According to the manufacturer, the SWI15007 - Zip Combs Spines fit an 8 1/2" x 11" piece of paper.
How many sheets do you need to bind? The easiest way to pick the right comb size is to choose according to sheet capacity. Manufacturers list the bindings' sheet capacity as well as diameter in the product name and description. For example, FEL52507 Plastic Comb Bindings have a diameter of 5/16." Instead of trying to convert that to mm, just look at the sheet capacity. In this case, the sheet capacity is 40 sheets.
You can use the Sheet Capacity filter on the right side of the the Binder Spines page, under "Narrow Your Choices," to select the size you need.
According to Fellows it is 1/4" comb bind
With a good supply of binding spines, your binding machines are always ready to use when you need them. On Time Supplies carries binding spines in a range of capacities and styles, including Fellowes plastic comb bindings. Thanks to fast shipping and bulk pricing from On Time Supplies, you always have binding spines when you need them. If you need more information on the supplies featured at On Time Supplies, or on GSA pricing, call us toll free at 1-866-501-6055. We're happy to answer any supply questions.