The gutter is the cheap part
Anyone can divide an eave length by ten. What sends people back to the
store is everything else: a hanger every two feet, an end cap on each
open end, a mitre at each corner, an outlet per downspout, elbows top and
bottom, and two bands on every length of spout. This counts all of it,
against the numbers the manufacturer publishes rather than against
guesswork.
hangers = eave ÷ 24 in, + 1 per run
sections = eave ÷ 10 ft, rounded up
end caps = 2 per run − 2 per corner
bands = 2 per 10 ft downspout length
fall = ¼ in per 10 ft toward the outlet
120 ft of eave, 4 runs, 4 corners, 3 × 20 ft spouts
12 lengths of gutter
64 hangers
4 mitres, 0 end caps
6 lengths of downspout, 12 bands
3 outlets, 6 elbows
Hangers, and why 24 becomes 18
Amerimax's instruction is one hanger every 24 inches, and every 18 inches
in heavy ice and snow load areas. That is not a decorative difference:
the 120 foot example goes from 64 hangers to
84, and a gutter full of ice is carrying far more weight than
one full of water. Tick the snow box and the tool recounts.
They screw through the fascia into the rafter tails. If the fascia is
soft — and on a house old enough to need new gutters it often is —
replacing it is part of the job, not an optional extra.
Parts for common runs
From this calculator at 24 in hanger spacing, 10 ft stock and a 20 ft drop. Corner runs form a closed loop, which is why the end cap count falls to zero.
A quarter inch in every ten feet
The guide's method is to tack a string, level it, then lower it a quarter
of an inch for every 10 feet toward the downspout and re-tack. On a
30 foot run that is 0.75 inches
end to end — enough to drain and little enough that nobody standing in
the garden sees it against the fascia line.
How many downspouts is not a calculation
A 2 x 3 in is published as draining
186 gallons a minute and a 3 x 4 in
as 434. Those are capacities of the pipe. What actually
decides how many downspouts a roof needs is rainfall intensity, the area
feeding each run, and how far the water has to travel along the gutter to
reach an outlet — and the manufacturer's own guidance is simply to
increase the size or the number for wide roofs, heavy downpour areas and
any system that overflows.
So this page counts parts for the number you are planning and publishes
their figures beside it. It does not convert a roof area into a downspout
count, because that conversion needs a rainfall figure for your postcode
that nobody here can supply.
Not on the list
- Drip edge. It is what gets roof runoff into the gutter rather than behind it. The shingle calculator counts it.
- Sealant and screws. Every joint and every hanger.
- What happens at the bottom. Splash blocks, extensions or a buried line. A downspout discharging at the base of the wall has moved the problem, not fixed it.
The rest of the roof
The shingle calculator works
the covering from the sloped area and already counts drip edge, and the
rafter calculator gives you the
pitch and slope factor that turn a footprint into that area. Where the
water goes after the downspout is the
french drain calculator's
problem, and for the fascia and the wall behind it the
wall framing calculator
and the stone veneer calculator
cover what a hanger is screwing into.
Common questions
How much gutter do I need?
Measure every eave that will carry a gutter and add them up — not the perimeter of the house, because gable ends usually get nothing. Divide by the length your supplier sells, normally 10 feet, and round up. A 120 foot total is 12 lengths. The gutter itself is the easy part of the order; the hangers, mitres, caps, outlets, elbows and bands are where a trip back to the store comes from.
How far apart do gutter hangers go?
Every 24 inches, or every 18 inches where ice and snow load are heavy — that is Amerimax's own installation instruction, not a rule of thumb. It is also the biggest number on the parts list: the 120 foot example takes 64 hangers at standard spacing and 84 at snow spacing. Hangers screw through the fascia into the rafter tails, so the fascia has to be sound before any of this matters.
How much fall should a gutter have?
A quarter of an inch for every 10 feet, sloped toward the downspout — the guide tells you to tack a level string and then lower it by that much per 10 feet before you fix anything. On the 30 foot run in the worked example that is 0.75 inches of drop end to end. Too little and it holds water; too much and it looks wrong against a fascia line everyone can see.
How many downspouts do I need?
This calculator will not tell you, and neither will the manufacturer. Their guide publishes a drainage figure for each size and then says plainly to increase the size or the number for wide roof areas, heavy downpour areas and any system that overflows. How many you need depends on your rainfall intensity, the roof area feeding each run and how far water has to travel along the gutter to reach an outlet. Enter the number you are planning and the tool counts the parts for it.
How much water does a downspout carry?
Amerimax publish 186 gallons a minute for a 2 x 3 in and 434 for a 3 x 4 in. Read those as the capacity of the pipe rather than as a design flow for a roof — in practice the constraint is usually the gutter's own carrying capacity and getting the water to the outlet, not the downspout choking. If a system overflows at the gutter lip rather than backing up at the spout, more downspouts is still the fix, but the reason is different.
How many end caps and mitres?
Two end caps per run, less two for every corner — because a corner is where two runs meet and a mitre goes in place of both their ends. On the worked example, four runs meeting at four outside corners is a closed loop around the house: 4 mitres and 0 end caps. Get this wrong the other way and you are up a ladder with two caps and a hole where a mitre should be.
How many elbows per downspout?
Two is usual — one at the top to bring the spout back against the wall from the gutter outlet, and one at the bottom to throw water clear of the foundation. That is a convention rather than a published figure, which is why it is a field you can change. A deep soffit, a bay window or a run that has to dodge a meter box all take more, and elbows are cheap enough that having a spare beats a second trip.
What is not on this list?
Sealant, screws, the drip edge that gets roof runoff into the gutter in the first place, and whatever moves water away from the foundation once it leaves the bottom of the downspout — splash blocks, extensions or a buried line. That last one matters most: a gutter that discharges at the base of the wall has moved the problem four feet, not solved it.
