It is often thought that the size of a manhole cover should be measured by its outside dimensions — this is wrong — although logical and most other things in life get by this way, it’s not the best way to measure a manhole cover.
Manhole covers are manufactured and sold by their clear opening size — the size of the manhole that’s being covered — and not the size of the cover itself. This is so the cover can sit securely over the top of the manhole.
It doesn’t matter if the cover is square or round — it is always the clear opening size you should be measuring.
As an example, one of our B125 circuluar manhole covers is actually 555mm in overall diameter (including its frame). Although, it’s designed to fit a manhole that has a diameter of 450mm, so it is listed as a 450mm B125 Circular Manhole Cover. It’s still worth keeping external dimensions in mind, as depending on whether you need to buy a new or replacement manhole cover it could impact how and where you can install it.
