The honest answer is that it depends on what you want lit, not on how big your house is. Most of the over-ordering we see comes from measuring the whole perimeter when the back of the house was never going to be lit, and most of the under-ordering comes from forgetting that corners eat length.
Measure the runs, not the house
Walk the elevations you actually want lit and measure along the fascia or soffit line where the track will sit. Include the returns and the gable runs; leave out anything hidden from the street if you are not lighting it.
As a sanity check against what other homes take: a front elevation on its own is usually about 120 to 150 ft, a full wrap of the main roofline is about 200 to 300 ft, and most homes land between 120 and 300 ft. If your number is a long way outside that, measure again before you order.
Then add 5 to 10 percent
Mitred corners consume length, a miscut costs a piece, and coming up ten feet short at the top of a ladder means a second order and another wait. The allowance is cheap insurance against both.
Work out your run
Measure the runs you want lit, not the perimeter of the house.
Pieces, and why the count matters
Track arrives as whole pieces that butt together along the run, so the piece count is really a joint count — it tells you how many mitres and caps you are dealing with. The Govee Original profile comes in 2.7 ft pieces and the Pro profile in 3.3 ft, and the two are not interchangeable, so check which profile fits the lights you own before you order.
What it costs
Track is one flat rate per foot at every length, so there is no saving in buying a longer run than you need and no penalty for a shorter one. Standard shipping is free. The shop page lists every length, and the calculator above shows the one your measurement rounds up to.
Before you order
Two things are worth settling first. One, the colour: the track is powder-coated to any RAL Classic shade and you approve a proof before anything is cut or coated, which is also why a custom colour cannot be returned for preference. Two, whether you are installing it yourself — the install guide is the whole method, and it is a fair way to judge before you commit.