When you first begin using OneNote for the web, notes you type on new pages you’ve created are formatted with the 11-point Calibri font.
Unfortunately, you can't currently change the default font setting from within OneNote for the web, but you can in OneNote for Windows 10, OneNote 2016, and OneNote for Mac. Once you set a new default font, future notes for OneNote for the web will reflect the change.
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For OneNote for Windows 10, open the notebook for which you want to change the default font.
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Click or tap Settings and more ( ... ) located in the upper right corner, and then click or tap Settings.
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Click or tap Options, and then on the Options pane, under Default Font, select a new font or font size.
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Learn how to change the default font in OneNote 2016 for Windows
Tip: If you find the font in OneNote for the web too small but you don’t want to actually change the text formatting of your notes, you can use your browser’s Zoom function to temporarily change how small or large you want your notes to appear on the current page.