A tree is a volume to fill and a railing is a line to cover, so they are answered differently. Pick the mode that matches what you are lighting, and change any field — including the lights-per-foot figure, which is a retail convention rather than a physical constant.
- Bulbs needed
- 750
- Strings to buy
- 8
7.5 ft x 100 lights/ft = 750 bulbs.
750 / 100 per string = 8 strings (rounded up).
The formulas, in plain text
For a tree:
- Bulbs = tree height in feet x lights per vertical foot.
- Strings = bulbs / bulbs per string, rounded up.
For a run:
- Lighted feet per string = (bulbs per string x spacing in inches) / 12.
- Strings = run length in feet / lighted feet per string, rounded up.
The density bands
| Look | Lights per foot | A 7.5 ft tree |
|---|---|---|
| Classic | 100 | 750 bulbs |
| Full | 150 | 1,125 bulbs |
| Showpiece | 200 | 1,500 bulbs |
These are the standard retail conventions rather than measurements. The bulb counts are height multiplied by density, computed by us.
Source: The 100-lights-per-foot rule is the common retail guideline for a full-profile tree; the calculator lets you override it.
Wrapping a trunk or a post
The run mode assumes the lights follow a line. A spiral wrap around a post, a trunk or a column covers far more distance than the object’s length, and the working rule is roughly three times the straight-line measurement. A 6-foot post wrapped in a spiral uses about 18 feet of lighted strand, so enter 18 rather than 6.
Where to find your bulb spacing
Most listings publish a bulb count and a lighted length rather than a spacing. Divide one by the other:
- 100 bulbs over 33 ft = 4 in spacing (the common LED mini string)
- 100 bulbs over 20 ft = 2.4 in spacing (a dense incandescent mini string)
- 25 bulbs over 25 ft = 12 in spacing (a standard C9 roofline set)
- 25 bulbs over 17 ft = 8 in spacing (a dense C9 porch set)
Then check it will run
A quantity answer is only half of it. Twenty-four hundred incandescent bulbs on a showpiece tree is a real electrical load, and the second question is whether the circuit will carry it. Put the result into the load calculator.
The full tables, including tree height to bulb count and lighted feet by spacing, are in how many Christmas lights do I need.