How many Christmas lights do you need?

For a tree: about 100 lights per foot of height. A 6 ft tree takes roughly 600, a 7 ft tree about 700. For a roofline: measure the run and divide by the set's published covered length - not by its bulb count.

Those are two different sums because they are two different jobs. A tree is wrapped, so density is what you are buying. A roofline is outlined, so length is what you are buying. Using the tree rule on a roof - or buying roof sets by bulb count - is how people end up with the wrong amount in both directions.

The rule is a proxy for surface area. A tree's lightable surface grows faster than its height - a 9 ft tree is not twice the job of a 4.5 ft one, it is more - but the rule holds in practice because taller trees are also usually wrapped less densely at the top, where nobody is standing close.

What the rule does not account for is width. A wide, full tree at a given height has considerably more surface than a slim or pencil tree of the same height. If your tree is unusually full, treat the height-based figure as the floor; if it is a pencil tree, the sparse column is probably right.

It also assumes small bulbs. The whole rule is written for mini and 5mm lights wrapped around branches. It does not transfer to C9, which is outlined at 8 to 12 in. spacing rather than wrapped - applying 100 per foot to C9 would have you buying roughly four times what the job takes.

Outdoors the question changes from density to length, and the number on the front of the box is the wrong one to use. Two sets both marked "100 count" can cover 16 ft and 32 ft - the bulb count tells you the spacing, not the coverage.

For a tree, in 100s: the JinBest green wire set. Warm white on green wire with UL certification stated. Buying in hundreds makes the per-foot arithmetic trivial - six boxes for a 6 ft tree.
Who it is for: anyone who wants the sum to be simple and the wire to disappear. Who should skip it: anyone with a white or flocked tree.

For a tall tree, in 200s: the 200 LED, 66 ft twinkle set. Halves the number of boxes and plugs for a 7 ft or larger tree.
Who it is for: tall trees on a budget. Who should skip it: anyone who will mind if it returns to a flashing mode after a power cut.

For a roofline: the Brizled C9 set. 16 ft of covered length per set - the figure to divide your measured run by.
Who it is for: outlining a roof. Who should skip it: anyone wrapping anything, where the per-foot rule and a small bulb apply instead.

For wrapping posts and trunks: the Brizled 5mm wide angle set. Connectable, so one outlet can serve several wraps.
Who it is for: trunks, railings, shrubs. Who should skip it: rooflines, where small bulbs blur into a line.

If you came here from the household side of this site, the same approach - measure, then buy to a number rather than an impression - is what how many lumens does a room need does for ordinary rooms.

Once the quantity is settled, C9 vs C7 vs mini decides the shape and warm white vs cool white decides the color.