Frame size vs clear opening
A simple guide to avoid sizing mistakes: how to choose frame size vs clear opening for RCC manhole covers and chamber covers.
Clear opening comes first
The clear opening is the usable inner opening of the frame. It must match the chamber access requirement. Once clear opening is fixed, select a compatible cover and then confirm the overall frame size fits the top surface.
Example (600 × 600 frame)
A 600 × 600 frame may have a smaller clear opening (for example 400 × 400). This is normal because the frame has walls and seating ledges. Always compare the clear opening values shown in the specifications table.
Checklist for correct fitment
- Measure the chamber opening (inner access).
- Select a clear opening that matches or slightly exceeds the access requirement.
- Confirm frame size fits the top slab and installation area.
- Confirm cover thickness/height and load class.
If you are unsure, send the site photo + measurement on WhatsApp from the product page and we will recommend the closest match.
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