What people use it for
- One number for both jobs The tree is wrapped, the roof is outlined, and they are two different sums. This adds them and tells you what to put in the cart.
- Sets, not bulb counts Bulbs come in 50s, 100s and 200s and none of them divide into 730. You see the sets to buy, with the spare already in them.
- The load, before the extension cords Watts and amps for the count you have, and how many plug groups it takes at the chaining limit of the bulbs you picked.
The rule every page gives you
A hundred lights for every foot of tree. Wirecutter prints it, the light sellers print it, the decorators on video say it, and for a six-foot tree that is 600 lights. It is a fair starting point and it is where nearly every page stops, which leaves you with a number that ignores the two things that actually decide the count: how wide the tree is, and the fact that half of what you are hanging is not going on a tree at all.
Wrapping a tree is a job about density. Outlining a gutter is a job about length. They are bought from the same shelf, hung on the same afternoon and counted with different sums, and no chart on the first page of Google does both.
Density on the tree, length on the roof
A tree gets wrapped, so what you are buying is bulbs per foot of height: fifty for a tree that reads as lit from the far side of the room, a hundred for the traditional look, a hundred and fifty for the display end that shows up in photographs.
A roofline gets outlined. The string runs along the gutter and each bulb keeps its own space, so thirty feet of roof with a bulb every foot is thirty bulbs, and twenty feet with a bulb every six inches is forty. Divide the run by the spacing. The bulb count of the set does not enter that sum at all, which is why people who buy roof lights by bulb count end up with twice the run they can hang or half the glow they wanted.
| Tree height | Sparse, 50 a foot | Full, 100 a foot | Display, 150 a foot |
|---|---|---|---|
| 4 ft | 200 | 400 | 600 |
| 6 ft | 300 | 600 | 900 |
| 7 ft | 350 | 700 | 1,050 |
| 8 ft | 400 | 800 | 1,200 |
| 10 ft | 500 | 1,000 | 1,500 |
| Roofline | Every 6 in | Every 9 in | Every 12 in |
|---|---|---|---|
| 20 ft | 40 | 27 | 20 |
| 30 ft | 60 | 40 | 30 |
| 40 ft | 80 | 53 | 40 |
| 50 ft | 100 | 67 | 50 |
The width is the number nobody asks for
A hundred per foot is a proxy for surface area, and surface area grows faster than height. A wide seven-foot tree has branches deep enough to take a string back and forth three times where a pencil tree of the same height takes it once. That is why two people with the same seven-foot tree argue about whether 700 is enough: one of them has a tree that is four feet across at the bottom.
So the tool asks for the shape. A slim tree lights with about three quarters of the bulbs, a full one takes a third more, and the difference on a nine-foot tree is two sets in the cart.
What the count pulls from the plug
Incandescent mini lights draw about 0.4 W a bulb, so a hundred-bulb set is 40 W and 730 lights are 292 W. LED minis draw around 0.05 W a bulb, the same set is 5 W, and the same 730 lights are 36 W. On a standard 15 A circuit that gives you 1,440 W of headroom, one is nothing and the other is a fifth of the circuit.
The difference matters on the roof, where you chain sets end to end. Incandescent sets are usually rated at three to six connected, LEDs at twenty or more, and the box prints the number for that exact set. The tool gives you the plug groups your count needs at the limit of the bulb you picked, so you know how many extension cords and outlet strips are in the plan before you are on the ladder.
Why the sets never divide evenly
Bulbs come in 50s, 100s and 200s. A display of 730 lights does not fit any of them, and the tool rounds up with ten percent spare instead of rounding to the nearest set. It is not padding: a strand that dies halfway, a branch wider than it looked from the ground, and a section of gutter that needs one more pass are the three reasons people drive to the store twice on the same Saturday.
The spare is also the difference between a display that gets finished and one that gets abandoned behind the shrubs. Nobody sees the back of the tree, and everybody sees the gap in the middle of the front.