How to use the Christmas lights calculator
List each run you want to light: a straight run such as an eave, a sloped run up a gable, the outline of a window or door, a spiral wrap on a tree, or a flat area for net lights. Type its size and how many of it you have, and pick the kind of lights for that run. The page adds up the feet, rounds up to whole strands, and shows the spare length. The Power tab turns the watts into amps, plugs and circuits. Pick a breaker, type anything else already on that circuit, and the page tells you how many circuits the whole display needs. Nothing you type leaves your device unless you make a share link.
How it is figured
strands = round up (feet × (1 + extra) ÷ lighted feet per strand)
amps = watts ÷ 120
one circuit holds 80% of its breaker's watts
A run up a gable is longer than it looks from the front, by the same factor as a roof: at 6 in 12 it is 1.118 times as long, and at 8 in 12 it is 1.202 times. A tree wrap is one spiral from the base to the top; the page unrolls the cone to find its length. A window outline is 2 × (width + height). Strands come in whole sets, so the page rounds up and shows the spare. Plugs are counted from how many strands the maker allows end to end, and circuits are filled with whole chains, so a chain is never split across two breakers.
A worked example
A gable-front house has a 40 foot eave, two gable edges that run 20 feet across at 6 in 12, four windows 3 by 4 feet and one 8 foot tree 5 feet wide at the base. The gable edges are 20 × 1.118 = 22.36 feet each, so the roofline is 40 + 44.72 = 84.72 feet. With 10 percent extra that is 93.19 feet, which takes 6 LED C9 strands of 16 lighted feet (about 2.8 feet spare) at 2.4 watts each. The window outlines are 4 × 14 = 56 feet and the tree wraps at 76.35 feet, which makes 132.35 feet. With 10 percent extra, 145.58 feet takes 5 LED mini strands of 33 feet at 7.2 watts. The whole display draws 14.4 + 36 = 50.4 watts, or 0.42 amp. It needs 2 plugs and 1 circuit.
The same roofline in 24 foot incandescent C7 sets at 125 watts needs 4 strands, which is 500 watts or 4.17 amps. The printed 210 watt note allows 1 set per plug, so it takes 4 plugs.
Plugs, fuses and the 80 percent rule
Every set has a limit on how many can share one plug. Makers print it as a count of sets, as a watt note, or both. The page uses the lower of the two. A circuit has a limit too. The code treats lights that run 3 hours or more as a continuous load and keeps them to 80 percent of the breaker, which is 12 amps (1,440 watts) on a 15 amp circuit and 16 amps (1,920 watts) on a 20 amp circuit at 120 volts. Anything else on that circuit counts against the same limit, so type it in. The edition of the code your town uses varies; ask your building department or an electrician.
Where the numbers come from
Bulb counts, lengths, watts and end-to-end limits are the ones printed on the retailer pages listed on the Limits tab, read on 1 Nov 2026. They are examples of common sets, not every brand. Type your own set's numbers under My own set. The 80 percent figure comes from NEC sections 210.19(A)(1) and 210.20(A) as described in an engineering article. The pitch factor is the roof factor used by the map tool.
Tips and common mistakes
- Count plugs, not only watts. The C7 example above needs 4 plugs for only 500 watts.
- Measure the slope, not the floor plan. A gable edge measured from above is short by the pitch factor.
- Leave room for the cord. Type the distance from the outlet to the lights and the page counts the cord for every plug.
- Do not cut the rope light. Cut only at the marks.
- Share the circuit honestly. If the garage freezer is on it, type its watts.
Questions
How many strings of Christmas lights can I plug together?
Whatever the box says, and no more. The sets on the Limits tab print from 1 to 87 on one plug, depending on bulb type. The page takes the lower of the printed set count and the printed watt note.
How many Christmas lights can one outlet or circuit hold?
A 15 amp circuit holds 1,440 watts of continuous load at 80 percent, and a 20 amp circuit holds 1,920 watts, minus anything else on it.
Do LED lights save a lot of power?
On the sets listed, yes: a 100 bulb LED mini strand prints 7.2 watts and the incandescent one prints 40.8.
Is there extra for waste?
The page adds 10 percent, the same as the map tool's lights job. Change it in the Extra box.
Can I use metric?
Yes. Press Metric to type meters. Watts and amps are the same in both systems.
Can I share my plan?
Yes. Share makes a link with a picture for Facebook, X and Pinterest, a link you can copy, a phone share button, and a short video for TikTok made on your device. The link keeps only the house size and the kinds of lights, for 90 days. It never keeps your price.