Check containment fill using simple geometric area method.
Indicative only. Verify against project specification, applicable standards, and manufacturer guidance.
For design guidance only. Always verify with a qualified engineer and manufacturer data.
About this containment fill calculator
This free containment fill calculator checks whether a cable tray, trunking or conduit has enough room for a given bundle of cables. You enter the containment dimensions, internal width and usable depth for tray or trunking, or the internal diameter for conduit. Then add each cable by its overall diameter and quantity. The tool returns a PASS or FAIL verdict, the percentage fill used, and the containment, allowed and remaining cable areas. It is aimed at UK building-services and electrical engineers, contractors and estimators who need a quick capacity sense-check at design or first-fix stage. Common UK cables (Twin & Earth, SWA, FP200 Gold, Cat5e/Cat6) are built in as presets, with representative diameters drawn from standards such as BS 6004, BS 5467/6346 and manufacturer data. Verify the exact figure against the cable datasheet.
How the fill is calculated
The calculator uses the simple geometric area method: it sums the cross-sectional area of every cable (π·(OD÷2)² × quantity) and compares the total against the available containment area. For tray and trunking the containment area is width × depth; for conduit it is π·(ID÷2)². You set a maximum fill (33, 40, 45, 50 % or custom), and an optional multiplicative reserve fraction reduces it further, effective allowance = max fill × (1 − reserve), so 40 % with a 10 % reserve allows 36 %. The check passes when total cable area is at or below the allowed area. This is more permissive than the BS 7671 conduit and trunking unit-factor tables, so for compliance you should refer to those; the result here is indicative only.
Frequently asked questions
Does this replace the BS 7671 fill tables?
No. The calculator uses the geometric area method, which is more permissive than the BS 7671 conduit and trunking unit-factor tables. Treat the result as an indicative sense-check and refer to those tables, the project specification and manufacturer guidance for compliance.
What does the reserve fraction do?
It applies a multiplicative spare allowance on top of the maximum fill. The effective allowance is max fill × (1 − reserve), so a 40 % limit with a 10 % reserve checks cables against 36 % of the containment area, leaving headroom for future additions.
What cable diameter should I enter?
Use the cable's actual overall diameter (OD) from its datasheet, and the true internal diameter for conduit rather than the nominal size. For flat Twin & Earth, which has no true circular OD, the method uses the major (width) dimension as an equivalent diameter. The method does not model cable type, installation method, segregation, thermal derating or grouping.