View > View 1:1 > Calibrate 1:1

Tool Palettes > View palette >

Lets you set an orthographic view so that your model shows up at true scale (1:1).

True scale (or 1:1 scale) means that a 10 centimeter long object will appear as 10 centimeters long when measured on the screen with a measuring tape.

To calibrate the view to be 1:1

Choose View > View 1:1 > Calibrate 1:1 - q
The option box opens, and a distance measurement (red line) appears in the current orthographic window.

NoteThis will also work in the Perspective window if Perspective is turned off in the Viewing Panel, and one of the preset orthographic views (horizontal or vertical arrows) is clicked in the Viewing Panel. (See Use the view panel).
Dolly in/out if necessary to have the distance measurement fill up most of your window.
Use a real ruler or measuring tape to measure the true length of the distance measurement on the screen.
Enter this value as the Displayed Length in the option box.
Press the Calibrate button in the option box.

Options

Reference Length: The world space length (in current linear units) of the distance measurement as it first appears on your screen. Choose between 1, 10, 100 or User defined, to adjust this value.
User Defined Length: If Reference Length is set to User defined, this field allows you to enter any value for the length of the initial distance measurement.
Displayed Length: True length of the distance measurement as measured on the screen with a measuring tape or ruler.