I recently tried to buy a 3-ring binder, and I was annoyed didn't list spacings on paper OR binders. This is my attempt to write a reference for future searchers.

The spacing between rings seems to be standard. The ring size itself isn't standard. Holes vary from 6-8mm. I live in the United States -- different binders may be standard where you live.

Here are the spacings I found. If you encounter a 3-ring binder not listed, or pre-punched paper that doesn't match the below, please email me with the measurements and brand name.

Paper sizes:

• US Letter: 215.9x279.4 mm / 8.5x11 inch
• A4: 210 x 297 mm / 8-1/4 x 11-3/4 in
• A5: 148 x 210 mm / 5-7/8 x 8-1/4 in
• A6: 105 x 148 mm / 4-1/8 x 5-7/8 in
• Filofax (brand name): 95x170 mm / 3.75x6.7 inch
• SMD "sampler" parts sheets (unclear if standardized): 117x175 mm; A6 6-ring spacing

US Letter, 3-ring

These are the "standard" 3-ring binders in the USA.

108mm hole spacing

A5, 3-ring

In Staples, I found these sold as "mini" binders.

70mm hole spacting

A5, 6-ring

19mm hole pitch, 70mm central gap

A6, 6-ring

19mm hole pitch, 40mm central gap

Filofax, 6-ring

19mm hole pitch, 50.8mm central gap