The base width matters as much as the height
Ask how many lights a seven foot tree needs and the question is missing half its inputs. A narrow seven foot tree three feet across and a full one five feet across are wrapping jobs that differ by two thirds, because the spiral has to travel around whatever girth is there at every level. Height sets how many times you go round. Width sets how far each of those trips is. The answer is the product.
That is why the per-foot rules of thumb are unreliable in both directions. On the default figures — a 7 ft tree, 4 ft across, starting half a foot off the floor, wrapped every 6 inches — the spiral around the branch tips is 76.5 feet, which is four 24 foot strings with 19.5 feet left on the last one. Narrow the same tree to 3 ft across and it drops to 57.6 feet. Nothing about the height changed.
Scaling up costs more than you would think
Because the length depends on height times girth, and a bigger tree of the same shape is bigger in both, the lights go up with the square of the scale. Going from a 7 ft tree to a 9 ft tree of the same proportions is a factor of 9 over 7 in height, which squares to 1.65, and the calculator reports 1.7 for the actual pair — 130.3 feet against 76.5. It comes out slightly above the clean square because the half foot at the bottom that stays unwrapped does not scale with the tree. A 10.5 ft tree, half again as tall, comes to 180.2 feet, or 2.36 times. People replace a tree with a taller one, buy one or two more boxes on the theory that it is a couple of feet taller, and come up short.
The calculator shows both of those comparisons directly, using your own tree as the starting point, so the multiplier is visible rather than something you have to trust.
Wrapping the tips is one job, wrapping in and out is another
The arithmetic here describes a spiral around the outside of the tree. A great many people do not light a tree that way at all — they run the string in along a branch toward the trunk and back out to the tip, branch by branch, which produces depth and a glow from inside the tree and uses somewhere between one and a half and two and a half times as much string for the same tree.
There is no honest constant for that. It depends on how far in you go, how many branches there are and how disciplined you stay by the third hour. The depth factor on this page is a field for exactly that reason, and the only reliable way to fill it is to light one branch the way you intend to light the whole tree, measure what it took, and compare that against the tip-only length for the same section.
Garland and ribbon are the same spiral, wound wider
Garland gets wound at a much wider spacing than lights — a foot or more rather than six inches — so it takes proportionally less. The same default tree at 12 inch spacing needs about 39 feet of garland, which at 9 feet a piece is five pieces with six feet left over. Ribbon on a roll works identically; put the roll length in the piece field.
The thing the arithmetic will not tell you is where the joints go. Five pieces of garland means four joints, and each one has to be tucked behind a branch or hidden under an ornament. Wind the garland before the ornaments go on, and start each new piece where you already know something will be hanging.
A trunk or a porch column is a cylinder, not a cone, and that is the column and trunk wrap calculator. Garland hung as a swag between two fixed points follows a curve rather than a spiral, and the length is on the garland swag calculator. Outdoor roofline is on the christmas light string calculator.
Questions people ask
How many lights for a 7 foot Christmas tree?
It depends on how wide it is and how tightly you wrap. A 7 ft tree 4 ft across, wrapped around the tips every 6 inches starting half a foot off the floor, takes 76.5 feet of lit string — four 24 foot strings. The same height at 3 ft across takes 57.6 feet. At 4 inch spacing instead of 6 the 4 ft tree jumps to 114.2 feet. The height on its own does not determine the answer.
Is the 100 lights per foot rule any good?
It has the wrong shape. It scales with height alone, and the actual length scales with height times girth, so it under-predicts full trees and over-predicts narrow ones, and it gets steadily worse as trees get bigger. It also depends on the bulb spacing of whatever strings you happen to own, which varies. Working from the spiral length in feet and dividing by the lit length printed on your boxes avoids all of that.
Why does a 9 foot tree need so much more than a 7 foot one?
Because it is bigger in two directions at once. A proportionally larger tree is both taller and wider, so you go round more times and each time round is longer. The lights scale with the square of the size ratio: 9 over 7 is 1.29, squared is 1.65, so a proportional 9 ft tree wants roughly two thirds more string than a 7 ft one rather than a third more. On the default figures the calculator puts it at 130.3 feet against 76.5, a factor of 1.7, a shade above the clean square because the unwrapped half foot at the bottom stays the same size. This is the most common way people end up short.
How much more do I need if I wrap in toward the trunk?
Somewhere between one and a half and two and a half times the tip-only length, and nobody can be more precise than that from a distance, because it depends on how deep you go and how many branches you actually do it on. The calculator has a depth factor field rather than a built-in number. Light one branch the way you plan to light the tree, measure the string it consumed, and compare it against what the tip-only spiral would have used for that same section.
How much garland do I need for a Christmas tree?
The same spiral arithmetic at a wider spacing. The default 7 ft tree, 4 ft across, wound every 12 inches, takes about 39 feet — five 9 ft pieces with 6 ft spare. Halve the spacing to 6 inches and it doubles. Remember that five pieces means four joints, and each joint needs somewhere to hide, which is easier if the garland goes on before the ornaments.