Before you can perform takeoff on a sheet, you must specify its scale.
Set the scale by selecting from scale options in the viewer toolbar, or by calibrating the sheet. You can also select the scale for multiple sheets at once.
You can calibrate the sheet by capturing a known distance between two objects and then adjusting your sheet to reflect these dimensions and measurement units.
To calibrate and set scale:
Click Scale not set to start calibration. Alternatively, select Calibrate
from the viewer toolbar.
Select two points on the sheet.
In the Calibration dialog that opens, define the size and unit type of the dimension you just drew, then click Set calibration.
The scale for the sheet is now set and is displayed in the viewer toolbar at the bottom of the sheet. The scale is set for this sheet for anyone viewing this project in Takeoff.
You can select a scale from the viewer toolbar:
You can set the scale for multiple sheets at once. Only sheets without takeoff can use this action.
To set the scale for multiple sheets:
Click the scale (or Scale not set if scale has not yet been set) in the toolbar at the bottom of a sheet in the viewer. A list of scale options will open.
Hover over the desired scale option in the list. An Apply to multiple sheets icon will be visible to the right of the scale.

Click the Apply to multiple sheets icon to open the Apply scale to selected sheets modal.
Select the sheets in the modal that you want to apply the selected scale to.
Click Apply scale to set the scale for the selected sheets.
The scale is now set for the selected sheets and is the scale for anyone viewing this sheet in the project in Takeoff.