Gutter outlets are the interface from the gutter to your downspout. The size of the outlet and its shape is critical to how quickly your downspout will plug.
Various gutter outlets laying in the bottom of a 5K gutter
Outlet Size and Shape
Outlets are based on the size of the downspout and must be smaller than the bottom of your gutter. A 2 inch outlet is used for a 2" x 3" rectangular downspout. 2 inch outlets can be round or rectangular. A rectangular outlet uses closer to the full 2" x 3" opening size of the downspout, but is more work to cut out. A round 2 inch outlet is easy to cut out using a hole saw but really makes for a small hole that plugs easily. Some gutter installers use punches and sheet metal snips to make an outlet. I often drill a small hole and then use sheet metal snips.
3x4 outlets will attach to a 5K gutter. 3x4 rectangular outlets have 5.7 sq inches of opening and are 50% larger than a 2x3 outlet. 3x4 rectangular outlets can also be modified to yield a 2 1/8 by 3.5 inch opening with total of 7.4 sq inches. This modified size is a 30% gain.
2x3 outlets will attach to a 4 K gutter.
Where the gutter outlet attaches to the gutter, it creates a lip that the water and debris must flow over. While water will flow over it easily, the lip often will hang up the needles and leaves. This causes them to back up and as water pressure builds, form a glob that suddenly gives way to clog the downspout opening (outlet). The most logical solution to this is to have an outlet that connects to the bottom of the gutter and provides a positive sloper towards the downspout without a lip.
Here's a quick cutaway sketch of the bottom of the gutter and outlet, showing the outlet lips on the gutter.
If you are looking to purchase a gutter outlet in Portland, check out Gutterman's Supply at 1620 Northeast Argyle Drive
Portland, OR 97211. You can reach them on the phone at (503) 285-2500.
I prefer the 5K gutter with the modified 3x4 rectangular outlet to minimize clogging. I also help keep them from plugging by using an over-sized downspout strainer necked down to the 3x4 outlet size. For two story gutters under pine trees, then I recommend corrugated 3x4 downspouts (with leaftraps installed near the ground) with over-sized outlets that have 3 1/4 X 8 inch openings.
Most homes have 5K or Fascia gutters installed with round outlets. This causes the gutters to plug frequently which is why I see a lot of these gutter systems. It only takes one or two leaves to plug a round gutter outlet that doesn't have a strainer installed.
See also Downspouts, Gutter Types