Need a new furnace vent cap

So the vent cap on my roof appears to be rusting away and I wanted to replace it. Currently, it looks like it has something like this:

https://www.homedepot.com/p/Master-Flow-7-in-Vent-Cap-VC7/100135124

However, the circumference when I measured is 21 inches exact, which is roughly a 6.7 inch diameter. Where the heck do I find one that measures by circumference instead of diameter?