How many Christmas lights for your house: outdoor lights calculator

How many lights for a roofline

Walk the house with a tape, or measure a plan, and add up the eaves, gutters and gable edges you want to light. The Christmas Light Emporium’s rule is one C9 bulb per foot of fascia at 12 inch socket spacing, plus 10% slack for corners and overlaps. A 60 ft roofline therefore takes 66 C9 bulbs. Counted in 25-bulb strings, that is three strings; check the count and spacing on yours, and pick mini lights if you prefer a finer line.

Windows, bushes and trees

Windows. The Christmas Light Emporium says a window’s perimeter in feet is the string length it needs, plus 10%: a 3 × 5 ft window takes 16 ft, 17.6 ft with the slack, or 53 mini lights at 4 inch spacing.

Bushes. The same guide says a 4 × 6 ft net covers about 24 square feet, so divide a bush’s surface area by 24 to estimate nets. The calculator works out that surface by treating the bush as a dome, half of a spheroid as wide and as tall as the bush, using the exact formula for its area. A 3 ft round bush 3 ft tall has about 24.2 sq ft of surface, a little more than one 4 × 6 ft net covers, so the calculator counts two. If you drape strings instead, rows 6 inches apart need about 48 ft of lights on that bush, around 145 mini bulbs.

Trees. Christmas Lights, Etc. says most outdoor trees are wrapped the length of the trunk and about 3 to 4 feet into the main branches. Its calculator divides the height by the gap between wraps and multiplies by the trunk’s circumference. The exact length of that spiral is √(height² + (wraps × circumference)²), which the calculator uses; the difference is small, because each wrap rises only a few inches. A 6 ft trunk 30 inches round, wrapped every 6 inches, takes 12 wraps and about 31 ft of lights, and four branches wrapped for 3.5 ft each add 23 ft.

LED or incandescent: the watts

Watts add up very differently. The calculator shows both, from published figures per bulb:

For LED C7 and C9 bulbs the calculator takes the higher figure, so its totals err on the high side. Christmas Lights, Etc. gives an even starker example: 300 C9 bulbs take 2,100 watts as incandescents and only 29 watts as LEDs. Your box or bulb packet has the real figure; use it if it differs.

How many lights on one circuit or plug

Amps are watts divided by volts: the example house draws 0.73 A at 120 V in LED, or 6.6 A in incandescent bulbs. In the US, Christmas Lights, Etc. recommends never going over 80% of a circuit’s capacity: 1,440 W of the 1,800 W a 15 A circuit carries, or 1,920 W of 2,400 W on a 20 A circuit. Everything else on that circuit counts too, such as a porch light or a garage freezer. In the UK, a 13 A plug or extension lead at 230 V carries about 2,990 W, and Electrical Safety First notes that some older extension leads are rated only 10 A. Watt Cottage’s watts, amps and volts calculator converts the figures for your supply, and its extension lead overload calculator checks a lead with several things on it.

End-to-end limits apply outside too. The plug or box says how many sets can be joined; Christmas Lights, Etc. says the maximum number of incandescent net lights you can connect is usually 3, against 14 to 21 LED nets.

Common mistakes

• Measuring the house’s width instead of the roofline. Gables run up and down, and the edges add up.
• Forgetting the run from the socket to the first light: that needs an outdoor-rated extension cord, not a lit string.
• Adding up bulbs but not watts, and putting several incandescent displays on one circuit.
• Plugging transformers or plugs in outdoors, where Electrical Safety First says they should be indoors.

Inside, the tree lights calculator does the same for the Christmas tree. Having the neighbours round for a gift swap? Sunlit Circle runs the Secret Santa draw.