units of heat

British thermal unit

A British thermal unit (Btu) is the amount of heat energy required to raise the temperature of one pound of water from 59.5° to 60.5°F at one atmosphere pressure. One Btu (or BTU) = 778.3 foot-pounds = 252 calories = 0.293 watt hours = 1,055 joules.


Btu conversion factors for site energy are as follows:


Electricity ..... 3,412 Btu/kilowatthour
Natural Gas ..... 1,031 Btu/cubic foot
Fuel Oil No.1 ..... 135,000 Btu/gallon
Kerosene ..... 135,000 Btu/gallon
Fuel Oil No.2 ..... 138,690 Btu/gallon
LPG (Propane) ..... 91,330 Btu/gallon
Wood ..... 20 million Btu/cord


A Btu per cubic foot is the total heating value, expressed in Btu, produced by the combustion, at constant pressure, of the amount of the gas that would occupy a volume of 1 cubic foot at a temperature of 60°F if saturated with water vapor and under a pressure equivalent to that of 30 inches of mercury at 32°F and under standard gravitational force (980.665 cm/s2) with air of the same temperature and pressure as the gas, when the products of combustion are cooled to the initial temperature of gas and air when the water formed by combustion is condensed to the liquid state. (Sometimes called gross heating value or total heating value.)



One calorie, or "small calorie", is the amount of heat needed to raise the temperature of one gram of water by 1 degree Celsius (centigrade). Since this amount varies slightly depending on the temperature of the water, various definitions have arisen to take account of this fact. For example, the 15°C calorie is the amount of heat needed to raise the temperature of one gram of water from 14.5 to 15.5°C (58.1 to 59.9°F) at standard atmospheric pressure. For most scientific purposes, the calorie has now been superceded by the SI unit of energy, the joule. One (15°C) calorie = 4.1855 joules.


The one area where calories are still much used is dietary science, but here the usage has become confusing. One kilocalorie or "big calorie" (Cal) equals 1,000 calories and is the unit usually used in describing the energy content of food. However, dietary Calories are almost never capitalized as they should be, and are often mistaken for calories.



See main entry.


The joule (J) is the unt of heat energy, and of energy in general, in the SI system of units. 4.1855 J of heat energy (or one calorie) is required to raise the temperature of a unit weight (1 gram) of water from 0°C to 1°C.