BTU Calculator
How the formula works
This calculator has three modes: cooling load of a room, heating load of a room, and unit conversion.
Cooling load
Room heat = Floor area × W/m² (climate) × (Height / 2.7)
People = Number of people × 125 W
Total = (Room heat + People + Equipment) × 1.10
BTU/h = Total W × 3.412
TR = Total W / 3517
Heating load
Heat loss (W) = Room volume × Temperature difference × Insulation factor
BTU/h = W × 3.412
Suggested heater = Heat loss × 1.15
Unit conversion
1 W = 3.412 BTU/h
1 kW = 3,412 BTU/h
1 TR = 12,000 BTU/h = 3.517 kW
1 kcal/h = 1.163 W
Where:
- BTU/h = British thermal unit per hour, a measure of heat flow
- TR = ton of refrigeration
- 2.7 m is the reference ceiling height for the typical W/m² values
Typical values
Cooling heat gain (W per m² of floor, without people): moderate climate 100, hot 140, very hot 180. Heating loss (W per m³ per °C): good insulation 0.6, average 1.0, poor 1.5. These are typical planning values in line with common ASHRAE-style estimating practice. Use the Custom option to enter your own.
How to use the calculator: choose the calculation, fill in the fields that appear, then click Calculate.
Important notes: standard AC sizes used are 9,000, 12,000, 18,000, 24,000, 30,000, 36,000, 48,000 and 60,000 BTU/h. For windows and sun exposure, use the AC Tonnage Calculator.
Worked example
Example 1: Cooling a bedroom
4 m × 3.5 m, height 2.7 m, hot climate, 2 people, 200 W equipment.
Room heat = 14 × 140 = 1,960 W. People = 250 W. Equipment = 200 W. With 10 % margin: 2,651 W = 9,046 BTU/h = 0.75 TR. Recommended: 1 TR (12,000 BTU/h).
Example 2: Living room in feet
15 ft × 12 ft, height 9 ft, moderate climate, 3 people, no equipment.
Floor area = 16.7 m². Room heat = 1,699 W. People = 375 W. With margin: 2,281 W = 7,785 BTU/h = 0.65 TR. Recommended: 0.75 TR (9,000 BTU/h).
Example 3: Large hall
10 m × 8 m, height 3 m, very hot climate, 10 people, 3,000 W equipment.
Room heat = 16,000 W. People = 1,250 W. With margin: 22,275 W = 76,005 BTU/h = 6.33 TR. This is above the largest standard size, so use 6.5 TR with ducted or multiple units.
Example 4: Heating a room
5 m × 4 m, height 2.7 m, average insulation, 22 °C inside and 5 °C outside.
Volume = 54 m³. Temperature difference = 17 °C. Heat loss = 54 × 17 × 1.0 = 918 W = 3,132 BTU/h. Suggested heater with 15 %: 3,602 BTU/h (1.06 kW).
Example 5: Convert 2 TR
2 TR = 24,000 BTU/h = 7,034 W = 7.034 kW = 6,048 kcal/h.
Example 6: Convert 18,000 BTU/h
18,000 BTU/h = 5,275 W = 5.275 kW = 1.5 TR = 4,536 kcal/h.
Common mistakes
- Confusing BTU with BTU/h. BTU is energy, BTU/h is power (rate of heat).
- Buying an AC by area only and ignoring sun, roof and windows. Use the AC Tonnage Calculator for that.
- Forgetting the heat from people and equipment in the room.
- Mixing feet and metres in the same calculation.
- Using a cooling BTU value to size a heater, or the other way round.
- Assuming 1 TR is 1,200 BTU/h. The correct value is 12,000 BTU/h.
- Choosing an oversized AC, which short-cycles and removes less moisture.
- Using the indoor temperature as the outdoor design temperature for heating.
- Ignoring poor insulation, which can double the heating load.
Frequently asked questions
What is a BTU?
A BTU is the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. BTU/h is the heat per hour.
How many BTU/h do I need for my room?
Enter the room size, climate, people and equipment. The calculator gives BTU/h and the nearest standard AC size.
How many BTU/h are in 1 TR?
One TR is 12,000 BTU/h, about 3.517 kW.
How do I convert BTU/h to kW?
Divide BTU/h by 3,412. For example, 12,000 BTU/h is about 3.52 kW.
How do I convert kW to BTU/h?
Multiply kW by 3,412. Use the Convert mode of this calculator for a quick answer.
How many BTU does a 1.5 TR AC have?
A 1.5 TR AC is 18,000 BTU/h, about 5.28 kW of cooling.
Is BTU the same as the electrical power of the AC?
No. BTU/h is the cooling output. The electrical input is lower and depends on the AC efficiency.
How much BTU do I need to heat a room?
It depends on volume, insulation and the temperature difference. Use the heating mode of this calculator.
Why is the heater size 15 % higher?
The extra margin helps the room warm up faster and covers small losses that are not counted.
What is the difference between this and the AC Tonnage Calculator?
This one is a quick estimate and unit converter. For window area, sun exposure and a detailed room heat load, use the AC Tonnage Calculator.
What if my load is above 60,000 BTU/h?
A single split AC is not enough. Use a ducted system or several units.
Is this a final design?
No. It is a planning estimate. Final design should follow the local code and a full heat load calculation.