What it calculates
Estimate branch-circuit cable length with waste.
Romex / NM-B Length Calculator
Estimate NM-B cable needed from branch-circuit runs, drops and waste allowance.
Planning and estimating aid only. Verify project requirements, manufacturer instructions, adopted codes, permits, and field conditions before performing regulated work.
Electrical planning
How to Use This Calculator
Use the field units shown below, then compare your result with the worked example. If the average run already includes vertical sections and service loops, avoid adding the same sections again as drops.
How the calculation works
Base cable = number of runs × (average run + routing/drop allowance). The waste/slack percentage is then added.
What the result means
Measure real routing paths whenever possible; straight-line room dimensions often understate cable needed.
Account for routing and drops once
Five runs with an average horizontal route of 20 feet and 6 feet of added drop allowance per run give 5 × (20 + 6) = 130 feet. A 10% allowance gives 143 feet of purchase planning length.
Contractor tip: If the average run already includes vertical sections and service loops, avoid adding the same sections again as drops. Compare the final footage with available package lengths.
Verify the final quantity
The drop value is added to EACH run. Measure routes around obstacles rather than a straight line across a floor plan. This estimates length only; it does not select a cable type, gauge or installation method.
Good practice: Compare the calculated quantity with plans, specifications, product packaging and current field conditions before purchasing material.