Christmas light calculator: how many lights do I need?

How to measure your house

1. Walk the front of the house with a tape measure or a measuring wheel. Measure each straight roof edge or gutter run you want to light, in feet.
2. Add every gable edge. A gable peak is two sloped edges, so measure each side.
3. Count windows and doors you want to outline.
4. Count trees to wrap and bushes to cover.
5. If you cannot measure from the roof, use the length of each wall below it. The roof edge is almost the same length as the wall under it, plus about 1 ft of overhang at the ends. For a gable, multiply the width by the pitch multiplier in the table below.

The formula and the strand lengths it uses

Strands needed = (length in feet x 1.1) divided by strand length, rounded up. The 10 percent covers sag, corners and the end of each run.

The calculator uses these common strand lengths: C9 strings at 100 ft, mini strings at 66 ft (200 count at 4 inch spacing), icicle sets at 25 ft and net lights at 4 by 6 ft. Your product may differ, so read the package for the actual length and for how many strings can be joined end to end.

Clips are one per foot for C9 and one per 8 inches for mini and icicle lights, plus the same 10 percent: 110 clips for 100 ft of C9, 170 for 100 ft of icicles. Each run is counted with its own bulb type. Cords depend on where your outlets are, not on how many feet are lit: the tool asks how many outdoor outlets you can use and how far the farthest run is, and gives one 25 ft cord per 25 ft of distance at each outlet. A trunk wrap is counted as 40 ft of mini lights per tree.

Example: a two-story colonial

A colonial about 40 ft wide with a one-story garage wing has about 40 ft of front roof edge, 60 ft of side and wing edge, and 12 ft of garage header, about 112 ft of runs. With 10 percent extra that is about 123 ft, so 2 C9 strings of 100 ft. Add 8 windows (112 ft) and the door (17 ft) in mini lights, 129 ft in all, and that is 3 more mini strings. A colonial guide has the full breakdown.

Prices in the tool are rough 2026 US retail for LED sets. A 100 ft LED C9 string costs about $45 to $80 per 100 ft reel depending on brand and wire; the tool uses the middle. Premium commercial-grade strings cost more and last longer.

Gable edges: the pitch multiplier

A sloped edge is longer than the width under it. The multiplier is the square root of 1 plus the rise over 12, squared. Take the full width of the gable, multiply, and you have both rake edges.

How much roofline is typical, and why people under-measure

Trimlight says most single-story houses need 100 to 150 linear feet of roofline lighting and larger or multi-story ones often 200 ft or more (Trimlight). It also notes that upper-story runs are hard to judge from the ground, so self-measured totals tend to come up short. Christmas Light Source suggests adding at least 10 percent, up to 15, and rounding up (Christmas Light Source). Measure each straight section on its own and add the totals instead of pacing the whole house at once.

What the calculator does not know

• Power. It does not know your outlets or circuit loads. See how many strings you can connect.
• Pitch and height. A steep roof makes edges longer than the wall below. Measure the roof edge itself if you can.
• Your own look. Dense looks use more lights. Double the strands for a very full icicle or window look.
• Trees indoors. Use the Christmas tree light calculator for an indoor tree.

Once you have a number, preview the look on your own house before you buy. Preview it on my house, free