Last updated: 2026-06-30

Specific Heat Capacity

Specific heat capacity (symbol c) is the amount of energy required to raise the temperature of one unit of mass by one degree. It is a fundamental material property — water resists temperature change far more than iron does because its specific heat is roughly nine times higher.

The SI unit is joule per kilogram-kelvin (J/(kg·K)), but engineers and scientists also work with kJ/(kg·K), cal/(g·°C), and BTU/(lb·°F) depending on the context.

Units covered

J/(kg·K) — the SI base unit. All conversions use this as the reference. Water at 25°C has a specific heat of 4,186 J/(kg·K).

kJ/(kg·K) — 1,000 J/(kg·K). Most engineering thermodynamics textbooks list material properties in kJ/(kg·K) to keep numbers in a convenient range (air ≈ 1.005 kJ/(kg·K), steel ≈ 0.49 kJ/(kg·K)).

cal/(g·°C) — the calorie-based unit. One cal/(g·°C) equals 4,186.8 J/(kg·K) under the International Table (IT) definition. Water's specific heat is almost exactly 1 cal/(g·°C), which is not a coincidence — the calorie was originally defined that way.

BTU/(lb·°F) — the US customary unit used in HVAC, process engineering, and steam tables. One BTU/(lb·°F) equals 4,186.8 J/(kg·K), the same as 1 cal/(g·°C).

Key conversion table

From 1 unit	J/(kg·K)
1 kJ/(kg·K)	1,000
1 cal/(g·°C)	4,186.8
1 BTU/(lb·°F)	4,186.8

Why cal/(g·°C) and BTU/(lb·°F) are numerically equal

Both units were originally defined by how much heat water needs: the calorie for raising 1 g by 1°C, the BTU for raising 1 lb by 1°F. Because both reference the same substance (water) at nearly the same conditions, the specific heat of water is 1 in both systems, and the two units are therefore equal in magnitude when expressed per unit mass per unit temperature interval.

Typical specific heat values

Material	J/(kg·K)	kJ/(kg·K)	BTU/(lb·°F)
Water (liquid)	4,186	4.186	1.000
Air (at 25°C)	1,005	1.005	0.240
Aluminum	900	0.900	0.215
Iron / Steel	449	0.449	0.107
Copper	385	0.385	0.092
Concrete	880	0.880	0.210

Usage

Enter any specific heat value in any field and all other units update instantly. The default value is 4,186 J/(kg·K) — the specific heat of water, a common reference point in thermodynamics.

Frequently Asked Questions (FAQ)

What is the difference between J/(kg·K) and kJ/(kg·K)?

Both express the same physical quantity — how much energy is needed to raise one unit of mass by one degree of temperature. J/(kg·K) is the SI base form; kJ/(kg·K) is simply 1000 times larger. Water has a specific heat of 4186 J/(kg·K) or 4.186 kJ/(kg·K). Engineering textbooks often prefer kJ/(kg·K) because the numbers are more manageable for common materials (metals typically fall in the 0.1–0.9 kJ/(kg·K) range).

How do I convert BTU/(lb·°F) to J/(kg·K)?

Multiply BTU/(lb·°F) by 4186.8 to get J/(kg·K). One BTU/(lb·°F) is the amount of heat needed to raise one pound of a substance by one degree Fahrenheit. Using the International Table definitions (1 IT BTU = 1055.056 J, 1 lb = 0.453592 kg, 1°F = 5/9 K), the conversion factor works out to exactly 4186.8 J/(kg·K) per BTU/(lb·°F).

What is the specific heat capacity of water?

Water has a specific heat capacity of approximately 4186 J/(kg·K) — or equivalently 4.186 kJ/(kg·K), 1.0 cal/(g·°C), and 1.0 BTU/(lb·°F). This is exceptionally high compared to most other materials: iron is about 449 J/(kg·K), aluminum about 900 J/(kg·K), and copper about 385 J/(kg·K). Water's high specific heat is why it is used as a coolant in engines and why coastal climates are milder than inland ones.

Why do cal/(g·°C) and BTU/(lb·°F) give the same number?

The calorie was originally defined as the heat needed to raise 1 gram of water by 1°C, and the BTU was defined as the heat needed to raise 1 pound of water by 1°F. Since water has the same specific heat regardless of which system you use, the numerical value comes out the same: 1 cal/(g·°C) = 1 BTU/(lb·°F).

Both equal 4186.8 J/(kg·K) under the International Table definitions. This makes the cal/(g·°C) / BTU/(lb·°F) pair a handy sanity check when switching between metric and US customary results.