Gutter Size Calculator
Size rain gutters and downspouts from the roof area, pitch and your design rainfall intensity: K-style or half-round size, number of downspouts, capacity check per downspout and a materials list.
How it works
watershed = roof plan area × pitch factor · downspouts per run = max(watershed ÷ downspout capacity, run ÷ max spacing, outlets needed by the biggest gutter) · gutter fits when area per downspout × rainfall (in/hr) ≤ its capacity at 1 in/hr (5 in K-style 5,520, 6 in K-style 7,960, 5 in half-round 2,500, 6 in half-round 3,840 sq ft)
A gutter has to carry the rain that falls on the roof planes above it during the heaviest short burst of a storm. The roof’s plan area is raised by a pitch factor (1.0 for low slopes up to 1.3 for 12/12 and steeper) because steep roofs catch wind-driven rain; that gives the watershed. Each gutter is rated by the watershed it can drain to one downspout at 1 inch of rain per hour, so the capacity at your design rainfall is that figure divided by the intensity — at 5 in/hr a 5 in K-style gutter handles about 1,100 sq ft per downspout, not the 5,520 sq ft quoted at 1 in/hr. Each downspout is limited to roughly 100 sq ft of watershed per square inch of its cross-section (about 600 sq ft for a 2 × 3 in), and runs are split so no section is longer than about 40 ft. Adding a downspout halves the area each gutter section carries, so the calculator adds outlets until the chosen style fits and then picks the smallest size that works. For a hip roof the area is split between the long and short sides by the hip geometry; a gable roof drains half to each eave.
Worked example
A 40 × 30 ft house with a 6/12 gable roof has a watershed of 40 × 30 × 1.1 = 1,320 sq ft, or 660 sq ft to each 40 ft eave. With 2 × 3 in downspouts (600 sq ft each), each side needs 2 downspouts, so each section drains 330 sq ft. At a design rainfall of 5 in/hr a 5 in K-style gutter handles 5,520 ÷ 5 = 1,104 sq ft per downspout, so 5 in K-style gutters with 4 downspouts in all are enough. You buy 80 ft of gutter, 40 ft of downspout pipe for a 10 ft eave, 12 elbows and 4 end caps.
Sources: Fine Homebuilding — Sizing Gutter Systems (1999) · This Old House — How to size gutters and downspouts · NOAA Atlas 14 Precipitation Frequency Data Server
Frequently asked questions
When do I need 6 inch gutters instead of 5?
When the area each downspout section carries, multiplied by your rainfall intensity, exceeds about 5,520 sq ft·in/hr — for example more than about 920 sq ft per downspout at 6 in/hr. You can often stay with 5 in gutters by adding a downspout, which halves the load on each section; the calculator shows both the size and the downspout count.
What rainfall intensity should I use?
Use the heaviest 5-minute rainfall expected about once every 10 years, in inches (or mm) per hour, from NOAA’s Atlas 14 precipitation frequency server or your local building department. It is far higher than daily rainfall: roughly 4–8 in/hr in most of the US and more on the Gulf Coast and in the tropics.
How many downspouts do I need?
At least one per gutter run, one for every 30–40 ft of gutter, and enough that each covers no more watershed than it can drain — about 600 sq ft for a 2 × 3 in downspout and 1,200 sq ft for a 3 × 4 in. Larger downspouts also clog less often.
Are half-round gutters smaller?
Yes, for the same width. A 5 in half-round gutter drains about 2,500 sq ft at 1 in/hr and a 6 in about 3,840, versus 5,520 and 7,960 for K-style, because the rounded profile holds less water. Choose half-round and the calculator sizes and counts downspouts for it.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.