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Cable Length Calculator — Estimate Total Cable Needed
Calculate the total cable length needed for your circuit run, including termination allowances and wastage. Works for any cable type.
On = ring final (cable returns to the consumer unit, BS 7671 Reg 433.1.204). Off = radial run (cable ends at the last outlet).
Actual route distance, not straight line
Sockets, switches, or junction boxes
Average cable distance between each outlet
Adds 10% extra for cutting waste and measurement errors
Estimating rule of thumb (not a standard): ~10% for most installs, ~15% for DIY. Adjust to suit your job.
Enter your cable price per metre for a cost estimate
Slack per connection point for stripping and terminating. Estimating convention, not a standard — 200–300mm is typical contractor practice (default 0.3m = 300mm).
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 estimates the total cable length needed for a circuit by adding up the distance from the consumer unit to the first outlet, the spacing between subsequent outlets, termination allowances at each connection point, and an optional wastage factor.
The formula
Total cable = (CU to first outlet + spacing between outlets) + termination allowances + wastage
Breakdown
- Run length: Distance from consumer unit to first outlet, plus the spacing between all subsequent outlets
- Ring return leg: For a ring final circuit (Ring toggle on), the cable returns to the same protective device, so a return leg from the last outlet back to the consumer unit is added (BS 7671 Reg 433.1.204)
- Termination allowance: 300mm per connection point (the consumer unit plus each outlet) for stripping, terminating, and neat connection — an estimating convention, not a BS 7671 figure
- Wastage: ~10% extra for cutting waste, measurement errors, and route changes (optional; a rule of thumb rather than a standard)
Choose radial (cable ends at the last outlet) or ring final(cable loops back to the consumer unit) with the circuit-type toggle. For accurate results, measure the actual cable route including vertical drops, horizontal runs through joists, and any detours around obstacles. The calculator works for any cable type — twin and earth, SWA, or flexible cable.
Frequently Asked Questions
Around 200-300mm of extra cable at each connection point — the consumer unit end and each socket, switch, or junction box — gives enough slack to strip, terminate, and neatly connect the conductors. This is an estimating convention rather than a figure fixed by BS 7671; the calculator defaults to 300mm and you can adjust it in the advanced options. For complex connections (e.g., inside a consumer unit), allow 400-500mm.
Yes. Adding around 10% wastage is common contractor practice — it is a rule of thumb, not a standard. Cable is wasted through measurement errors, route changes on site, off-cuts that are too short to use, and damage during installation. For DIY installations, consider ~15%. It is always cheaper to have a bit extra than to run short and need a separate delivery.
Measure the actual route the cable will take, not just the straight-line distance. Cable runs typically go up walls, across ceilings, through floors, and around obstacles. Allow for the vertical drop from ceiling or joist level down to the accessory: in new dwellings, socket-outlets and switches sit between 450mm and 1200mm above finished floor level (Approved Document M), so drops are typically about 1.2-2.0m depending on the accessory and ceiling height. A laser measure helps for longer distances.
In the UK, twin and earth cable typically comes in 50m or 100m drums/rolls. SWA cable is usually supplied cut to length from a drum. Flexible cable comes in 50m or 100m rolls. When ordering, round up to the next standard length — a spare partial roll is useful for future repairs or additions. Once you know the length, check voltage drop compliance with our Voltage Drop Calculator.
Yes — switch on the 'Ring final circuit' toggle. A ring final circuit (the standard 2.5mm² 32A domestic socket circuit) starts and returns to the same protective device, so the cable forms a loop: consumer unit, around all the sockets, and back to the consumer unit (BS 7671 Regulation 433.1.204 / Appendix 15). With the toggle on, the calculator adds a return leg from the last socket back to the CU and counts both cable ends as terminations at the consumer unit. Enter the return-leg distance if you know it, or leave it blank to assume it equals the CU-to-first-socket distance. Leave the toggle off for a radial run, where the cable simply ends at the last outlet.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.