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The Christmas Lights Calculator turns your house into a shopping list. The roofline is every eave and rake you want to outline plus the vertical drops, divided by the lit length of a strand: C7 and C9 strands are commonly 25 ft with a bulb every 12 inches, and a 100-light mini set is about 22 ft. Trees use the retailer rule of 100 mini lights per foot of height, so a 7 ft tree takes 700 lights, seven sets. Bushes take the common 4 × 6 ft net light with 150 bulbs, one per 24 sq ft of shrub face. Power uses typical incandescent ratings — 0.4 W per mini bulb, 5 W per C7 and 7 W per C9 — or ENERGY STAR's ceiling of 0.2 W per LED bulb, about 92% less. A 15 amp, 120 volt circuit carrying a load for hours should stay under 80% of its rating, 1,440 W. Running cost is watts × hours × nights at EIA's 2025–26 winter average of 17.3¢ per kWh: 100 incandescent C9 bulbs (700 W) lit 6 hours a night for 40 nights use 168 kWh, about $29.
Strand lengths are common retail sizes. LED watts are ENERGY STAR’s ceiling of 0.2 W per bulb (energystar.gov, Decorative Light Strings); incandescent watts are typical label ratings. Running cost uses EIA’s 2025–26 winter electricity price, 17.3¢ per kWh.
| Light | Lit length | Bulbs | Incandescent | LED |
|---|---|---|---|---|
| Mini lights (100-light set) | 22 ft | 100 | 40 W | 20 W |
| C7 bulbs (25 ft, 12 in spacing) | 25 ft | 25 | 125 W | 5 W |
| C9 bulbs (25 ft, 12 in spacing) | 25 ft | 25 | 175 W | 5 W |
Measure every eave and rake you want to light, add the vertical drops at the corners and divide by the lit length of one strand. C7 and C9 strands are commonly 25 ft with a bulb every 12 in, so 100 ft of roofline takes 4 strands and 100 bulbs.
The common retailer rule is 100 mini lights per foot of tree height: 700 for a 7 ft tree, 900 for a 9 ft tree. Use 150 per foot for a dense, bright look or fewer for a soft glow.
A 15 amp, 120 volt circuit should carry no more than 80% of its rating for a load that runs for hours: 1,440 watts. Separately, every light set has a maximum connected wattage printed on its tag — never connect more sets end to end than the tag allows.
Incandescent minis draw about 0.4 W per bulb (40 W per 100-light set) and C9 bulbs about 7 W each. ENERGY STAR LED light strings use no more than 0.2 W per bulb, about 92% less. At EIA's 17.3¢/kWh, 100 incandescent C9 bulbs (700 W) run 6 hours a night for 40 nights use 168 kWh, about $29; the same in LED costs well under $1.
Net lights are the fastest: a common 4 × 6 ft net carries 150 bulbs and covers 24 sq ft of shrub face. Measure each bush's width × height and divide by 24 to count nets.
Icicles along the same eaves? The roof heat tape calculator sizes de-icing cable for that roofline.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.