Gutter calculator

Gutter linear feet along the eave, the peak flow your gutters have to carry, and the number of downspouts that keeps each run inside its rated capacity — sized to your roof area and your local rainfall, the way the plumbing code does it.

01 How the math works

Two numbers come out of a gutter takeoff, and they answer different questions. The first is linear feet: how much gutter to buy. That is just the length of the eaves the gutter runs along, rounded up to the section length your supplier stocks. Gutters hang level, so there is no pitch factor here — the run equals the horizontal eave.

The second is how many downspouts, and that is a flow problem. Rain lands on the roof at a rate, the gutters collect it, and each downspout drains one sloped length of gutter. If that length collects more water than the gutter can carry, it overtops — so the number of downspouts is set by how much flow one gutter can move. The plumbing code puts the peak flow at:

The 0.0104 converts inches of rain over a square foot per hour into gallons per minute. Gutter capacity comes from the code table: a 5 in gutter at the usual 1/8 in/ft slope carries 74 gpm, a 6 in gutter 110 gpm. The metric form is tidier, because 1 mm of rain over 1 m² is exactly 1 litre: Q (L/min) = R (mm/hr) × A (m²) ÷ 60.

The input most calculators skip is R. A gutter in Mobile is carrying more than twice the flow of the same gutter in San Diego, because the rain falls more than twice as hard. The downspout count has to follow the rainfall, not a fixed spacing rule.

02 Worked example

Take a house with a 2,400 ft² roof footprint and 80 ft of eave carrying gutter (223 m² and 24.4 m). Put it on the central Gulf Coast, where the 100-year, 1-hour rainfall rate runs about 4.5 in/hr (114 mm/hr).

Gutter run is the eave length: 80 ft, which is 8 ten-foot sections. Peak flow is 4.5 × 2,400 × 0.0104 = 112 gpm (425 L/min). A 5 in gutter carries 74 gpm, so 112 ÷ 74 = 1.5, rounded up to 2 downspouts — one gutter run topping out near 1,580 ft² of roof at this rainfall.

Move the same house to Los Angeles, where the rate is about 2.0 in/hr, and the flow drops to 50 gpm — one downspout covers it. Same roof, same gutter, half the downspouts. That difference is the whole point of putting the rainfall rate in the math.

03 When this calculator is wrong

The capacity figures come from the code table for semicircular (half-round) gutters. The K-style gutter on most houses has a different cross-section, so treat the gutter capacity here as the code baseline rather than an exact figure for your profile. If your takeoff lands right on the edge between one downspout and two, it is on the edge — add the second one.

• The downspout can be the bottleneck, not the gutter. A 2×3 in downspout drains a much smaller run than a 3×4 in. Stepping a gutter up to 6 in without also stepping the downspout up to 3×4 in just moves the restriction to the outlet. Pair a 6 in gutter with a 3×4 in downspout.
• Pitch does not enlarge the drainage area. Flow follows the roof's horizontal footprint, not its sloped surface, so a steep roof sheds the same volume as a flat one over the same footprint. This is the reverse of a shingle or underlayment takeoff, where the pitch factor (6/12 = 1.1180, 8/12 = 1.2019) does enlarge the material area. Use the footprint area here, not the roof-surface area.
• Spacing is a second limit, separate from flow. Even where the flow math allows one downspout, a long level-looking run drains and self-cleans better split between two. Common practice caps a run at roughly 35–40 ft (10–12 m) per downspout; take whichever number is larger, the flow count or the spacing count.
• The rate is the 100-year, 1-hour value, and it moves. The example rates here are from a 1961 rainfall atlas, used only to show the spread. Look up the current figure for your exact location on NOAA Atlas 14 (the NWS Precipitation Frequency Data Server) — it has replaced the old atlas and generally reports harder rain.
• Valleys concentrate flow. Where two roof planes drain into one gutter, that section carries both — size the downspout at that outlet for the combined area, not the average.

04 What to do with the result

Order the gutter by the linear-foot number, rounded up to whole sections, and place downspouts to hit the count above — spread evenly, one at each low end of a run, and an extra wherever a valley dumps in. When the flow math and the spacing rule disagree, follow the larger count.

When the number comes out borderline, round up. The cost of one more downspout and a few feet of pipe is small; the cost of a gutter that overtops onto the fascia and the foundation in the storm it was supposed to handle is not. That holds across this site — size for the order you need, not the one the raw math barely allows. The exception is the genuinely dry end of the range (an arid-climate roof well under its single-downspout capacity), where a second downspout buys nothing.

05 Common questions

How many downspouts do I need?

Divide the peak flow (R × A × 0.0104 gpm) by the gutter's capacity and round up. For a 2,400 ft² roof at 4.5 in/hr on a 5 in gutter, that's 2. Then check spacing — no run much over 35–40 ft on one downspout — and use whichever count is larger.

What size gutter do I need, 5 inch or 6 inch?

A 5 in K-style gutter handles most houses. Step up to 6 in when the roof is large, the rainfall rate is high, or a lot of area drains to one run — the 6 in carries 110 gpm against the 5 in's 74 gpm, so it lets one downspout cover about 50% more roof. Pair the 6 in with a 3×4 in downspout.

Does roof pitch change how much gutter or how many downspouts I need?

No. Drainage follows the horizontally projected footprint, so pitch doesn't change the flow. It's the opposite of shingles, where a steeper roof needs more material. Use the footprint area for gutters.

How many feet of gutter per downspout?

There's no single number — it depends on rainfall. The flow a gutter carries, not its length, sets the limit, so a run in a rainy region reaches capacity in fewer feet. As a spacing backstop, keep a run under about 35–40 ft (10–12 m) per downspout, then let the flow math raise the count where the rain is heavy.

Where do I find my rainfall rate?

Use the 100-year, 60-minute value for your location from NOAA Atlas 14 (the NWS Precipitation Frequency Data Server). It ranges from roughly 1 in/hr on the West Coast to about 4.5 in/hr on the Gulf Coast, which is why the downspout count can't be a fixed rule.