How many manhole rings do I need?

The number of rings depends on the manhole depth. Standard precast concrete rings are 500mm high. Deduct approximately 300mm for the cover slab and base slab, then divide the remaining depth by 0.5m and round up. For example, a 1.2m deep manhole needs approximately 2 rings (1.2m - 0.3m = 0.9m, 0.9 / 0.5 = 1.8, rounded up to 2).

What is the difference between a manhole and an inspection chamber?

An inspection chamber is typically shallower (up to 1m deep) and smaller diameter, used for access to drainage runs at junctions and changes of direction. A manhole is deeper (over 1m) and requires step irons or a ladder for access. Both serve the purpose of allowing inspection and maintenance of underground drainage. Part H of the Building Regulations defines the requirements for each.

What size manhole cover do I need?

Manhole cover size depends on the loading class and location. For gardens and pedestrian areas, a Class A15 cover (450x450mm or 600x450mm) is sufficient. For driveways and car parks, use Class B125 (600x450mm or 600x600mm). For roads and areas with HGV traffic, Class D400 (600x600mm or larger) is required. The cover must be appropriately rated for the expected traffic loading.

When do I need step irons in a manhole?

Step irons are required in manholes deeper than 900mm to provide safe access for inspection and maintenance. They should be spaced at approximately 300mm intervals and staggered for safe climbing. Step irons must comply with BS EN 13101 and be made of corrosion-resistant material (typically polypropylene-coated steel or galvanised iron).

What is benching in a manhole?

Benching is the concrete shaped formation at the base of a manhole that guides the flow of water through the channels. It is formed to a smooth finish with a fall towards the channel to prevent standing water and debris accumulation. Benching should rise to at least the top of the pipe (soffit level) and be finished with a smooth surface. It typically uses a 1:1 or 1:12 slope depending on the situation.

Manhole Calculator — How Many Rings Do I Need for an Inspection Chamber?

Manhole Calculator — How Many Rings for an Inspection Chamber?

Calculate precast concrete rings, bricks, benching volume, cover and frame requirements for manholes and inspection chambers to Part H Building Regulations.

Manhole Depth (invert level)

Depth from finished ground level to invert

Number of Branches

Number of incoming pipe connections

Manhole Type

Precast rings are most common for domestic drainage

Cover / Loading Class

Select based on location — garden, driveway, or road

Price Per Ring (optional)

Price per precast concrete ring for cost estimate

How We Calculate This

This calculator determines the materials needed for a manhole or inspection chamber based on the invert depth, construction type, and loading requirements.

The formula

Rings = ceil((Total depth - Cover slab - Base slab) / Ring height)

For precast concrete ring manholes, the usable depth is calculated by deducting approximately 150mm for the cover slab and 150mm for the base slab from the total manhole depth. The remaining depth is divided by the ring height (typically 500mm) and rounded up.

Brick manholes

For brick-built manholes, the calculator determines the number of courses and bricks per course based on the internal diameter. Engineering bricks (Class B per BS EN 771-1) are required for below-ground drainage work. Each course is 75mm high (65mm brick + 10mm mortar joint), and the number of bricks per course depends on the circumference.

Benching volume

Benching volume is estimated based on approximately 60% of the base area multiplied by an average benching height of 150mm. The actual volume will vary depending on the number and size of incoming and outgoing pipes.

Standards

Manholes and inspection chambers must comply with Part H of the Building Regulations, BS EN 752 (Drain and sewer systems outside buildings), and BS EN 1917 (Precast concrete manholes and inspection chambers). Cover loading classes are defined in BS EN 124.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Manhole Calculator — How Many Rings for an Inspection Chamber?

How We Calculate This

The formula

Brick manholes

Benching volume

Standards

Frequently Asked Questions

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