In thermodynamics, the specific heat capacity of a substance is the slope (derivative) of the internal energy due to random motion of atoms in a sample as a function of temperature, normalized by dividing by the mass of the sample. Because the internal energy curve is normally almost linear, it can be approximated by measuring the heat required to raise the temperature of 1 kg of a substance by 1 °C (or one kelvin). The SI unit of measurement for this is the joule per kilogram per kelvin (J·kg^-1·K^-1). 1 J/(kg·K) is identical to 1 m²/(s²·K), which is non-intuitive, but involves only SI base units.

Factors that influence heat capacity measurements:

• The temperature of the substance. For example, measuring the heat capacity of water produces different results if you start at 20 °C or 60 °C.
• Intermolecular forces. If a fluid has stronger intermolecular forces (such as hydrogen bonding in water) then the heat capacity is likely to be higher.

Related concepts are specific melting heat and specific heat of vaporisation, see also Phase (matter), Heat#Changes of state, Latent heat.