Christmas lights come in fixed-length strands, and a roofline runs however long it runs. Divide the roofline by the strand length and round up, and you know how many strands to bring so you never come up a gable short.
A 140-foot roofline in 25-foot strands needs six strands.
The example
Three homes, each with a roofline length in feet and the length of one light strand.
| A | B | C | D | |
|---|---|---|---|---|
| 1 | Home | Roofline ft | Strand ft | Strands |
| 2 | Ranch | 140 | 25 | 6 |
| 3 | Two-story | 220 | 25 | 9 |
| 4 | Cottage | 90 | 25 | 4 |
The formula
Roofline over strand length is the raw count; round up for whole strands:
How it works
Two inputs:
B2is the total roofline to light in feet — add up every eave and gable run.C2is the length of one light strand as sold.ROUNDUP(...,0)rounds up, because a partly-used strand is still a whole strand you have to buy.- Strands connect end to end only up to a maximum run — a long roofline may need multiple power drops even if the strand count fits.
Measure the roofline from the ground with the pitch in mind — a steep gable is longer than its horizontal span, so add a little for the rake edges.
Try it: interactive demo
Enter the roofline length and the length of one strand, in feet.
Variations
Strand cost
Multiply the strands by the price each.
Total bulbs
Strands times bulbs per strand is the bulb count for a load check.
Pitfalls & errors
There is a limit to how many strands connect end to end before the wire is overloaded. The strand count tells you what to buy, not how many can share one plug — check the max-connected rating.
Roofline measured from the ground understates steep pitches. The rake edge of a gable is longer than the horizontal run, so add a bit for pitched sections.
Practice workbook
Frequently asked questions
Should I measure the roofline from the ground?
Can I just connect all the strands together?
Stop fighting formulas. Learn them in a day.
This recipe is one of hundreds of real-world formulas we teach. Our Excel Formulas & Functions class covers lookups, logic, text, and dynamic arrays hands-on — live in Dallas–Fort Worth, Houston, Austin, Oklahoma City, Denver, or online.
See the Formulas & Functions Class