Manhole Calculator — How Many Rings for an Inspection Chamber?

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.