The easiest measuring job on a roof, because it is linear rather than area. What people get wrong is not the length. It is the count of corners, outlets and downspout drops.
Gutters run along the eaves only, the horizontal bottom edges of the roof. They do not run along rakes, the sloping edges at gable ends.
So on a simple gable roof you have two gutter runs, one along each long side, and nothing on the gable ends. On a hip roof, where every edge is horizontal, gutter runs all the way round. Sketching the roof outline and marking which edges are eaves takes a minute and prevents the most common ordering error.
Measure each eave run at ground level along the wall below it, which is the same length. There is no need to be at gutter height, and a laser measure braced against the corner of the building handles a long run in one reading.
Total length tells you how much gutter to buy. The fittings are what get missed.
Seamless gutter, formed on site from a roll, removes the joiners but not any of the other fittings.
Each downspout needs the vertical drop from the gutter to the ground or to wherever it discharges, plus the offset pieces at the top that bring it back to the wall under the eave overhang, plus a shoe or extension at the bottom.
The vertical drop is a straightforward measurement from the ground up to the gutter line, and this is a natural indirect measurement if the eave is high. Measuring height you cannot reach is covered here.
Do not forget the overhang offset. The gutter sits at the edge of the roof, which may be a foot or more out from the wall, and the downspout has to travel back in before it runs down. That is two elbows and a short connecting section per downspout, and on a house with four downspouts it adds up.
Undersizing here is the most consequential mistake, because it causes overflow rather than just looking wrong.
The number needed depends on the roof area draining into each run and on local rainfall intensity, which is why guidance varies by region. As a general principle, longer runs and larger roof areas need more outlets, and a long run with a single downspout at one end will overflow in heavy rain regardless of how well the gutter is fitted.
Position matters as much as count. Downspouts should sit where the water is going anyway, which means at the low end of the fall, and discharge somewhere that carries water away from the foundation rather than pooling against it.
Gutters are not fitted level. They are fitted with a slight fall toward the outlet so water actually moves.
A commonly used figure is around a quarter inch of drop per 10 feet of run. On a 40 foot run that is a full inch of height difference between one end and the other, which is visible if you look for it and essential if you want the gutter to drain.
The measuring consequence is that on a long run you may need to split the fall, running from a high point in the middle down to outlets at both ends, rather than falling continuously across the whole length. Otherwise the far end sits too low and looks wrong against the roofline. Deciding that before installation is a planning question, and it depends on the run lengths you just measured.
Add a small allowance to the total length for cuts and for the overlap at joiners. Gutter is inexpensive relative to the labour of a second trip, and an offcut is not a problem.
Take the fitting counts from your sketch rather than from memory, since inside and outside corners are easy to mix up when you are looking at a list instead of the building. The full roof method is here.
You will find our top laser measure picks on the front page, ranked with the reason for each.