Last updated: 2026-06-19

String Lights

String lights are sold in fixed-length strands, and the number a job needs is set by the length of the run to cover, the lit length of one strand, and the spacing between bulbs. This calculator estimates how many strands and bulbs are required to span a roofline, fence, or set of eaves, including a slack allowance for sag and overlap.

Run length and slack

The starting point is the measured run, but the lights rarely follow a perfectly straight line. They sag between fixing points, overlap at corners, and need extra length to reach the nearest outlet. A slack allowance expands the measured run into the total length that the strands must actually cover:

where $L$ is the run length and $k$ is the slack allowance as a fraction. The number of strands is the total length divided by the lit length of one strand, rounded up to the next whole strand, and the bulb count is the total length divided by the bulb spacing:

Rounding up matters: a run that needs 2.2 strands still requires three physical strands, because a strand cannot be cut to length without losing the unlit lead.

Worked example

Choosing inputs

Use the lit length quoted by the manufacturer rather than the total cable length, since the unlit lead at each end is not part of the covered span. Closer bulb spacing gives a denser look but raises both the bulb count and the connected load, so confirm the maximum number of strands that can be linked end to end before planning a long run.

For estimating supplies for the gathering itself, see Party Drinks Calculator.