# Rulers

You can overlay horizontal and vertical rulers on a web page:

```{image} viewport_rulers.png
:class: center
```

The units are in pixels.

The dimensions of the viewport are displayed near the top-right corner of the viewport.

To be able to toggle rulers for a page, you first need to enable the button by going to the settings page for the Developer Tools and checking "Toggle rulers for the page" under Available Toolbox Buttons.

```{image} settings_available_buttons.png
:class: center
```

Once enabled, the "Toggle rulers for the page" button appears at the top right of the Toolbox, in the same place as the Settings/Options button.

```{image} ruler_toggle_button.png
:class: center
```

Behavior to keep in mind when using rulers:

- The rulers command must be reapplied in **new tabs** and after **each page refresh.**
- The command isn't permanent.