If you take one thing from this guide, make it this:
Manhole covers are sized by the clear opening, not the lid.
Once that clicks, everything else becomes straightforward.
This short guide walks you through the process in the same order we would on the phone.
Start Here, What Are You Actually Replacing?
Before measuring anything, ask yourself one question:
- Are you replacing an existing cover, or
- Buying a cover for a new opening?
In both cases, the measurement you need is the same.
Step 1, Ignore the Lid Completely
This is where most mistakes start.
The lid size is irrelevant.
The visible top plate is never the reference dimension.
Manhole covers are specified by the clear opening, meaning:
- The internal opening of the chamber
- The hole the cover drops into
- The structural opening below the frame
Different names, same thing.
Step 2, Can You Lift the Lid?
If yes, this is the best and most accurate method.
Remove the lid safely and measure:
-
Square or rectangular chambers
Measure internal width × internal length -
Round chambers
Measure the internal diameter
Always measure in millimetres.
That is how every cover on the site is listed.
That number is the size you order.
Lid Won’t Come Out? Do This Instead
If the lid is stuck, sealed or unsafe to lift, you can still get close enough to order correctly.
- Measure the overall size of the cover, edge to edge
- Subtract 50 to 100 mm from each dimension
This gives a reliable estimate of the clear opening.
Example
Overall size: 670 × 530 mm
Estimated clear opening: 600 × 450 mm
This works because frame widths are fairly consistent across standard covers.
Quick Sense Check Before You Order
Ask yourself:
- Does this size sound like a standard chamber?
- Is it close to common sizes like 300, 450, 600 or 750 mm?
If yes, you are almost certainly measuring the right thing.
If not, pause and recheck before ordering.
Shape Matters, But Less Than You Think
Round chambers
Measure the internal diameter.
Square recessed covers can still be used if the clear opening is large enough, commonly 300 mm or 450 mm.
Rectangular chambers
Measure at the narrowest internal points, not across chamfers or slopes.
Measuring for Recessed Manhole Covers
Same rules apply.
Recess depth does not change how you measure.
Measure the clear opening first.
Then choose tray depth to suit the surface finish.
Typical guidance:
- Tiles or slabs: 20 to 45 mm
- Block paving: 80 to 100 mm
Internal locations usually need sealed and locking or triple sealed covers.
Final Check, Read This Before Buying
- Do not measure the lid
- Measure the internal clear opening
- Use millimetres
- Round chamber, measure diameter
- Rectangular chamber, measure width × length
- Lid stuck, overall size minus 50 to 100 mm
- Order by clear opening, not overall size
Why This Matters
Most returns happen for one reason only.
The cover was ordered to the lid size, not the opening.
Get the clear opening right and everything else becomes a specification choice, not a guessing game.
Every product page clearly states the clear opening. Match that number and the cover will fit as intended.
