Excel doesn’t currently support watermarks. But as a workaround, you can use WordArt to create a watermark.
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Go to Insert, select
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Type something like Draft or Confidential, and rotate it as needed.
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Control-Click the text and select Format Text Effects.
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Under Text Fill & Outline, set Transparency under Text Fill to 50% or more.
For more info, see Add a watermark to a sheet in Excel for Mac.
Creating a watermark isn't currently possible in Excel for the web. You can, however, create a watermark in the Excel desktop app.
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To open the desktop app, at the top of the ribbon, select Editing > Open in Desktop App.
Don't have the Excel desktop app? Try or buy Microsoft 365.
