Christmas tree lights calculator: how many lights you need

How many lights does a Christmas tree need?

There is no standard, only rules of thumb, and they agree on a starting point. Christmas Lights, Etc. says to plan for a minimum of 100 lights per vertical foot when wrapping an indoor tree. 1000Bulbs calls 100 mini lights a foot the conservative approach and suggests 200 a foot for a brighter look. The Christmas Light Emporium gives the same two figures and adds two useful details: the count is for the body of the tree, not the topper, and a slim tree needs 25 to 30% fewer. Christmas Light Source starts lower, at 50 to 100 a foot. So the calculator uses 100 a foot for the classic look and 200 for the full look, and lets you type your own.

In metric, 100 a foot is about 330 lights a metre of height, so a 2.1 m tree takes about 690. Measure the height from the lowest branches to the tip, without the star; that is the part the lights go on.

Classic, full, wrapped or branch-wrapped

• Classic (100 a foot) and full (200 a foot) are counts from the rules of thumb. You weave the strings in and out of the branches, so the lights sit at different depths and the tree glows from inside.
• Wrapped trunk to tip is a spiral round the outside, evenly spaced. Here the count comes from geometry: the length of the spiral divided by the spacing of the bulbs on your string.
• Branch-wrapped means running the lights out along each main branch and back. Christmas Lights, Etc. recommends strings with 6 or 8 inch bulb spacing for this. The calculator models tiers of branches every few inches, each wrapped out to its tip and back; count a few real branches and adjust the two settings.

The spiral: how far a string goes round the tree

Treat the tree as a cone of height H and base radius R (half the width). A spiral that climbs evenly from the bottom to the tip in N full rounds has an exact length:

L = ½·√(s² + k²) + (s² ÷ 2k)·asinh(k ÷ s), where s = √(R² + H²) and k = 2πNR

With no rounds it is just the slant height s, the line straight up the side. With many rounds it comes close to πNR: the number of rounds times the circumference half-way up, because the rounds shrink from the full width at the bottom to nothing at the tip. The tests behind this page check the formula against a numerical integration for dozens of heights, widths and round counts.

For the classic and full looks the calculator turns this round: it shows how many rounds your lit wire would make if you wound it only round the outside. The 700 lights on the example tree, on 4 inch spacing, are 233 ft of lit wire: 35 rounds about 2.4 inches apart. Nobody wraps that tightly, which shows how much wire disappears into the branches when you weave it.

Worked example: a 7 ft tree

A 7 ft artificial tree, 4.2 ft wide, with 100-light LED sets at 4 inch spacing. Classic look: 7 × 100 = 700 lights, which is 7 strings. Full look: 1,400 lights, 14 strings. Wrapped trunk to tip with 8 inches between rounds: 10.5 rounds, 70 ft of wire, 211 bulbs, so 3 strings. Branch-wrapped, with 8 branches every 8 inches and 6 inch bulb spacing: 80 branch runs, 183 ft, 366 bulbs, 4 strings.

LED or incandescent: watts and strings end to end

Power is the number of strings times the watts printed on the box. As an example, Northlight’s 100-light mini sets sold by Christmas Central are 4.8 W for the LED version and 40.8 W for the incandescent one, so the LED set uses about 88% less. ENERGY STAR says a certified LED light string uses approximately 70% less energy than an incandescent one. Seven LED strings draw 33.6 W, about 0.28 A at 120 V; seven incandescent strings draw 285.6 W.

How many strings you can plug end to end is printed on the box or the plug, and it is the number that counts. The same Northlight sets allow up to 5 incandescent or 43 LED sets, in both cases not more than 210 W in all; Christmas Lights, Etc. explains that 210 W is the usual limit for mini sets with thin 22-gauge wire. The US Consumer Product Safety Commission is stricter for incandescent lights: never string together more than three sets. So the calculator assumes 3 for incandescent sets, and shows how many plugs, or separate runs, you need.

Common mistakes

• Counting the topper in the height. A 7.5 ft tree with a 9 inch star still has about 7.5 ft of branches.
• Buying by the number of lights without checking the spacing. 100 lights at 4 inches is 33 ft of wire; at 6 inches it is 50 ft.
• Mixing LED and incandescent sets on one run, or adding sets beyond the box’s limit.
• Forgetting the extension lead: the first string must reach a socket, so allow a cord from the bottom of the tree.
• Leaving the lights on overnight. NFPA says to turn Christmas tree lights off before leaving home or going to bed.

Before the lights go on

Check every set as it comes out of the box. Electrical Safety First says to make sure lights are not damaged or broken and have no loose wires, and to buy from a reputable retailer rather than a third-party seller online. NFPA says to replace any string with worn or broken cords or loose bulb connections. A real tree should stand at least three feet from heat sources, lights included in NFPA’s list; see real Christmas tree care for water and safety, and the tree size calculator if you haven’t bought the tree yet. With the tree lit, Paper Lanternly prints place cards and banners for the table.