How many Christmas lights do I need for my house?

Short answer: measure the roof edges you intend to light, then divide by your bulb spacing. At the standard 12-inch C9 spacing, one foot of roofline is one bulb, so a 180-foot roofline is about 180 bulbs — roughly seven 25-bulb sets, or eight once you add a margin. For mini LED string at 6-inch spacing, the same roofline is about 360 bulbs.

That is the whole calculation. The part people get wrong is the first step, because the length of your roofline is not the width of your house and it is usually much more than you would guess from the driveway.

Step 1: measure the roofline you are actually lighting

Walk out to the street and decide which edges get lights. On a typical house that list is longer than expected:

• The main front gutter line
• The two rake lines of any front-facing gable (the sloping edges)
• The garage roof edge, plus its own gable if it has one
• The porch or entry roof
• Any dormers

Measure each one and add them up. A laser measure does this from the ground in about ten seconds per edge; a 25-foot tape and a helper works too and costs nothing. The full method, including how to handle a sloping rake without getting on the roof, is in measuring your roofline.

Step 2: choose a spacing, which chooses your bulb count

Twelve-inch spacing on C9 is the convention for a reason: it is the spacing at which individual bulbs read as individual bulbs from across the street, which is the entire point of choosing C9 over mini string. Going tighter than eight inches on C9 starts to look crowded and roughly doubles the cost.

Step 3: convert bulbs to sets

Light sets are sold by bulb count and by length, and the two do not always match the spacing you want. A 25-bulb C9 set at 12-inch spacing is 25 feet long. A 100-bulb mini set at 6-inch spacing is about 50 feet long. Read the length, not just the bulb count.

Then add a margin. Fifteen percent is the number we use, and it covers three real things: the edge you forgot when you measured, the set that arrives faulty, and the fact that runs almost never end exactly where a set ends.

A worked example: 2,200 sq ft, two stories, attached garage

At 12-inch C9 spacing that is 156 bulbs. Add 15 percent and you are buying for 180 bulbs, which is eight 25-bulb sets (200 bulbs) — the nearest whole number of sets above your requirement, with 20 bulbs of genuine spare.

The same house in mini LED at 6-inch spacing is 312 bulbs, plus 15 percent is 360, which is four 100-bulb sets rounded up to four rather than 3.6.

Step 4: the two numbers everyone forgets

You now know how many lights to buy. Two more numbers fall straight out of the same measurement and both of them cause problems if you skip them.

Clips. One per bulb for C7 and C9, or one per 18 to 24 inches for plain wire. Our 156-foot example needs around 160 clips for C9 and about 100 for mini string. Nobody buys enough; the full method is in how many clips do I need.

Amps. Add the wattage printed on each set's tag, divide by 120, and check it against the 12-amp working budget of a standard 15-amp circuit. Eight LED C9 sets will not come close. Eight incandescent sets might well exceed it. The arithmetic is in Christmas light amps and watts, and you can run it directly in the power and wattage calculator.

Common mistakes

• Estimating from the ground. Guesses on roofline length are usually 30 to 50 percent low, because people count the gutter and forget the rakes.
• Buying by bulb count instead of length. A '300-count' set tells you nothing about how many feet it covers until you know its spacing.
• Buying in two trips. The second trip is where the color mismatch comes from, because the store has restocked with a different product line.
• Forgetting the spare set. One faulty set on install day with no spare is a wasted weekend.

With the footage and the set count settled, go and choose the product: the best outdoor Christmas lights for the general case, or best C9 if you have decided on a traditional roofline.