How much extra cable should I allow for terminations?

Allow approximately 300mm (30cm) of extra cable at each connection point — that includes the consumer unit end and each socket, switch, or junction box. This gives enough slack to strip, terminate, and neatly connect the conductors. For complex connections (e.g., inside a consumer unit), you may want 400-500mm.

Should I add wastage to my cable order?

Yes. Adding 10% wastage is standard practice. 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% wastage. It is always cheaper to have a bit extra than to run short and need a separate delivery.

How do I measure cable run length accurately?

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. Account for vertical drops from ceiling to socket height (typically 2.1m) and horizontal runs through joists or behind walls. A laser measure helps for longer distances.

How much cable comes in a standard roll?

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.

Does the calculator work for ring final circuits?

For a ring final circuit, the cable goes from the consumer unit, around all the sockets, and back to the consumer unit. Enter the distance to the first socket, the number of sockets in the ring, the spacing between them, and remember the cable must return to the CU. You may need to add the return distance separately or plan the ring route carefully.

Cable Length Calculator — How Much Cable Do I Need?

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.

Distance from Consumer Unit to First Outlet

Actual route distance, not straight line

Number of Outlets in Run

Sockets, switches, or junction boxes

Spacing Between Outlets

Average cable distance between each outlet

Include wastage allowance

Adds 10% extra for cutting waste and measurement errors

Wastage Percentage

10% recommended for most installations, 15% for DIY

Price Per Metre (optional)

Enter your cable price per metre for a cost estimate

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
Termination allowance: 300mm per connection point (CU + each outlet) for stripping, terminating, and neat connection
Wastage: 10% extra for cutting waste, measurement errors, and route changes (optional)

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

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Cable Length Calculator — Estimate Total Cable Needed

How We Calculate This

The formula

Breakdown

Frequently Asked Questions

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