Heat Transfer

What Is Specific Heat Capacity?

Specific heat capacity (Cp) is the energy required per unit mass per degree of temperature change. Learn why it matters in heat duty calculations and how units relate.

TypeEngineering guide — concept explainer

Definition

Specific heat capacity (Cp) is the amount of energy required to raise the temperature of one unit mass of a substance by one degree. It is a material property — different substances absorb different amounts of heat for the same temperature change. Common units are J/(kg·K), kJ/(kg·K), and BTU/(lb·°F).

Why it matters

Cp is a required input for any sensible heat calculation. When sizing heaters, coolers, or heat exchangers, the formula Q = m × Cp × ΔT governs the heat duty. An incorrect Cp value directly scales the error in heat duty — a 10% overestimate in Cp produces a 10% overestimate in required heat transfer. Engineers must obtain Cp from reliable references for the specific material and temperature range.

Formula

Heat duty (batch)

Q = m × Cp × ΔT

Heat duty (continuous)

Q̇ = ṁ × Cp × ΔT

Units involved

•Cp — specific heat capacity in J/(kg·K), kJ/(kg·K), or BTU/(lb·°F)
•Q — heat energy in J, kJ, or BTU
•Q̇ — heat duty (power) in W, kW, or BTU/h
•m — mass in kg or lb
•ṁ — mass flow rate in kg/s, kg/h, or lb/h
•ΔT — temperature change in K, °C, or °F

Concept diagram

Worked example

How much energy is needed to heat 50 kg of water from 20 °C to 60 °C? Cp of water at this temperature range is approximately 4.184 kJ/(kg·K).

01m = 50 kg
02Cp = 4.184 kJ/(kg·K)
03ΔT = 60 − 20 = 40 K
04Q = 50 × 4.184 × 40
05Q = 8,368 kJ

Result

Heat energy required = 8,368 kJ

Common mistakes

•Using Cp in J/(kg·K) when the rest of the calculation expects kJ/(kg·K) — this introduces a factor-of-1000 error.
•Treating Cp as constant across large temperature ranges — Cp varies with temperature, especially for gases. Use an average value over the interval.
•Confusing Cp (constant pressure) with Cv (constant volume) — for liquids the difference is small, but for gases it matters significantly.
•Applying Cp across a phase change — Cp applies only to sensible heat (temperature change). Phase changes require latent heat calculations.
•Using Cp for one material but calculating heat duty for a different fluid — Cp is material-specific.

When to use the calculator

Use the Heat Duty calculator when you know the mass flow rate, specific heat capacity, and temperature change. The calculator handles unit conversions between kW, BTU/h, and other power units. You must supply the Cp value — ProcessConvert does not include a Cp lookup.

Heat Duty Calculator →

FAQ

What is a typical Cp value for water?

Water at approximately 25 °C has Cp ≈ 4.184 kJ/(kg·K) or approximately 1.0 BTU/(lb·°F). This value changes slightly with temperature — at 80 °C it is about 4.196 kJ/(kg·K). For many engineering estimates, 4.18 kJ/(kg·K) is used as a round value.

Why does ProcessConvert not look up Cp automatically?

Cp depends on the specific material, its temperature, and sometimes its pressure. Providing a lookup requires a property database with defined accuracy and source citations. ProcessConvert currently requires the user to supply Cp from a trusted reference such as Perry's Chemical Engineers' Handbook or the NIST webbook.

Is Cp the same in °C and K?

Yes. A temperature difference of 1 °C equals a temperature difference of 1 K, so Cp values in J/(kg·K) and J/(kg·°C) are numerically identical. However, °F uses a different interval size — conversion between J/(kg·K) and BTU/(lb·°F) requires a unit conversion factor.

How do I convert Cp between SI and imperial units?

Use the specific heat capacity conversions on ProcessConvert. For example, 1 BTU/(lb·°F) = 4,186.8 J/(kg·K). The conversion accounts for differences in mass (lb vs kg) and temperature interval (°F vs K).