Subtract the amount of enthalpy present prior to the chemical or physical process taking place from the amount of enthalpy remaining after the chemical or physical process has taken place. The result is the delta H of the process.
Subtract the enthalpy of the substances consumed in a physical reaction from the enthalpy of the substances produced by the reaction. The result is the delta H of the reaction.
Multiply the pressure (in pascals) by the change in volume (in cubic meters) of the substances involved in a physical or chemical process. Add this value to the change in internal energy (in joules per mole) from the beginning of the process to the end of the process. The result is the delta H of the process.