In short: for a roof section of about 1,200 sq ft draining to two downspouts at a typical 4 in/hr design rainfall and standard 1/16 in per foot slope, you need a 6-inch gutter. Bigger roofs, heavier local rainfall, fewer downspouts, or a flatter slope all push you toward a larger size — use the calculator above for your own numbers.
This is a planning aid based on standard hydraulic formulas. For final design, check your local plumbing code and manufacturer capacity tables, or consult a licensed engineer.
How to calculate gutter size
Gutter sizing is a two-step hydraulics problem. First, find how much water the roof sheds during a design storm — the peak runoff. Second, find the smallest standard gutter that can carry that much water at the slope you're installing it on.
That flow then has to fit inside a channel. Gutters are open channels, so their carrying capacity follows Manning's equation, which relates flow to the channel's cross-section, slope, and roughness. This calculator models a gutter as a smooth metal half-round channel and — because gutters shouldn't run brim-full — derates the full-channel capacity to about 70% as a safety margin for splash-over and debris.
Worked example. Take a 1,200 sq ft roof section, a 4 in/hr design rainfall, C = 1.0, two downspouts, and a 1/16 in per foot slope. Peak runoff is Q = 1.0 × 4 × 1,200 ÷ 43,200 = 0.111 cfs, or about 49.9 GPM. A 5-inch gutter at that slope carries roughly 46.2 GPM — not quite enough — while a 6-inch gutter carries about 75.2 GPM, a comfortable 51% margin. So 6 inches is the recommendation.
Downspouts are a separate sizing step. Once you know the flow reaching each downspout — shown as "flow per downspout" above — size the downspout itself with the Downspout Size Calculator. A common rule of thumb is about 1 sq in of downspout area per 100 sq ft of roof at 1 in/hr, scaled up for your actual rainfall rate.
Why roof area, rainfall and slope change the answer
Roof area sets how much water arrives — it's a straight multiplication in the Rational Method, so doubling the drainage area doubles the required capacity. Rainfall intensity works the same way: a roof in a Gulf Coast climate with an 8 in/hr design storm needs roughly twice the gutter capacity of the same roof under a 4 in/hr design storm elsewhere.
Slope behaves differently. Because Manning's equation includes the square root of the slope, capacity doesn't scale linearly with it — doubling the slope only increases capacity by about 41% (√2). That's still worth having: a gutter hung at 1/8 in per foot instead of the code-minimum 1/16 in per foot buys meaningful extra margin without changing the gutter size at all.
Number of downspouts doesn't change how much water the gutter itself must carry along its run, but it does change how much reaches each downspout — more downspouts, spaced along the run, mean smaller, cheaper downspouts and shorter runs to any one outlet.
Gutter capacity by size
Usable capacity (about 70% of full-channel Manning flow, smooth metal) for standard half-round / K-style gutter sizes, recalculated live for the slope you've selected above.
| Nominal size | Capacity (GPM) | Capacity (cfs) | Fits your roof? |
|---|
Worked examples
Small addition, one downspout
4-inch gutter
600 sq ft at 4 in/hr with 1 downspout and 1/16 in/ft slope needs about 24.9 GPM. A 4-inch gutter carries about 25.5 GPM — it fits, but with very little margin, so a 5-inch gutter is worth the small upcharge.
Mid-size home, steeper hang
6-inch gutter
2,400 sq ft at 4 in/hr with 3 downspouts and a steeper 1/8 in/ft slope needs about 99.7 GPM. A 6-inch gutter at that slope carries about 106.3 GPM, roughly a 7% margin.
Gulf Coast design storm
8-inch gutter
1,800 sq ft at a 7 in/hr design rainfall with 2 downspouts and 1/16 in/ft slope needs about 130.9 GPM. That exceeds a 7-inch gutter's ~113.4 GPM capacity, so an 8-inch gutter (~162 GPM) is the minimum that fits.
What else affects gutter sizing
Roof pitch and wind-driven rain. Very steep roofs shed water faster and can throw rain past a narrow gutter in wind, so many designers add a margin on steep-slope roofs beyond what the plan-area calculation alone suggests.
Valleys and roof intersections. A roof valley concentrates the runoff from two roof planes into one gutter run, effectively multiplying the contributing area for that section — size valley-fed gutter runs using the combined area, not just the length of gutter beneath them.
Leaves and debris. Clogged gutters lose capacity fast. The 70% usable-capacity assumption in this calculator already builds in some margin, but gutters in heavy tree cover benefit from guards and more frequent cleaning rather than oversizing alone.
Outlet and downspout restrictions. A well-sized gutter still backs up if its downspouts or the drainage it empties into are undersized — check the Downspout Size Calculator and, if water discharges into a pipe or drain, the Storm Drain Size Calculator.
Frequently asked questions
What size gutter do I need for my roof?
It depends on your roof's plan area, the local design rainfall intensity, and how steeply the gutter is hung. A typical 1,200 sq ft roof section draining to two downspouts at a 4 in/hr design rainfall and a 1/16 in per foot slope needs about a 6-inch gutter. Larger roofs, heavier rainfall, or fewer downspouts push you to a larger size.
Should I install 5-inch or 6-inch gutters?
5-inch gutters suit smaller roof sections — roughly up to 900–950 sq ft per downspout at a typical 4 in/hr design rainfall and standard slope. Beyond that, or in heavier rainfall areas, 6-inch gutters carry meaningfully more flow and give a larger margin against leaf and debris buildup.
How many downspouts does my roof need?
Enough that the gutter run feeding each one stays within its capacity, and that no single run gets too long to drain quickly. As a starting point, most homes use one downspout per 30–40 linear feet of gutter, then check the actual flow with a calculator like this one.
Does gutter slope really matter?
Yes. Gutter capacity scales with the square root of the slope, so doubling the slope increases capacity by about 41%. Most codes call for a minimum of 1/16 inch of drop per foot of gutter; installing at 1/8 inch per foot gives noticeably more margin for the same gutter size.
What rainfall rate should I use to size gutters?
Use your local short-duration design storm intensity, typically a 5-minute, high-return-period rate from NOAA Atlas 14 for your area. In the absence of local data, 4 in/hr is a common moderate default in much of the continental US, but Gulf Coast and mountain regions can see 6–8 in/hr or more.
Sources & references
- Kuichling, E. (1889). The relation between the rainfall and the discharge of sewers in populous districts. Transactions of the ASCE, 20, 1–56. (Origin of the Rational Method, Q = C·i·A.)
- SMACNA. Architectural Sheet Metal Manual — industry gutter and downspout sizing practice.
- Manning, R. (1891). On the flow of water in open channels and pipes. Transactions of the Institution of Civil Engineers of Ireland.
- NOAA National Weather Service. Atlas 14 Point Precipitation Frequency Estimates.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)