Replacement Plug O Ring Seal - Which size?

[HenryGregory]

I need to change the seals on my bathroom sinks as the O Rings have started to become very loose.
I have taken some measurements and I am getting 33mm wide for where the rubber actually attaches to:

The outer measurements of the plug are 34mm:

Can anyone recommend some suitable O Rings that will work as replacements? I am finding a lot of 38 or 39mm ones online but think they would be too large.
Not sure if I should be going with the smaller 33 r larger 34 mm size when ordering.

Thanks in advance

 

[Roddy Young]

Your caliper gauge is being used wrongly and giving you the wrong reading. Put the thing you're measuring between the 2 parallel jaws ( facing down in your photo) which will measure the diameter .... assuming the jaws reach the middle ie diameter of the " plug".

 

[Madrab]

I really need to refresh the page before I press post Wot he said ^^

I hate to tell you this but you're not using your calipers properly, you are using the internal diameter side of the calipers, the outside diameter is measured by the lower set of caliper jaws.

 

[HenryGregory]

Thank you BOTH ever so much. Durrr, silly me. I was given it as a gift, first time ever using it and the battery has gone flat so was trying to do it manually.

I have re-done thanks again to both of you.
I am getting for the inner measurement 35mm:

And for the outer 39mm:

I have notices that the O Ring has a bit of a lip to it on the edge. I have not seen many like it online. The sinks are Roca. Not sure if they have a different type to normal ones?

 

[HenryGregory]

Maybe something like this would be a good match?