Hello, I'm an expert in kitchenware and home goods. When it comes to the size of the top of a Mason jar, it's important to note that there are several standard sizes for Mason jars, and the diameter of the opening can vary depending on the size of the jar.

The most common sizes for Mason jars are:

1. Regular Mouth: This is about 2 3/8 inches (60mm) in diameter.
2. Wide Mouth: This is about 2 7/8 inches (72mm) in diameter.

These measurements are for the inner diameter of the jar's opening where the lid will be placed. The actual size of the jar's mouth, including the threads, may be slightly larger.

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In the United States, standard-size Mason jars are made of soda-lime glass and come in two sizes: regular mouth, 2 3?8 in (60 mm) inner and 2 3?4 in (70 mm) outer diameter, and wide mouth, 3 in (76 mm) inner and 3 3?8 in (86 mm) outer diameter, versions.read more >>