Units of Heat

Heat is a form of energy that is transferred between systems or bodies with different temperatures. This fundamental concept is critical in the fields of thermodynamics, engineering, and environmental science.

What are Units of Heat?

Heat, as a form of energy, is quantified using several units that are essential in various scientific and practical applications. The Joule (J), the SI unit for energy, is also used to measure heat, reflecting the amount of energy transferred as heat in scientific contexts.

Transitioning to more traditional measures, the Calorie (cal) is particularly prominent in nutritional science, where it describes the energy content in food, indicating how much heat energy the body can obtain from consuming it.

SI Unit of Heat

joule (J)

The SI unit of heat is the joule (J). Heat, a form of energy transfer, quantifies how much energy is exchanged between systems due to temperature differences. The joule, named after James Prescott Joule, measures this energy transfer effectively. It signifies the amount of energy used when a force of one newton is applied over a distance of one meter.

BTU Unit of Heat

one degree Fahrenheit

The British Thermal Unit (BTU) is a unit of heat used primarily in the United States. It measures the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. The BTU is commonly employed in various applications including heating and cooling systems, cooking appliances, and energy consumption in industrial processes.

CGS Unit of Heat

calorie (cal)

In the CGS (centimeter-gram-second) system of units, the unit of heat is the calorie (cal). One calorie is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius at atmospheric pressure. This unit is particularly useful in chemistry and the food industry to describe the energy content of substances and reactions.

Difference between Temperature, Heat, and Internal Energy

Temperature is a measure of the average kinetic energy of the particles in a substance. It is an indicator of how hot or cold a substance is and is typically measured in degrees Celsius (°C), Fahrenheit (°F), or Kelvin (K). Temperature provides a relative measure, allowing us to assess whether energy will transfer from one object to another. It does not, however, indicate the total energy present in a substance.

Heat refers to the transfer of thermal energy between systems or bodies that are at different temperatures. Heat is a form of energy in transit; it is not contained within an object but moves from one place to another. Heat can be measured in joules (J) or British Thermal Units (BTU) and is only evident when there is a temperature difference that causes energy to flow from a warmer area to a cooler one.

Internal Energy is the total energy stored within a system or substance. It encompasses all the kinetic and potential energies of the particles making up the system. This includes energies related to the motion of particles (kinetic) and energies due to the arrangement of particles or their interaction with each other (potential). Internal energy is a state function, which means it is dependent only on the current state of the system, not on how that state was achieved.

List of Heat Units

Unit	Symbol
Joule	J
Kilowatt-hour	kWh
Calorie	cal
Kilocalorie	kcal
Electronvolt	eV
British Thermal Unit	BTU
Foot-pound	ft-lb

Joule (J)

1J = 1N⋅m = 1kg⋅m²/s²

The Joule is the SI unit of energy, named after James Prescott Joule. It measures the energy transferred when a force of one newton is exerted over a distance of one meter. It is universally used in science and engineering to quantify energy, work, or heat.

Kilocalorie (kcal)

1kcal = 1000cal = 4184J

Known also as a food calorie, the kilocalorie is primarily used to express the energy content in foods, equivalent to 1000 smaller calories. It is important in dietary energy contexts to determine caloric intake.

British Thermal Unit (BTU)

1BTU ≈ 1055J

The British Thermal Unit is predominantly used in the United States within heating and cooling systems. It defines the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.

Therm (US)

1 therm = 100,000 BTU

A therm is a unit of heat commonly used in the energy industry, particularly in natural gas billing in the United States. It represents 100,000 BTUs, measuring large-scale energy consumption.

Watt-hour (Wh)

1Wh = 3600J

The watt-hour is a smaller unit of energy typically used in electricity measurements. It represents the energy produced or consumed by a power of one watt sustained for one hour.

Conversion of Heat Units

Here’s a conversion table for various units of heat, presenting how different heat units relate to each other, specifically Joules, Calories, BTUs, and Kilowatt-hours:

Unit	Joules (J)	Calories (cal)	British Thermal Units (BTU)	Kilowatt-hours (kWh)
Joules (J)	1	0.2390	0.000948	2.77778E-07
Calories (cal)	4.187	1	0.00397	1.16279E-06
BTUs	1055	252	1	0.000293
Kilowatt-hours	3.6E6	8.6E5	3412	1

Joule to Calorie

• Conversion: 1 calorie = 4.184 joules.
• Example: To convert 100 joules to calories, divide by 4.184.

100 J / 4.184 = 23.88 cal

Joule to British Thermal Unit (BTU)

• Conversion: 1 BTU = 1055 joules.
• Example: To convert 500 joules to BTUs, divide by 1055.

500 J / 1055 ≈ 0.474 BTU

Joule to Kilowatt-hour

• Conversion: 1 kilowatt-hour = 3.6 × 10^6 joules.
• Example: To convert 18000 joules to kilowatt-hours, divide by 3.6 × 10^6.

• 18000 J / 3.6 × 10^6 ≈ 0.005 kWh

Calorie to Joule

• Conversion: 1 joule = 0.2390 calories.
• Example: To convert 50 calories to joules, multiply by 4.184.

• 50 cal × 4.184 = 209.2 J

Calorie to British Thermal Unit (BTU)

• Conversion: 1 calorie = 0.00397 BTUs.
• Example: To convert 100 calories to BTUs, multiply by 0.00397.

• 100 cal × 0.00397 = 0.397 BTU

Calorie to Kilowatt-hour

• Conversion: 1 kilowatt-hour = 860,420 calories.
• Example: To convert 1000 calories to kilowatt-hours, divide by 860,420.

• 1000 cal / 860,420 ≈ 0.00116 kWh

British Thermal Unit to Joule

• Conversion: 1 joule = 0.000948 BTUs.
• Example: To convert 2 BTUs to joules, multiply by 1055.

• 2 BTU × 1055 = 2110 J

British Thermal Unit to Calorie

• Conversion: 1 BTU = 252 calories.
• Example: To convert 5 BTUs to calories, multiply by 252.

• 5 BTU × 252 = 1260 cal

British Thermal Unit to Kilowatt-hour

• Conversion: 1 kilowatt-hour = 3412 BTUs.
• Example: To convert 10 BTUs to kilowatt-hours, divide by 3412.

• 10 BTU / 3412 ≈ 0.00293 kWh

Kilowatt-hour to Joule

• Conversion: 1 joule = 2.77778E-07 kilowatt-hours.
• Example: To convert 1 kilowatt-hour to joules, multiply by 3.6 × 10^6.

• 1 kWh × 3.6 × 10^6 = 3.6 × 10^6 J

Kilowatt-hour to Calorie

• Conversion: 1 calorie = 1.16279E-06 kilowatt-hours.
• Example: To convert 0.5 kilowatt-hours to calories, multiply by 860,420.

• 0.5 kWh × 860,420 = 430,210 cal

Kilowatt-hour to British Thermal Unit

• Conversion: 1 kilowatt-hour = 3412 BTUs.
• Example: To convert 0.1 kilowatt-hour to BTUs, multiply by 3412.

• 0.1 kWh × 3412 = 341.2 BTU

FAQ’S

What are the specific units of heat?

Heat is measured in joules (J) in the SI system, but calories (cal) and British Thermal Units (BTU) are also widely used.

What is the biggest unit of heat?

The biggest common unit of heat is the kilocalorie (kcal), primarily used in dietary energy contexts, translating large amounts of heat energy.

What is the old unit of heat?

The calorie, an older unit, was traditionally used to measure heat based on the energy needed to raise the temperature of water by one degree Celsius.