How to Calculate Holiday Lights for Christmas in 5 Steps

Holiday lights in one line

Holiday lights are counted in four moves: measure the roofline in feet, multiply by twelve and divide by the bulb spacing for the bulbs, divide the bulbs by the strand size and round up for the strands, and multiply the bulbs by the watts per bulb and the hours by the rate for the monthly cost. Sixty feet at 6 in spacing is 120 bulbs, 5 strands, 60 watts, and $1.53 a month at 5 hours a night. Everything else in this guide is about getting the roofline, the spacing, and the hours honest, because the formula is simple and the measurements are where light counts go wrong.

Step 1: Measure the roofline you actually hang

Measure the path the lights follow, not the width of the house: the worked example uses 60 ft. Walk the eaves with a tape or a measuring wheel, and add every run together — the front eave, the side returns, and both slopes of any gable the lights climb. A gabled roofline is longer than its footprint, because the lights go up one slope and down the other, and a straight-line measure across the front of the house misses exactly that extra length. Measure twice where the line turns a corner, because a corner measured short is a strand bought short, and a strand bought short is a dark stretch on the night the lights go up. The number that results is the roofline input.

Step 2: Turn feet into bulbs with the spacing

Turn the feet into bulbs: 60 × 12 ÷ 6 = 120 bulbs. The spacing is the distance between bulbs on the strand you are buying, printed on the box — 6 in on the defaults, 12 in on wider strands, 4 in on dense ones. Multiply the feet by twelve to get inches, then divide by the spacing: the result is the bulb count, and it moves fast with the spacing, because the same 60 ft at 12 in spacing needs only 60 bulbs while at 4 in it needs 180. Never borrow another strand's spacing: a count planned at 6 in and bought at 12 in dresses only half the roofline, and a count planned at 12 in and bought at 6 in buys twice the lights the roofline holds.

Step 3: Turn bulbs into strands by rounding up

Turn the bulbs into strands: 120 ÷ 25 = 4.8, rounded up to 5 strands. Strands are sold whole, in a fixed bulb count — 25 on the defaults — so the division almost never lands whole, and the rule is always to round up, never down: four strands cover only 100 bulbs, leaving 20 bulbs of roofline dark, while five strands cover all 120 with 5 spare bulb positions on the last strand. Count the spare as the price of a fully lit line, not as waste: the last strand's extra bulbs simply sit unused at the end of the run, and the alternative is a gap in the middle of the display. If the division lands exactly whole, buy exactly that many — rounding up changes nothing there.

Step 4: Work out the watts before you plug in

Work out the watts: 120 × 0.5 = 60 watts. Each LED bulb on the defaults draws half a watt, so the whole display draws less than a single old light bulb, and a standard outdoor circuit carries it with room to spare. Check the watts anyway before plugging in, because the maths changes completely with older strands: incandescent mini-lights draw several watts a bulb, and 120 of those can approach the limit of a single outlet circuit, especially alongside other decorations. Add the watts of every strand on the same circuit together, and keep the total comfortably below the circuit's rating — the watts are the safety number, not just a cost number.

Step 5: Turn watts and hours into the monthly cost

Turn the watts and the hours into the monthly cost: 60 × 5 × 30 ÷ 1000 = 9 kWh, and 9 × $0.17 = $1.53 a month. The hours are the hours the lights actually burn each night, and the honest figure is the timer's figure, not the intention's: a timer that cuts the lights after 5 hours gives the $1.53 month, while lights left on all night burn more than twice as much over the same strands. Use your own electricity rate in place of $0.17 if you know it — the rate is printed on the bill — and count 30 nights for a full month of display. For the interactive version, use the holiday lights calculator, which runs the same four moves on any roofline, spacing, and hours.

Worked example, start to finish

Roofline 60 ft, spacing 6 in, hours 5 a night, rate $0.17 a kWh. 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. The same five steps on 90 ft of roofline give 180 bulbs, 8 strands, 90 watts, and $2.30 a month; at 12 in spacing they give 60 bulbs, 3 strands, 30 watts, and $0.77 a month; and at 8 hours a night they give the same 5 strands but $2.45 a month. Roofline, spacing, and hours are the only three levers, and only the hours move the bill without moving the lights.

Three mistakes to avoid

The first mistake is measuring the house instead of the path: a straight width across the front misses the gable slopes and the side returns, and the count comes up short on the night the lights go up. The second mistake is rounding strands down: a division of 4.8 is five strands, not four, because four strands leave a dark gap and no shop sells eight-tenths of a strand. The third mistake is forgetting the timer: lights counted at 5 hours a night but left burning until dawn cost more than twice the planned month, and the timer, not the strand count, is the cheapest fix on the whole display. Measure the path, round up, set the timer — in that order, every time the lights are counted.