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