---
title: "Outdoor Christmas Lights Calculator | Deco Sprig"
description: "How many Christmas lights for your house? Roofline, windows, bushes and trees to bulbs and strings, LED and incandescent watts, and the circuit load."
url: https://decosprig.me/outdoor-christmas-lights-calculator/
---

Roofline, windows, bushes and trees

# How many Christmas lights for your house: outdoor lights calculator

Measure each part of the house and turn it into bulbs. A roofline needs about one C9 bulb per foot at 12 inch spacing, plus 10% for corners and overlaps, and each window its perimeter plus 10% (The Christmas Light Emporium). A 60 ft roofline, four 3 × 5 ft windows, three bushes and one tree come to 875 bulbs: about 88 watts in LED, or 792 watts incandescent.

## 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.

What to measure: the roofline edges, each window’s perimeter, each bush’s width and height, and the trunk height and girth of each tree.

## 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:

| Bulb | LED | Incandescent | From |
| --- | --- | --- | --- |
| Mini light | 0.048 W | 0.408 W | Northlight 100-light sets: 4.8 W and 40.8 W |
| C7 | 0.5 W | 5 W | 1000Bulbs: LED 0.48–0.5 W, incandescent 5 W |
| C9 | 0.74 W | 7 W | 1000Bulbs: LED 0.5–0.74 W, incandescent 7 W |

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](https://wattcottage.me/watts-amps-volts-calculator/) converts the figures for your supply, and its [extension lead overload calculator](https://wattcottage.me/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.

**Outdoor safety.** Check the labels to be sure the lights are certified for outdoor use, and plug them only into GFCI-protected outlets, as the US Consumer Product Safety Commission advises. In the UK, Electrical Safety First says lights plugged in outdoors must be connected via a 30 mA RCD protected socket (or an RCD plug), should be marked for outdoor use with a water drop symbol or an IP code whose last digit, for protection against water, is 3 or higher, and should have their plugs and transformers plugged in indoors. Check every set for damage before it goes up, and switch the lights off when you go to bed or go out; a plug-in timer helps. This is general information, not electrical advice: for new outdoor sockets or wiring, use a qualified electrician.

### Light-show projectors from a shop you don’t know

Social media is full of Christmas decorations and light shows from unfamiliar shops at surprising prices. Electrical Safety First advises buying Christmas lights from a reputable retailer rather than a third-party seller online. Before ordering from a shop that is new to you, [WebTraceCheck’s DNS & domain lookup](https://webtracecheck.com/dns-lookup) shows when its domain was registered, one signal to weigh rather than proof either way, and [its disposable cloud browser](https://webtracecheck.com/) opens the link on an isolated desktop that is deleted after the session.

## 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](https://decosprig.me/) does the same for the Christmas tree. Having the neighbours round for a gift swap? [Sunlit Circle](https://sunlitcircle.cyou/) runs the Secret Santa draw.

## Questions

**How many Christmas lights do I need for my house?**

Add up each part: the roofline in feet, about one C9 per foot plus 10%; each window’s perimeter plus 10%; bushes by surface area; trees by trunk height and girth. A 60 ft roofline, four 3 × 5 ft windows, three 3 ft bushes and one tree come to 875 bulbs.

**How many C9 lights do I need for my roofline?**

About one per foot at 12 inch spacing, plus 10% for corners and overlaps (The Christmas Light Emporium). A 60 ft roofline takes 66 bulbs, three 25-bulb strings.

**How many lights for a bush?**

A 3 ft round, 3 ft tall bush has about 24 sq ft of surface. A 4 × 6 ft net covers about 24 sq ft (The Christmas Light Emporium), so allow one or two nets; with strings in rows 6 inches apart it takes about 145 mini lights.

**How many watts do outdoor Christmas lights use?**

An incandescent C9 bulb uses about 7 W and an LED C9 0.5 to 0.74 W (1000Bulbs); a mini light about 0.4 W incandescent or 0.05 W LED. The example house totals about 88 W in LED or 792 W incandescent.

**How many Christmas lights can I plug into one outlet?**

Keep the whole circuit to 80% of its capacity, about 1,440 W on a 15 A, 120 V circuit (Christmas Lights, Etc.), counting everything else on it. A UK 13 A plug carries about 2,990 W at 230 V. Also follow the end-to-end limit printed on each set.

**Do outdoor Christmas lights need a GFCI or RCD?**

The US Consumer Product Safety Commission advises plugging outdoor lights into GFCI-protected outlets. In the UK, Electrical Safety First says lights plugged in outdoors must be connected via a 30 mA RCD protected socket, or through an RCD plug.

## Sources

- One C9 per foot plus 10%, window perimeters, 4 × 6 ft nets covering about 24 sq ft: [The Christmas Light Emporium, How Many Christmas Lights Do You Need? A Planning Guide](https://thechristmaslightemporium.com/blogs/all-about-christmas-lights/how-many-christmas-lights-calculator) (undated; checked 5 November 2026).
- Wrapping trunks and 3–4 ft of branches: [Christmas Lights, Etc., How to Wrap a Tree with Christmas Lights](https://www.christmaslightsetc.com/pages/Wrapping-Trees-with-Lights.htm) (updated 13 August 2026); the trunk formula: [Christmas Tree Light Calculator](https://www.christmaslightsetc.com/pages/LightCalculation.htm) (updated 13 August 2026) (both checked 5 November 2026).
- 80% of a circuit, 1,440 W and 1,920 W, 300 C9s at 2,100 W or 29 W: [Christmas Lights, Etc., Christmas Lights Wattage](https://www.christmaslightsetc.com/pages/how-many-watts-amps-do-christmas-lights-use.htm) (updated 28 January 2026; checked 5 November 2026).
- Net lights end to end: [Christmas Lights, Etc., How to Install Christmas Lights on Bushes](https://www.christmaslightsetc.com/pages/net-lights-installation-guide.htm) (updated 13 August 2026; checked 5 November 2026).
- C7 and C9 watts: [1000Bulbs, C7 vs. C9 Christmas Lights](https://blog.1000bulbs.com/home/c7-vs-c9-christmas-lights) (undated; checked 5 November 2026).
- Mini-light watts: Northlight via Christmas Central, [100-light LED set](https://www.christmascentral.com/set-of-100-red-led-mini-christmas-lights-4-spacing-white-wire-32605401/) and [100-light incandescent set](https://christmascentral.com/set-of-100-clear-mini-christmas-lights-2-5-spacing-white-wire-32602375) (product pages, checked 5 November 2026).
- Outdoor-certified lights and GFCI outlets: [US Consumer Product Safety Commission, Holiday Season Decorating Safety Tips](https://www.cpsc.gov/Newsroom/News-Releases/2006/CPSC-Announces-Holiday-Season-Decorating-Safety-Tips) (release 06-046, published 8 December 2005; checked 5 November 2026).
- 30 mA RCD, IP rating, plugs indoors, reputable retailers: [Electrical Safety First, Christmas lights](https://www.electricalsafetyfirst.org.uk/guidance/product-safety/christmas-lights/) (last updated 28 June 2026); 13 A and 10 A leads: [Extension leads, cable reels and block adaptors](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/extension-leads-cable-reels-and-block-adaptors/) (last updated 30 September 2026) (both checked 5 November 2026).

Counts are retailers’ rules of thumb and exact geometry; watts are published figures for particular products. General information, not electrical advice.