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Lighting Circuit Calculator — Light Fittings Per Circuit
Calculate how many light fittings fit on a circuit. Work out fittings needed, total wattage, circuits required, and cable run estimates for UK electrical installations.
Total surface area to cover
Wattage per light fitting
Default 100 lm/W. Typical LED downlights 80–110; use the figure on your fitting’s datasheet
Default 0.66 (BS 7671 domestic lighting)
Default 0.8 for clean environments
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
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How We Calculate This
This calculator determines how many light fittings are needed for a room based on the required lux level, then works out how many lighting circuits are needed to serve them.
Fittings calculation
Total lumens = (Lux x Area) / (Utilisation Factor x Maintenance Factor)
Fittings needed = Total lumens / Lumens per fitting
Circuit calculation
Circuit capacity (W) = Rating (A) x 230V
Design load (W) = Total wattage x Diversity factor
Circuits required = Design load / Circuit capacity
Per BS 7671, the diversity factor reduces the design demand (the load the circuit must actually supply at once), not the circuit's current-carrying capacity. So diversity is applied to the load, then compared against the full 6A/10A/16A capacity — it is not used to derate that capacity.
Defaults
- Utilisation factor: 0.6 (typical domestic room)
- Maintenance factor: 0.8 (adjustable)
- Diversity factor: 0.66 (BS 7671 domestic lighting)
- LED efficacy: ~100 lumens per watt (adjustable — typical LED downlights are 80–110 lm/W; check your fitting’s datasheet)
Note: the fittings figure is a lumen-method estimate. For circuit design the IET On-Site Guide assumes a minimum 100W per lighting point regardless of LED wattage, which typically limits a 6A circuit to about 12–18 points.
Frequently Asked Questions
A 6A lighting circuit can handle up to 1,380W (6A x 230V). For circuit design, however, the IET On-Site Guide (Appendix A, Table A1) tells you to assume a minimum 100W per lighting point regardless of the actual LED wattage. On that basis a 6A circuit takes around 12 points, or roughly 18 once the lighting diversity allowance is applied. That is why most electricians limit a single domestic lighting circuit to about 10-12 fittings, even though tiny LED lamps draw far less than 100W each.
The CIBSE/SLL Code for Lighting recommends roughly: 100 lux for bedrooms and circulation areas, 150 lux for living rooms and bathrooms, 300 lux for kitchens and general task areas, 400 lux for home offices, and 500 lux or more for detailed work (workshops, reading areas). Commercial spaces may require higher levels. Use our Lux Level Calculator for detailed lux calculations, and our Lighting Layout Calculator for fitting placement.
The diversity factor (typically 0.66 or 66%) accounts for the fact that not all lights on a circuit will be on simultaneously. BS 7671 allows a 66% diversity factor for lighting circuits in domestic installations when calculating maximum demand.
Cable runs depend on the room layout, fitting positions, and switch locations. As a rough guide, allow 3-4 metres per fitting for the loop between fittings, switch drops, and connections to the consumer unit. Always measure actual routes for final cable ordering.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.