Product review (5 stars): Perfect Fit for Sinkmaster 850 Heavy Duty Disposal
Thanks to other Reviewers here, I was able to measure the drain to be sure this would fit and it's the perfect size.  My drain opening is 3-1/8" & this "stopper" has a 3.1" diameter, just as others have stated.  It's an ideal fit; not overly tight and not loose; great replacement and the price is excellent.  You get TWO pieces with this kit; the drain insert that snugs down into the bottom of the drain to prevent blowback from the running disposer & a "stopper" to help keep water dammed up in the sink for dishwashing tasks.  Fit my Sinkmaster Model 850 perfectly & ordering it from Amazon sure beats a drive to my local hardware store!