | 36,000 - 48,000 | 3.5 - 4.0 | Central AC |
| 2,000 - 2,500 | 48,000 - 60,000 | 4.0 - 5.0 | Central AC |
Taller ceilings mean more volume to heat or cool. Add 4% for each foot above 8 feet.
Poor insulation can increase BTU needs by 30% or more. Well-insulated spaces need less.
Large or single-pane windows increase heat gain/loss. South-facing adds cooling load.
Direct sunlight adds significant heat. South and west exposures need more cooling.
Each person adds ~400 BTU/h. Kitchens with cooking add 1,000-1,500 BTU/h.
Hot climates need more cooling; cold climates need more heating capacity.
12' × 14' bedroom with average conditions
Room size: 12 × 14 = 168 sq ft
Base BTU: 168 × 20 = 3,360 BTU
Adjustments: ×1.0 (standard conditions)
Result: ~5,000 BTU window unit
20' × 25' living room, sunny, Arizona climate
Room size: 20 × 25 = 500 sq ft
Base BTU: 500 × 20 = 10,000 BTU
Sun exposure: ×1.15
Hot climate: ×1.30
Result: 10,000 × 1.15 × 1.30 = 14,950 BTU
Recommended: 18,000 BTU mini-split
2,000 sq ft home in cold climate, average insulation
House size: 2,000 sq ft
Base BTU: 2,000 × 35 = 70,000 BTU
Cold climate: ×1.20
Result: 70,000 × 1.20 = 84,000 BTU
Recommended: 90,000 BTU furnace
Heating typically requires more BTU per square foot than cooling because the temperature differential is usually greater in winter (heating from 0°F to 70°F) than summer (cooling from 90°F to 70°F).
| Application | BTU per sq ft | Notes |
|---|---|---|
| Cooling (Mild) | 18-22 BTU | Northern states, well-insulated |
| Cooling (Hot) | 22-30 BTU | Southern states, desert |
| Heating (Mild) | 25-35 BTU | Moderate winters |
| Heating (Cold) | 40-60 BTU | Northern states, Canada |
An oversized AC will cool quickly but cycle on/off frequently (short cycling). This reduces dehumidification, wastes energy, and wears out the compressor faster.
For cooling, plan on 20 BTU per square foot as a baseline. For heating, plan on 30-45 BTU per square foot depending on climate and insulation.
One ton of cooling equals 12,000 BTU per hour. This comes from the amount of energy needed to melt one ton of ice in 24 hours.
Size for the greater of the two loads. In cold climates, heating usually dominates. In hot climates, cooling dominates. A Manual J calculation provides accurate sizing for both.
Kitchens generate significant heat from cooking. Add 1,000-1,500 BTU for residential kitchens. Commercial kitchens may need 2,000+ BTU per appliance.