A string is a module, and the module decides the count
Light strings cannot be cut. Whatever you measure, you are buying whole units of the lit length printed on the box, and the only question is how many times the rounding-up happens. Once, if the whole front of the house is one continuous run and the strings carry from the gutter around the corner and up the rake. Once per run, if every window frame and every porch column gets its own strings.
The default list on this page is 42 ft of gutter, two 26 ft rakes, four 9 ft columns and three 14 ft window frames — 172 ft in total. As one continuous run of 24 ft strings that is 8 strings, 192 ft of lit length, 20 ft over. Broken into whole strings per run it is 2 + 2 + 2 + 4 + 3 = 13 strings, 312 ft of lit length, 140 ft over. Same house, same tape measure, five extra strings, and the difference is entirely a decision about how the display is wired rather than anything you can measure.
Neither answer is wrong. Continuous runs are cheaper and they look continuous, which is usually what people want along a gutter. Separate runs let you take one column down without touching anything else, and they mean a failure in the middle of the display does not put half of it out. The point is that the choice costs about 60 percent more string on a list like that one, and it is worth making it deliberately rather than discovering it at the register.
Measure the rake along the rake
The single most common measuring error on a roofline is taking the gable from the ground. The sloped edge of a gable is longer than the horizontal span underneath it, by exactly the factor the pitch dictates. A 20 ft span on a 6:12 roof is a 22.4 ft rake — 12 percent more. On a 12:12 it is 28.3 ft, 41 percent more. Two rakes on a front gable and you have quietly lost a full string. If you know the pitch and the span, the roof pitch calculator converts one to the other; if you can get a tape on the edge itself, do that instead.
The other quiet loss is the drop at the corners. A gutter line that turns a corner and comes back is not two straight measurements — there is a foot or two of string going around each return, and there is usually a downspout it has to get past. Add the corners to the list as their own short runs rather than assuming the straight measurements swallow them.
The lead and the tail do not light anything
Most strings have a length of plain wire with no bulbs on it at each end. It is what lets one string reach the next outlet or the next segment, and on a long chain it adds up: 18 inches at each end of 8 strings is 24 ft of wire in a display that only lights 192. That matters in two places. It means a string will physically reach further than its lit length, which is how a 24 ft string covers a 25 ft segment when the extra falls where nobody looks. And it means the total wire in the air is longer than the lit total, which is worth knowing when you are working out what has to be tied off and kept out of standing water.
Clips
Clip count follows the run, not the string. At a clip every 6 inches, 172 ft of run is 344 clips before you add one at each end of every run, and the ten separate runs in the default list bring it to 354. Clipping at every third bulb on 100-bulb strings is a different number entirely — 34 per string, and it follows the strings rather than the feet, so all the leftover string in the per-run mode drags a leftover clip count along with it.
Clips come in bags. Whatever this page gives you, the shelf will round it up again, and a handful of spares is worth more in January than the two dollars it cost, because the ones that break are always on the run you cannot reach.
Once you have the string count, the two limits that decide how the display gets fed are the maximum number you may join end to end and what may share one circuit. Those are worked through on the light string chain and circuit calculator. For a trunk or a porch column wrapped rather than outlined, the column and trunk wrap calculator handles the helix, and a tree is on the tree light and garland calculator. Cord runs out to the display are sized on the extension cord size calculator.
Questions people ask
How many feet of lights do I need for my house?
There is no per-square-foot answer worth having, because it depends entirely on what you outline. Measure it. The front gutter, the two gable rakes, the porch columns, the window and door frames, and the corners where the run turns — each as its own line. The default list on this page comes to 172 ft, which is a fairly typical single-storey front, and that becomes 8 strings of 24 ft as one run or 13 as separate ones.
Why does the calculator give two different string counts?
Because rounding up once is not the same as rounding up ten times. If the strings carry from the gutter into the rake and around onto the next segment, the whole display is one division and one round-up. If every window frame gets its own strings, every frame rounds up on its own and the leftovers add together. On the default list that is 8 strings against 13. Pick the one that matches how you actually intend to run the wire.
Do I measure the gable along the slope or across the bottom?
Along the slope, because that is the edge the clips go on. The horizontal span underneath is always shorter, and on anything steeper than a shallow roof the difference is a whole string. A 20 ft span on a 6:12 pitch is a 22.4 ft rake; on a 12:12 it is 28.3 ft. If you cannot get a tape on the edge, work it out from the span and the pitch rather than guessing.
How many clips per string?
That depends on which thing you are following. At a fixed spacing the clip count follows the feet of run: every 6 inches on 172 ft is 344 clips, plus one at each end of each run. Clipping at every bulb follows the strings instead, so 100-bulb strings give 100 clips each and the count rises with every extra string rather than with every extra foot. Both modes are on the calculator because both are how people actually do it.
How many strings can I plug into each other?
That number is printed on the box or in the product listing and it varies enormously between products — it is set by the wire and the fuse inside the string, not by anything on this page. This calculator deliberately does not hold a figure for it. Take the string count from here to the chain and circuit calculator, type in the maximum the maker gives, and it works out how many separate chains you end up with and what that adds up to in watts and amps. It will not tell you whether the result is acceptable; that is a question for a licensed electrician.