Roofline to bulb count
C9 bulb calculator
Enter the feet of roofline you are lighting and the socket spacing on your wire. The calculator returns the bulb count and prices it with packs, cases or a mix.
The formula
Bulbs = roofline feet × 12 ÷ spacing in inches, rounded up. With spares: bulbs × (1 + spare %), rounded up. The calculator then prices every possible combination of 500-bulb cases and 25-bulb packs that covers the quantity and shows the one with the lowest total.
Worked example
A ranch house with 96 ft. of front eave and two 18 ft. gable rakes has 132 ft. of roofline. On 15 in. spacing: 132 × 12 ÷ 15 = 105.6, rounded up to 106 bulbs. Add 3% spares and the order is 110 bulbs, which is five 25-packs (125 bulbs). On 12 in. spacing the same house takes 132 bulbs before spares.
Picking spacing for the look you want
Closer spacing reads as a continuous line of light, especially from a distance. Wider spacing shows each bulb as a distinct point and uses fewer bulbs per foot. The spacing is fixed by the socket wire, so decide before you buy spools: 12 in., 15 in. or 24 in. bulk spools are the three options priced here. Most residential rooflines use 12 in. or 15 in.; 24 in. tends to appear on long commercial fascia.
Measuring tips
Measure along the edge where the lights will sit, not the wall below it. Gutters usually run a few inches longer than the wall at each corner. For sloped gable edges, the material cost calculator and a simple pitch factor help: a 6/12 roof adds about 12% to the horizontal run, an 8/12 roof about 20%, and a 12/12 roof about 41%.
Frequently asked questions
How many C9 bulbs per foot of roofline?
One bulb per socket, so it depends on spacing: 1 bulb per foot at 12 in. spacing, 0.8 per foot at 15 in., and 0.5 per foot at 24 in. Multiply your total feet by that number and round up.
Should I buy spare bulbs?
A small allowance covers bulbs dropped on the ladder or cracked during install. Many installers keep 2% to 5% extra on the truck; the calculator adds the percentage you choose to the order quantity.
When is a 500-bulb case cheaper than 25-packs?
At the snapshot prices, 25-packs are $19.95 ($0.80 per bulb) and the 500-bulb case is $319.00 ($0.64 per bulb). Once you need more than 375 bulbs, one case costs less than the packs, and the calculator switches automatically.
Do I need a bulb for every socket?
Yes. An empty socket leaves a gap in the line and exposes the contacts to rain. If you want wider spacing, buy a spool with wider socket spacing rather than skipping sockets.