# Hartree Energy Constant

Use Hartree Energy in Calcul.io. See what this built-in constant represents, enter it in an editable calculation, and review related values and units.

## hartreeEnergy

Hartree energy

Try it yourself:

```calculio
hartreeEnergy
```

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## Hartree energy: an atomic energy scale

The **hartreeEnergy** constant is the Hartree energy, a natural unit of energy in atomic physics and quantum chemistry. It is approximately 4.36 × 10⁻¹⁸ joules, or about 27.2 electronvolts.

## Practical use

Hartrees make electronic-structure energies compact. Convert a result with [to()](https://calcul.io/function/to/index.md) or choose a readable unit with [toBest()](https://calcul.io/function/toBest/index.md). Compare it with the [Planck constant](https://calcul.io/constants/planckConstant/index.md) in quantum formulas.

## Caveat

Keep units attached through a calculation; Hartree is an energy unit, not a voltage or frequency.

[All constants](https://calcul.io/constants/index.md)