BTU Calculator
Calculate the BTU needed to heat or cool your room. Get the right size air conditioner or heater based on room size, insulation, and climate.
Cooling Requirements
BTU Required
25,000
BTU/hour
Tonnage
2.1
tons
Calculation Type
Room Size
Building Factors
Additional Heat Sources
Calculation Results
BTU Required
25,000
BTU/hour
Tonnage
2.08
tons
Watts
7327
W
Kilowatts
7.33
kW
Recommended Unit Size:
30,000 BTU / 2.5 ton
How It's Calculated
Base BTU:
1000 sq ft × 20 BTU × 1.00 (height factor)
Adjustments Applied:
- • Insulation (average): multiplier applied
- • Climate (moderate): multiplier applied
- • Sun exposure (medium): multiplier applied
- • Windows (4): +4000 BTU
- • Occupants (2): +1000 BTU
Final Result:
25,000 BTU/hour = 2.08 tons
Quick Reference: BTU by Room Size
| Room Size | BTU (Cooling) | BTU (Heating) | Tons |
|---|---|---|---|
| 150 sq ft | 3,000 | 5,250 | 0.3 |
| 300 sq ft | 6,000 | 10,500 | 0.5 |
| 500 sq ft | 10,000 | 17,500 | 0.8 |
| 750 sq ft | 15,000 | 26,250 | 1.3 |
| 1000 sq ft | 20,000 | 35,000 | 1.7 |
| 1500 sq ft | 30,000 | 52,500 | 2.5 |
| 2000 sq ft | 40,000 | 70,000 | 3.3 |
| 2500 sq ft | 50,000 | 87,500 | 4.2 |
| 3000 sq ft | 60,000 | 105,000 | 5.0 |
* These are estimates for average conditions. Your actual requirements may vary based on the factors above.
Cooling Requirements
BTU Required
25,000
BTU/hour
Tonnage
2.1
tons
Quick Answer
To calculate BTUs needed: multiply room square footage by 20 BTU for cooling, then adjust for factors like ceiling height, sun exposure, and occupants. Basic formula: BTUs = Square Feet x 20. A 300 sq ft room needs approximately 6,000 BTUs for cooling. For heating, use 30-60 BTU per square foot depending on climate and insulation. Oversized units short-cycle and waste energy; undersized units run constantly.
Key Facts
- Basic cooling formula: Room Sq Ft x 20 BTU = Minimum BTUs needed
- Heating formula: Room Sq Ft x 30-60 BTU depending on climate
- Add 10% for sunny rooms; subtract 10% for shaded rooms
- Add 600 BTU per person beyond 2 occupants
- Add 4,000 BTU if room contains a kitchen
- High ceilings (over 8 ft): add 10% per additional foot
- Poor insulation: add 10-20% to calculation
- Window AC units range from 5,000-25,000 BTU; central AC measured in tons (1 ton = 12,000 BTU)
Frequently Asked Questions
BTU (British Thermal Unit) is a unit of energy. One BTU is the energy needed to heat one pound of water by one degree Fahrenheit. In HVAC, BTU/hour measures heating or cooling capacity. Higher BTU = more powerful unit.
General rule: 20 BTU per square foot for cooling, 30-40 BTU for heating. However, this varies significantly based on ceiling height, insulation, climate, windows, and sun exposure. A 500 sq ft room typically needs 10,000-12,000 BTU for cooling.
1 ton = 12,000 BTU/hour. The term comes from ice cooling - one ton of ice provides 12,000 BTU of cooling per hour. Central AC units are often rated in tons (2 ton, 3 ton, etc.). A typical home needs 2-5 tons.
Too small: runs constantly, can't maintain temperature, higher energy bills, shorter lifespan. Too big: short cycles (turns on/off frequently), doesn't dehumidify properly, uneven temperatures, higher initial cost. Proper sizing is crucial.
Improve insulation, seal air leaks, add weatherstripping. Use window treatments (blinds, curtains). Plant shade trees on south/west sides. Use energy-efficient windows. Add ceiling fans. These can reduce BTU needs by 20-40%.
Cooling Requirements
BTU Required
25,000
BTU/hour
Tonnage
2.1
tons