The calorie (cal) is a small energy unit defined as the amount of heat needed to raise one gram of water by one degree Celsius, primarily used in physics and chemistry. In everyday language, 'calorie' usually refers to the kilocalorie (kcal), but the true calorie is 1/1,000th of that amount. One calorie equals exactly 4.184 joules, providing a direct bridge between thermal and mechanical energy.

The BTU (British Thermal Unit) equals approximately 1,055.06 joules, defined as the heat needed to raise one pound of water by one degree Fahrenheit. It is the primary unit for rating heating and cooling equipment in the United States, appearing on air conditioner specifications, furnace ratings, and energy efficiency labels. One BTU equals about 0.2931 watt-hours or roughly 0.252 kilocalories.