Manhole & Inspection Chamber Calculator
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Manhole & Inspection Chamber Calculator
Find the right access type, chamber size and cover class for a given depth and pipe — then take off the precast rings, slab, base, bricks and excavation. Built on UK Building Regulations (Approved Document H1, Tables 11 & 12), cover classes to BS EN 124:2015 and precast units to BS EN 1917 / BS 5911.
What access chamber do I need?
Approved Document H sets the access type and minimum size by depth to invert and the largest pipe. The cover class comes from where the chamber sits (BS EN 124:2015).
Ring & material take-off
Estimate the precast rings (or plastic risers), cover slab, base, seating bricks, benching concrete and excavation for a chamber of a given depth.
Reference & rules
The access types, minimum sizes and cover classes behind this calculator.
| Type | Depth to invert | Min. internal size |
|---|---|---|
| Rodding eye | – | as pipe (min 100) |
| Access fitting (small) | ≤ 0.6 m | 150 dia / 150×100 |
| Access fitting (large) | ≤ 0.6 m | 225×100 |
| Inspection chamber (shallow) | ≤ 0.6 m | 190 dia / 225×100 |
| Inspection chamber | ≤ 1.2 m | 450 dia / 450×450 |
| Inspection chamber (deep) | > 1.2 m | 450 dia (access ≤ 350) |
| Depth to soffit | Pipe DN | Min. diameter |
|---|---|---|
| < 1.5 m | ≤150 | 1000 (sewer 1200) |
| < 1.5 m | 225–300 | 1200 |
| < 1.5 m | >300 | 1800 or DN+450 |
| 1.5–3.0 m | ≤300 | 1200 |
| 1.5–3.0 m | 375–450 | 1200 (1350×1225 rect) |
| 1.5–3.0 m | >450 | 1800 or DN+775 |
| > 3.0 m (shaft) | – | 1050 steps / 1200 ladder |
| Class | Test load | Typical use |
|---|---|---|
| A15 | 15 kN | pedestrian / no vehicles |
| B125 | 125 kN | footways, domestic driveways |
| C250 | 250 kN | kerbside / car parks |
| D400 | 400 kN | road carriageways |
| E600 | 600 kN | industrial / docks |
| F900 | 900 kN | heavy industry / airfields |
Benching: channels benched up to the top of the outgoing pipe at a slope of 1 in 12, rounded with a radius of at least 25 mm (AD H 2.53). Minimum clear opening for a manhole is 600×600 mm. Person-entry chambers need step irons/ladder to BS EN 13101 and a confined-space safe system of work.
This inspection chamber calculator finds the right access type, chamber size and cover class for a given depth and pipe, then takes off the precast rings, slab, base, bricks and excavation. It follows Approved Document H1 (Tables 11 and 12), cover classes to BS EN 124:2015 and precast units to BS EN 1917 / BS 5911. See Approved Document H for the underlying guidance.
What access chamber do I need?
Approved Document H sets the access type — rodding eye, inspection chamber, or full manhole — and the minimum size from the depth to invert and the largest pipe through the chamber. Enter those and the calculator returns the access type and minimum dimensions, so you do not over- or under-specify the chamber.
Cover class by location
The cover class comes from where the cover sits and the traffic over it, to BS EN 124:2015, from A15 in a garden up to F900 in heavy industrial areas. A domestic driveway is usually B125, a car park or kerbside C250, and an adopted carriageway D400. The calculator sets the class from the location you choose.
Ring and material take-off
Once the chamber is sized, the calculator takes off the precast rings, cover slab, base, brick courses and the excavation volume, giving you a quick bill of materials for pricing or ordering.
Further reading: foul, surface and combined sewers explained · drainage information needed for planning permission · Approved Document H.
Frequently asked questions
Depth and access drive it. Shallow chambers can be inspection chambers; deeper ones, or those needing person entry, become full manholes under Approved Document H. The calculator sets the access type from the depth and pipe size.
A domestic driveway taking cars is normally B125 to BS EN 124. A trafficked road or HGV access needs D400.
It depends on the depth to invert and the largest pipe. The calculator returns the minimum size from Approved Document H Tables 11 and 12.
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