Confined Space Volume Calculator
The tool computes the internal geometric volume.
Enter Values
How to use this calculator
- Pick the shape: for a rectangular pit or vault enter Length and Width; for a round tank or silo enter Diameter instead (leave the other blank). Then enter the Depth/height. All in metres.
- Optionally enter the number of air changes you plan to purge before entry (commonly 5 or more) to get the total air volume required.
- Optionally add your ventilation fan's flow in m³/min to estimate the minimum fan run-time to move that purge volume.
How it works
The tool computes the internal geometric volume. A rectangular space uses V = length × width × height; a round tank or silo uses V = π × (diameter ÷ 2)² × height. The result is shown in cubic metres and litres (1 m³ = 1000 L). This volume is the starting point for planning atmospheric testing points and ventilation.
If you enter air changes, the purge air = volume × changes — the total fresh air you want to push through before entry. Dividing that by the fan flow gives a theoretical run-time (run-time = purge air ÷ fan flow). Real purging never mixes perfectly (short-circuiting and dead pockets leave stale air), so the true safe purge time is longer. Always confirm the atmosphere with a calibrated multi-gas monitor and a valid entry permit — the maths never replaces testing.
Worked example
Round wastewater tank, 2.4 m across and 4 m deep, purged 5 times at 8 m³/min. Enter Diameter 2.4 m and Depth 4 m. Internal volume = π × (2.4 ÷ 2)² × 4 = 18.096 m³ (about 18,096 L). Add 5 air changes → purge air = 18.096 × 5 = 90.5 m³. With a fan moving 8 m³/min, estimated fan run-time = 90.5 ÷ 8 = 11.3 min. This is a theoretical minimum — allow extra run-time and verify with a calibrated gas monitor before anyone enters.
Common mistakes
- Mixing shapes — entering both a Width and a Diameter. Fill only the set of dimensions for one shape and leave the other blank.
- Using the tank's outside dimensions. Confined-space volume is the internal clear space; subtract wall thickness and any internal fittings, sludge or product for a realistic figure.
- Treating the fan run-time as a safe-to-enter time. It is a theoretical minimum for a perfect mix. Poor airflow paths mean you must purge longer and always verify with continuous gas monitoring before and during entry.
Frequently asked questions
Does a bigger calculated volume mean the space is safe to enter?
No. Volume only helps you plan ventilation and testing. A space is only safe to enter once a calibrated multi-gas monitor confirms acceptable oxygen (typically 19.5–23.5%) and no flammable or toxic gases, under a valid confined-space entry permit with a competent person and rescue plan in place.
How many air changes should I purge before entry?
There is no universal number — it depends on the contaminant, space geometry and airflow. Five or more changes is a common rule of thumb, but poorly mixed spaces need many more. Purge, then TEST; keep ventilating and monitoring continuously while anyone is inside. Follow your regulator's confined-space code.
Why is the real purge time longer than the calculated fan run-time?
The calculation assumes every cubic metre of stale air is perfectly replaced. In practice air short-circuits from inlet to outlet and leaves dead pockets, so it takes several nominal air changes to actually clear the space. Add a generous safety margin and confirm with atmospheric testing.
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Explore more in Safety, Workplace Risk & Compliance Helpers.
Tip: Enter any known values to calculate the remaining results.
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