How this calculator works
This runs our tree, roofline, circuit and running cost calculators on one display. The tree needs the height times the lights per foot, in strands of your size; the roofline needs the length divided by the strand length (with 10% spare) and a clip for each bulb. The circuit check adds up the watts, and the running cost multiplies the kWh by your price.
Worked example: a 7½ ft tree at 100 per foot is 750 lights (8 strands of 100), and an 80 ft roofline takes 4 strands of 25 ft with 88 clips and 15 ft of cord. In LED strands the 12 strands draw about 60 W, and cost $1.84 to run for 30 nights of 6 hours at 17 cents. The same display in old incandescent strands would cost $14.69.
Strands by tree and roofline size (100 lights per foot, 100-light and 25 ft strands, C7 or C9 bulbs 12 in apart)
LED strands of 5 W.
| Tree and roofline | Tree lights | Tree strands | Roof strands | Clips |
|---|---|---|---|---|
| 6 ft tree, 60 ft roofline | 600 | 6 | 3 | 66 |
| 7.5 ft tree, 80 ft roofline | 750 | 8 | 4 | 88 |
| 9 ft tree, 100 ft roofline | 900 | 9 | 5 | 110 |
| 10 ft tree, 150 ft roofline | 1,000 | 10 | 7 | 165 |
Tips before you buy
- Use lights rated for where they hang: indoor, outdoor or both, as the tag says.
- Plug outdoor lights into a GFCI outlet, use outdoor-rated cords, and keep connections off the ground and out of snow and puddles.
- Test every strand on the ground before you hang it, and throw away any with cracked sockets or bare wire.
- Water a real tree every day, and turn the lights off when you go to bed or leave the house. A timer makes that automatic.
- Use a stable ladder with a helper, and never work near power lines.
Tools & materials for this project
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Frequently asked questions
How many Christmas lights do I need for a 7.5 foot tree?
About 750 at the standard 100 per foot, which is 8 strands of 100. For a fuller look use 150 per foot.
How many lights do I need for an 80 foot roofline?
4 strands of 25 ft (with 10% spare), 88 bulbs and 88 clips if the bulbs are 12 inches apart.
How much does it cost to run Christmas lights?
The 12 LED strands of this plan, on 6 hours a night for 30 nights at 17 cents per kWh, cost about $1.84 for the season. The same display in old-style incandescent strands would cost about $14.69.
How big a display for a 9 ft tree and a 100 ft roofline?
14 strands in all: 9 for the tree and 5 for the roofline.
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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.