# Using Math & Scientific Constants

Learn how to enter built-in constants such as pi, e, Avogadro's constant, and the speed of light in Calcul.io expressions and unit conversions.

## Constants

Calcul.io offers plethora of useful constants that you can use in your calculations.

Currently, there's over 60 constants available

[Complete overview of all available constants](https://calcul.io/constants/index.md)

```calculio
speedOfLight
speedOfLight to km/h
```

[Next: **Currencies, Crypto and Stocks**](https://calcul.io/instructions/currency/index.md)

**Table of content**

- [Introduction](https://calcul.io/instructions/index.md)
- [Units and conversions](https://calcul.io/instructions/conversions/index.md)
- [List of units](https://calcul.io/instructions/units/index.md)
- [Functions](https://calcul.io/instructions/functions/index.md)
- [Constants](https://calcul.io/instructions/constants/index.md)
- [Currencies, crypto, stock](https://calcul.io/instructions/currency/index.md)
- [Time and dates](https://calcul.io/instructions/time/index.md)

## Choose the right built-in constant

Enter a constant by its exact Calcul.io name, either on its own or inside a larger expression. Constants can be combined with numbers, units, operators, and functions just like other values. Keeping the unit in the expression makes scientific calculations easier to check and reduces accidental conversions between incompatible quantities.

Use the [constants reference](https://calcul.io/constants/index.md) to review available names, meanings, units, and editable examples before adding one to a calculation.