# Enthalpy Calculator

URL: https://speedycalculator.com/calculators/physics/enthalpy/
Last updated: 2026-10-30

> The enthalpy calculator finds the change in enthalpy of a process from the change in internal energy and the pressure-volume work, or the enthalpy of reaction from the enthalpies of formation of up to three reactants and three products.

## Inputs

- Method
- Change in internal energy ΔU (J, kJ, MJ, kWh, cal, kcal, BTU)
- Pressure p (kPa, Pa, bar, atm, psi, MPa)
- Change in volume ΔV (L, m³, mL, ft³)
- Reactant 1 coefficient
- Reactant 1 ΔH_f (kJ/mol)
- Reactant 2 coefficient
- Reactant 2 ΔH_f (kJ/mol)
- Reactant 3 coefficient
- Reactant 3 ΔH_f (kJ/mol)
- Product 1 coefficient
- Product 1 ΔH_f (kJ/mol)
- Product 2 coefficient
- Product 2 ΔH_f (kJ/mol)
- Product 3 coefficient
- Product 3 ΔH_f (kJ/mol)

## Results

- Enthalpy change ΔH
- Process

## Formula

```
ΔH = ΔU + p·ΔV · ΔH_rxn = Σν·ΔH_f(products) − Σν·ΔH_f(reactants)
```

## Explanation

Enthalpy is the heat content of a system at constant pressure. Negative ΔH means a reaction releases heat (exothermic), like burning fuel; positive means it absorbs heat (endothermic), like cold packs.

### How to use the Enthalpy Calculator

- **Method**
- **Change in internal energy ΔU**
- **Pressure p**
- **Change in volume ΔV**
- **Reactant 1 coefficient**
- **Reactant 1 ΔH_f (kJ/mol)**
- **Reactant 2 coefficient**
- **Reactant 2 ΔH_f (kJ/mol)**
- **Reactant 3 coefficient**
- **Reactant 3 ΔH_f (kJ/mol)**
- **Product 1 coefficient**
- **Product 1 ΔH_f (kJ/mol)**
- **Product 2 coefficient**
- **Product 2 ΔH_f (kJ/mol)**
- **Product 3 coefficient**
- **Product 3 ΔH_f (kJ/mol)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Choose the method. For reactions, enter the stoichiometric coefficient and standard enthalpy of formation (kJ/mol) of each species; leave unused coefficients at 0.

### Tips

- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.

### Common mistakes to avoid

- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).

### Inputs explained

FieldWhat to enterMethodChoose one: ΔH = ΔU + pΔV, Enthalpy of reaction from ΔH_fChange in internal energy ΔUUnits: J, kJ, MJ, kWh, cal, kcal …Pressure pUnits: kPa, Pa, bar, atm, psi, MPaChange in volume ΔVUnits: L, m³, mL, ft³Reactant 1 coefficientA numberReactant 1 ΔH_f (kJ/mol)A numberReactant 2 coefficientA numberReactant 2 ΔH_f (kJ/mol)A numberReactant 3 coefficientA numberReactant 3 ΔH_f (kJ/mol)A numberProduct 1 coefficientA numberProduct 1 ΔH_f (kJ/mol)A numberProduct 2 coefficientA numberProduct 2 ΔH_f (kJ/mol)A numberProduct 3 coefficientA numberProduct 3 ΔH_f (kJ/mol)A number

### Example values

How the main result changes when **Change in internal energy ΔU** changes and everything else stays at the example values above:

Change in internal energy ΔU (J)Enthalpy change ΔH (kJ)5-890.37.5-890.310-890.312.5-890.315-890.320-890.3

## Worked example

Inputs — Method: Enthalpy of reaction from ΔH_f; Reactant 1 coefficient: 1; Reactant 1 ΔH_f (kJ/mol): -74.8; Reactant 2 coefficient: 2; Reactant 2 ΔH_f (kJ/mol): 0; Reactant 3 coefficient: 0; Reactant 3 ΔH_f (kJ/mol): 0; Product 1 coefficient: 1; Product 1 ΔH_f (kJ/mol): -393.5; Product 2 coefficient: 2; Product 2 ΔH_f (kJ/mol): -285.8; Product 3 coefficient: 0; Product 3 ΔH_f (kJ/mol): 0.

Results — **Enthalpy change ΔH = -890.3 kJ**; Process = Exothermic (releases heat).

## Key terms

- **Enthalpy of formation**: ΔH to form 1 mol of a compound from its elements.

## FAQ

**What does a negative ΔH mean?**
The reaction releases heat.

**Why is the ΔH_f of O₂ zero?**
Elements in their standard state are the reference.

**What is the example?**
Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O(l), about −890 kJ/mol.

**Is enthalpy the same as energy?**
Enthalpy includes the pV work at constant pressure.

**What formula does the Enthalpy Calculator use?**
ΔH = ΔU + p·ΔV · ΔH_rxn = Σν·ΔH_f(products) − Σν·ΔH_f(reactants).


## Sources

- [OpenStax – Chemistry 2e](https://openstax.org/details/books/chemistry-2e)
- [NIST – Fundamental physical constants](https://physics.nist.gov/cuu/Constants/)