# Pi Constant

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

## pi

Try it yourself:

```calculio
pi
sin(pi/2)
```

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

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## Pi: the circle constant

**pi** is the ratio of a circle’s circumference to its diameter, approximately 3.14159. It is exact and dimensionless.

## Use

Circle area is pi times radius squared. Use pi with [sin()](https://calcul.io/function/sin/index.md), [cos()](https://calcul.io/function/cos/index.md), and [tan()](https://calcul.io/function/tan/index.md); use [phi](https://calcul.io/constants/phi/index.md) for a separate algebraic constant.

## Caveat

Trigonometric functions normally use radians. Preserve pi symbolically until numeric output.

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