A "tab" of butter is not a standard measurement. It's likely a colloquial term, and different people could have different ideas of how much butter a "tab" represents.

To get a more accurate answer, you need to specify:

* What kind of butter? Is it a stick of butter, a pat of butter, or something else?

* What size is the "tab"? Is it a small piece, a larger chunk, or something specific?

If you can give more context, I can help you figure out how many ounces are in your "tab" of butter.