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Holiday lights calculator
Put your roofline length, bulb spacing and hours per day in: 60 ft at 6 in spacing needs 120 bulbs — 5 strands, 60 watts, $1.53 a month.
★ EVERYDAY · HOLIDAY · LIGHTS
Holiday Lights Calculator: Strands, Watts and Cost
On the defaults — 60 ft of roofline at 6 in spacing, burning 5 hours a night — the lights need 120 bulbs, 5 strands, 60 watts, and $1.53 a month. This is the holiday lights tool: turn feet into bulbs with the spacing, turn bulbs into strands by rounding up, and turn watts and hours into the monthly cost — a strand count guessed from the box tells you nothing until the roofline and the spacing behind it are known.
What does 5 strands, 60 watts, $1.53 a month mean?
It means a 60 ft roofline dressed at 6 in spacing carries 120 bulbs, and 120 bulbs do not split evenly into 25-bulb strands, so the fifth strand is bought for the last 20 bulbs. The watts stay small because each LED bulb draws only half a watt, and the monthly cost stays small because 60 watts over 5 hours a night is only 9 kilowatt-hours. The length sets the bulbs, the spacing sets how fast they accumulate, and the hours set only the bill: a longer night costs more over the same lights, never more lights.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Bulbs | length × 12 ÷ spacing | 120 — 60 × 12 ÷ 6. |
| Strands | bulbs ÷ 25, rounded up | 5 — 120 ÷ 25 = 4.8, rounded up. |
| Watts | bulbs × 0.5 | 60 — 120 × 0.5. |
| Monthly cost | watts × hours × 30 ÷ 1000 × $0.17 | $1.53 — 9 kWh × $0.17. |
Three things to check before you trust the number
First, check the length honestly: 60 ft must be the path the lights actually follow — along the eaves, up the gable and back down — not the straight width of the house, because a gabled roofline runs longer than its footprint and a straight-line measure buys too few strands. Second, check the spacing against the strand you are actually buying: 6 in is the spacing on the box, and a strand sold at 12 in spacing puts half the bulbs on the same roofline, so a count planned at one spacing and bought at another is a roofline half dressed. Third, check the hours against the timer, not the intention: 5 hours suits a timer that cuts the lights at bedtime, while lights left on all night burn more than twice the monthly cost on the same strands.
Worked example
60 ft of roofline, 6 in spacing, 5 hours a night. Bulbs: 60 × 12 ÷ 6 = 120. Strands: 120 ÷ 25 = 4.8, rounded up to 5. Watts: 120 × 0.5 = 60. Monthly energy: 60 × 5 × 30 ÷ 1000 = 9 kWh. Monthly cost: 9 × $0.17 = $1.53. In practice that means five strands on the roofline, a 60-watt load a child could carry, and a monthly bill smaller than a single coffee — the number to set against the price of the strands themselves, which is where the real cost of holiday lights sits.
Every branch, checked
With 90 ft of roofline instead of 60, the bulbs rise to 180 — 8 strands, 90 watts, 13.5 kWh, $2.30 a month. With 12 in spacing instead of 6, the bulbs fall to 60 — 3 strands, 30 watts, 4.5 kWh, $0.77 a month. With 8 hours a night instead of 5, the bulbs and strands stay put at 120 and 5, but the energy rises to 14.4 kWh and the cost to $2.45 a month. With 30 ft of roofline instead of 60, the bulbs fall to 60 — 3 strands, 30 watts, 4.5 kWh, $0.77 a month.
The reverses, worked backwards
The length behind 5 strands is 60 ft: at 30 ft the count is 3 strands (60 bulbs, 30 watts, $0.77 a month); at 90 ft it is 8 strands (180 bulbs, 90 watts, $2.30 a month). The spacing behind 5 strands is 6 in: at 4 in the count is 8 strands (180 bulbs, 90 watts, $2.30 a month); at 12 in it is 3 strands (60 bulbs, 30 watts, $0.77 a month). The hours behind $1.53 are 5 a night: at 3 hours the cost is $0.92 a month (5.4 kWh); at 8 hours it is $2.45 a month (14.4 kWh) — the hours never touch the strands, only the bill.
What the defaults assume
The defaults assume a single straight-ish roofline of 60 ft, LED bulbs at 0.5 watts each, strands of 25 bulbs, a flat $0.17 per kilowatt-hour, and 30 nights a month at 5 hours a night. They do not assume trees, bushes, windows, or porch rails — every extra run needs its own measure — and they do not assume old incandescent strands, which draw far more per bulb and would make the same 120 bulbs a much larger bill.
Who uses this
- A homeowner dressing the eaves: the 5-strand count settles the shopping list before the first trip to the store.
- A renter lighting a balcony rail: the same maths on a short run shows why one strand is usually enough.
- A decorator quoting a job: the watts and the monthly cost settle the power plan and the running-cost line.
- A household checking the bill: the $1.53 month settles whether the lights or the timer is the thing to change.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the roofline length, the true spacing, and the hours the timer actually runs. Real rooflines jog around gables and dormers, real strands vary a little in bulb count, and real rates vary by utility — so treat 5 strands as the planning count for a 60 ft run at 6 in spacing, and check it against one measured run before buying.
Cite this calculator
Bright Side Kit, “Holiday Lights Calculator: Strands, Watts and Cost”, https://brightoolbox.com/holiday-lights-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults so the result can be checked.
Frequently asked questions
How many strands do I need for 60 feet of roofline?
5 strands on the defaults: 60 ft at 6 in spacing is 120 bulbs, and 120 bulbs into 25-bulb strands is 4.8, rounded up to 5. The fifth strand carries only 20 bulbs.
Why round strands up instead of buying exactly 4.8?
Because strands are sold whole: 4 strands cover only 100 bulbs, leaving 20 ft of roofline dark. The spare bulbs on the last strand are the price of a fully lit line.
How much do holiday lights cost to run each month?
$1.53 on the defaults: 60 watts over 5 hours a night for 30 nights is 9 kWh, and 9 kWh at $0.17 is $1.53. The strands cost far more to buy than to run.
What if my bulbs are spaced 12 inches apart?
Then the same 60 ft needs only 60 bulbs — 3 strands, 30 watts, $0.77 a month. Wider spacing halves the bulbs, the strands, and the bill together.
Do I count the gable peak in the roofline?
Yes: measure the path the lights follow, up one side of the gable and down the other, not the straight width of the house. A gabled run is longer than its footprint.
How is this different from the Christmas tree size calculator?
That page sizes a tree to a ceiling; this page sizes strands to a roofline. Use that page for the tree, this page for the lights outside.