HVAC Tool

BTU Calculator

Calculate British Thermal Units for heating and cooling. Convert between BTU, watts, and other energy units for HVAC planning.

Using the BTU Calculator

Buying an air conditioner or heater that is too small won't cool or heat effectively, while one that is too large will waste energy and feel "clammy." This BTU (British Thermal Unit) calculator helps you find the perfect appliance size for your room.

How it Works

Enter the length, width, and height of the room.

Input the "Insulation Quality" of your home.

Adjust for "Sun Exposure" and the number of people regularly in the room.

Review the "Total BTU Requirement" for both cooling and heating.

Technical Formula


                        BTUs = Volume (Cu Ft) x Temperature Difference x 0.133

Calculation Type

Room Length

Room Width

Ceiling Height

Room Units

Climate Zone

Insulation Quality

Sunlight Exposure

Quick Reference

• Small bedroom (100-150 sq ft): 5,000 BTU
• Medium bedroom (150-250 sq ft): 7,000 BTU
• Large bedroom (250-350 sq ft): 9,000 BTU
• Living room (350-450 sq ft): 12,000 BTU
• 1 BTU/hr = 0.293 W • 1 Ton = 12,000 BTU/hr

Understanding BTU and HVAC

What is a BTU?

A British Thermal Unit (BTU) is a traditional unit of heat energy — the amount of heat required to raise one pound of water by 1°F. BTUs rate the heating and cooling capacity of HVAC systems and appliances.

BTU Conversion Factors

From	To BTU/hr	To Watts	To Tons
1 BTU/hr	1	0.293	0.000083
1 Watt	3.412	1	0.000284
1 Kilowatt	3,412	1,000	0.284
1 Ton	12,000	3,517	1
1 HP	2,545	746	0.212

Common Appliance BTU Ratings

Appliance	BTU/hr	Watts	Tons
Window AC (5,000 BTU)	5,000	1,465	0.42
Window AC (12,000 BTU)	12,000	3,517	1.00
Central AC (3-ton)	36,000	10,551	3.00
Space Heater	5,120	1,500	0.43
Electric Oven	12,000	3,517	1.00
Water Heater (40 gal)	34,000	9,961	2.83

Energy Efficiency Ratings

SEER — Seasonal Energy Efficiency Ratio for cooling (higher is better)
AFUE — Annual Fuel Utilization Efficiency for furnaces (percentage)
HSPF — Heating Seasonal Performance Factor for heat pumps
Energy Star — Products meeting strict energy efficiency criteria

Professional Sizing Factors

Load Calculation

Manual J calculations consider all heat sources and losses

Ductwork Design

Proper duct sizing affects system efficiency and performance

Air Changes

Ventilation requirements impact BTU needs significantly