Ordering the wrong size of manhole cover is one of the most common mistakes on drainage and utility projects. The cover rocks in its frame, or it doesn't fit the chamber at all. This guide lists the standard sizes of FRP manhole covers Rawji Exports moulds, explains how to measure a chamber, and shows how size links to the EN-124 load class you need.
First, know the difference: clear opening vs overall size
- Clear opening is the usable hole through the frame, the space a person or inspection camera passes through. Covers are specified by clear opening.
- Overall frame size is larger, because it includes the frame flange that sits on, or is cast into, the chamber wall.
When you order, always state the clear opening. If you are replacing a cover in an existing frame, also measure the frame's seating rebate.
Circular FRP manhole cover sizes
Standard clear openings for circular FRP manhole covers:
- 300 mm (12")
- 450 mm (18")
- 500 mm (20")
- 600 mm (24"): the most popular size, because a 600 mm opening gives maintenance workers and camera inspection crawlers enough clearance to enter sewer chambers
- 750 mm (30")
- 900 mm (36")
- 1000 mm (40")
Circular covers come with a rigid composite frame 50–100 mm deep, with anchoring ears for bedding into concrete.
Square and rectangular cover sizes
Standard sizes for square and rectangular manhole and inspection chamber covers:
- Square: 300 × 300, 450 × 450, 600 × 600, 900 × 900 and 1000 × 1000 mm
- Rectangular: 450 × 600, 450 × 900, 900 × 600, 1200 × 900 and 1500 × 900 mm
- Large openings such as pump pits and cable trenches: 2-leaf, 3-leaf and continuous trench-run modular covers with removable support beams, so each section can be lifted by one person
Gully grating and drain cover sizes
For stormwater inlets, FRP gully gratings are made in 300 × 300, 450 × 450, 500 × 500, 600 × 600, 600 × 450 and 900 × 450 mm, plus custom sizes.
Size and load class go together
Choose the EN-124 load class from the traffic that will drive over the cover, then check that the class is available in your size. Typical size ranges by class:
- A15 (1.5 tonnes): about 300–600 mm, for footpaths, gardens and cycle tracks.
- B125 (12.5 tonnes): about 450–700 mm, for car parks, driveways and kerbside lanes.
- C250 (25 tonnes): about 500–800 mm, for urban roads, bus bays and service corridors.
- D400 (40 tonnes): about 600–900 mm, for highways, container yards and heavy industrial areas. Standard D400 covers come in 500, 600, 750 and 900 mm circular, and 600 × 600 and 900 × 900 mm square.
Not sure which class you need? Read our EN-124 load class guide. Always specify the class in writing, never just "heavy duty".
How to measure an existing chamber
- Measure the inside of the chamber opening at the top, in both directions for square or rectangular chambers.
- If the old frame is staying, measure the rebate the cover sits in, including its depth.
- Note the shape, and whether the cover sits in a road, footpath or yard.
- Take a photo of the chamber and the old cover with a tape measure in the frame.
Weight
FRP covers are about 80% lighter than equivalent cast iron covers. A standard D400 FRP cover weighs roughly 22–28 kg, against roughly 90–120 kg for a cast or ductile iron cover it replaces. That makes lifting for inspection quicker and safer. See FRP vs cast iron covers.
What to send for a quote
- Shape and clear opening (or chamber measurements and a photo)
- EN-124 load class
- Quantity, and whether frames are needed
- Locking requirement, and any logo or utility marking (such as SEWER, WATER or FIBRE OPTIC)
- Delivery location
Indicative prices by class are on the FRP manhole cover price list, or see the full FRP manhole covers range.
