How Many Lights for My Christmas Tree? The Holiday Math Riddle

December 8, 2014

Remember those riddles from math class?

“What digit is the most frequent between the numbers 1 and 1,000 (inclusive)? Don’t solve this riddle by doing all of the math; rather try to figure out a pattern.”

“A merchant can place 8 large boxes or 10 small boxes into a carton for shipping. In one shipment, he sent a total of 96 boxes. If there are more large boxes than small boxes, how many cartons did he ship?”

And there are also math tricks. “Think of any number. Double the number. Add 9 to the result. Subtract 3 from that. Divide by 2. Subtract that number from the first number you started with. Your answer is 3.” (Really, it is 3.)

Figuring out how many lights you need for your Christmas tree can be like solving a math riddle. There are plenty of methods out there to solve the riddle, many with different variables. “For every foot and a half of tree, buy 100 lights.” Other calculators have you count the number of branches on your entire tree (because you have time for that) and multiply by 60%. More formula based calculations will have you measure the height and width of the tree to solve the riddle with the following formula (where h = height and w = width).

H x W x .18 = number of sets needed
2006 Blue Room Christmas tree - being decorated

By Shealah Craighead (The White House) [Public domain], via Wikimedia Commons

Height and Tree Type

The number of lights required, not only depends on the height of the tree, but the type of tree as well. Slender trees require the least amount of lights, while especially full tree will require more. The technique used (how you put the lights on the tree), will also vary the number of lights needed. Branch wrapping will generally require the most lights, while a lay-in technique or swaging technique will require 10%-30% less.

Bulb Style and Spacing

The number of lights needed will also depend on the bulb style selected for the tree. An 8 foot tree will need 560 to 840 mini lights, but only 325 to 425 C7s.
If you love lights and like a super bright tree, start at the higher end of this range, and add sets as needed.
If you prefer as more subtle look, start at the lower end of the range, and add or subtract sets as needed.
All of these calculations are even further complicated, by the fact that the strands differ in length depending on the bulb spacing. Some strands with the same bulb count can have 6″ light spacing, and others can have 12″ spacing, which means one set will be twice as long as the other set. The bulb calculations given below, are based on 4”-6” bulb spacing for the 5MM, G12, M5, or C6 light bulb shapes, and are based on 8”-12” bulb spacing for the C7 or C9 bulb shapes.

It’s easy to see that calculating the number of lights for your Christmas tree is a both a math riddle and an art.

It helps to have a clear chart like the one below.

Tree Height 5MM, G12, M5, or
C6 Bulb Shapes
C7 or C9 Bulb Shapes
2′ – 3′ Full 140 – 280 50 – 150
4′ – 5′ Full 280 – 420 150 – 225
6′ – 7′ Slender 210 – 350 150 – 225
6′ – 7′ Full 420 – 630 225 – 325
8′ – 9′ Slender 350 – 490 225 – 325
8′ – 9′ Full 560 – 840 325 – 425
10′ – 11′ Slender 490 – 630 325 – 450
10′ – 11′ Full 700 – 1050 425 – 550
12′ – 14′ Slender 630 – 770 450 – 600
12′ – 14′ Full 840 – 1400 550 – 675
15′ – 17′ Full 1050 – 1610 675 – 800
18′ – 20′ Full 1260 – 1890 800 – 950


Once you solve the riddle for the number of lights, you can calculate the amount of garland and ornaments. Professionals suggest having 9 feet of garland for every foot of tree and 12 ornaments for every foot of tree. (Thankfully there are no calculations needed for the one tree skirt!)

Now it’s time decorate the tree. It helps to make a plan and choose your approach.

First, test the lights to ensure that all the bulbs are working. Plug your lights in as you decorate. It’s easier to eliminate any defective sets at this point, and you can see how the lights are spaced on the tree if they’re on. Next, choose a light stringing technique.
Laying lights onto the individual branches, swaging the lights into the tree, or swirling around the tree, are the more common lighting techniques for indoor evergreen trees. For additional tips on decorating your Tree, please see our article Decorate Your Christmas Tree Like A Pro This Year

When you’ve finished applying the lights, turn off the room lights, and stand back to look for dark spots on the tree where the lights need to be re-arranged. After that, place your tree topper, garland, ornaments, and tree skirt.

Decorating the Christmas tree

By Ernst Vikne (Decorating the Christmas tree) [CC-BY-SA-2.0], via Wikimedia Commons

Easier numbers

No matter what sort of math you applied to solve the riddle of how many lights for your tree, here are some numbers that are easier to understand. Only 3 to 5 strands of incandescent lights can be safely joined, but LED lights are much safer and energy efficient, so up to 45 strands of Premium Grade LEDs can be joined end-to-end (Check the tags on your specific light set). And that’s just what you can plug end-to-end. Imagine what that means for your entire holiday display. No more adding up watts and amps, and hoping that breaker does not blow when you plug it all in. Just light it and enjoy!

If you’re still scratching your head to figure out how many strands of lights you need, call us and we’d be happy to help you calculate it—for minis, classic C7s or C9s, rope lights, and more.