How to tie PVC to cast iron

[opey541]

Been trying to change my kitchen sink plumbing to PVC. It ties into cast iron that I thought was 3" but maybe 2 1/2" connection. Any help would be great right now I'm running 2" PVC to the connection shown.

 

[Reach4]

If there is a cast iron hub, you can maybe put a "donut" there. If you will be cutting a pipe, measure the OD of the pipe. Consider a long-reach caliper to measure. https://www.harborfreight.com/6-in-long-reach-digital-caliper-63714.html

You can also wrap a string around the pipe. Mark the string. Measure the string length between marks, and divide by pi (3.14); that will give the diameter.