How this calculator works
Outline lights follow a line, so the quantity comes from the length. We add an allowance for corners and spares, divide by the length of a strand to get the number of strands, and divide by the spacing between bulbs to get the number of bulbs (and clips, since each bulb usually has its own clip).
Worked example: an 80 ft roofline with C9 bulbs 12 inches apart and 10% spare is 88 ft, which needs four strands of 25 ft (100 ft of lights), 88 bulbs and 88 clips.
Tips before you buy
- Measure the roofline from the ground with a laser measure or from photos, rather than climbing up. Use a ladder safely and have someone hold it, or hire it out.
- Count in each corner: lights wrap around them, so the line is longer than the straight edge.
- Use clips designed for gutters, shingles or siding, and never staple or nail through the wire.
- Plan the outlets before you hang anything, so you do not need a long run of cord. Test all the lights on the ground first.
- LED lights use a fraction of the power, which lets you link more strands, and cuts the cost of running them for weeks.
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How many Christmas lights do I need for my roofline?
Measure the length you will outline, add 10%, and divide by the strand length. An 80 ft roofline needs about four 25 ft strands of C9 bulbs.
How far apart should C9 bulbs be?
Twelve inches is the most common spacing. Fifteen-inch strands cover more distance per strand with fewer bulbs, and 6-inch spacing looks fuller and more expensive.
How many strands can I connect together outside?
The maximum is printed on the box. For LED strands it is often 20 to 40 or more, while incandescent strands may only link three or four.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.