What is a confined space?

A confined space is any enclosed or partially enclosed place that is not designed for continuous occupancy, has limited entry/exit, and poses a risk from hazardous atmospheres, engulfment, or restricted movement. Examples: manholes, tanks, trenches over 1.2m, roof voids, and service ducts.

When is a permit to work needed?

A permit to work (PTW) should be used for all high-risk confined space entry: where there is a risk of hazardous gas, where the space is not naturally ventilated, for entries over 1.5m deep, or where rescue would be difficult. The PTW documents safety precautions and emergency procedures.

What gas monitoring is required?

At minimum, test for oxygen levels (19.5-23.5% normal). For sewers/drains: add methane (LEL), hydrogen sulphide (H2S), and carbon monoxide (CO). A 4-gas monitor covers these. Test before entry, at multiple heights, and monitor continuously during work.

How many people are needed for confined space entry?

Minimum team: one entrant and one standby person at the entry point at all times. For high-risk entries, add a supervisor and rescue team. The standby person must NEVER enter the space to attempt rescue without proper equipment - this is how most multiple fatalities occur.

What ventilation rate is needed?

Target 10-20 air changes per hour using forced ventilation. For a 5m3 manhole, this means 50-100 m3/hr airflow. Fan should blow clean air in from the surface, not extract from below. Retest the atmosphere after ventilating and continuously during work.

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Confined Space Assessment Calculator

Classify confined spaces and determine entry requirements

Space Type

Depth / Height

Entry Duration

Number of Entrants

How We Calculate This

Select the space type and enter dimensions. The calculator classifies the confined space based on hazards present (gas risk, flood risk, depth, ventilation) and determines the risk level. It then specifies gas monitoring requirements, ventilation needs (air changes per hour), rescue provisions, and whether a formal permit to work is required under the Confined Spaces Regulations 1997.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.