C9 / C7 bulb count
Convert roofline or outline feet into socket/bulb counts for C9 or C7 stringers at your spacing.
sockets = ceil(linear_feet ÷ (spacing_inches ÷ 12)) · add spare % for peaks and replacements
When to use
This tool helps when you want a C9 / C7 bulb count planning figure with the assumptions written down. You will enter linear feet to outline, socket spacing (inches), and spare bulbs / buffer (%).
Convert roofline or outline feet into socket/bulb counts for C9 or C7 stringers at your spacing. Keep units consistent (feet vs inches, watts vs amps), then use the formula box to see how each term contributes.
Stop and ask a licensed electrician when loads, hardwired outlets, or roof work go beyond temporary plug-in holiday displays.
12 in ≈ 1 bulb/ft (common C9). 6 in doubles density.
Math is the same; label helps you match packs and clips.
Worked example
Using the form defaults (Linear feet to outline = 100, Socket spacing (inches) = 12), walk the formula: sockets = ceil(linear_feet ÷ (spacing_inches ÷ 12)) · add spare % for peaks and replacements.
With all defaults (100 ft, 12 in spacing, 10% spare): 100 ÷ (12 ÷ 12) = 100 sockets, and ceil(100 × 1.10) = 110 bulbs to order — the same numbers the Calculate button returns.
If your hand math disagrees, check units (inches vs feet) first.
Limits / assumptions
Units: keep every length in one system (feet or inches end-to-end) and every electrical quantity in the units the labels name. Mixing without converting invents a wrong answer.
Where the formula fails: odd roof geometry, wind on inflatables, worn sockets, and conditions not listed on the page are outside the model. Zero or nonsense inputs should error — do not force a number.
Not a substitute: the c9 / c7 bulb count result is a planning aid for holiday decorating — not an install quote, structural opinion, or product warranty.
FAQ
How many C9 bulbs per foot?
At 12-inch spacing you need about one socket per foot. At 6-inch spacing you need about two per foot.
Should I order extras?
A 5–15% spare buffer covers broken bulbs, peaks, and color-pattern leftovers.
Does this work for C7 too?
Yes — enter the same linear feet and your C7 stringer’s socket spacing.
What about gables?
Measure each rake and ridge run, then sum the linear feet before dividing by spacing.