What size gutters do I need?
Three numbers decide it: your effective roof area, your local rainfall intensity, and the gutter profile. Get those, and the size falls out.
Most homes take 5-inch K-style gutters. Some need 6-inch. A few need 7-inch or half-round. The size is not a guess — it is a drainage calculation, and it comes down to how much roof feeds the gutter versus how hard it rains where you live.
The one rule
Pick the smallest gutter size whose max drainage area ≥ your effective roof area. That is the whole method. Everything below is just how to get those two numbers.
Step 1 — effective roof area (not the ground footprint)
Gutters catch the plan-view footprint of the roof, but a steeper roof throws more wind-driven rain into the gutter, so SMACNA multiplies the footprint by a pitch factor = √(1+(rise/run)²). A flat 0/12 roof drains its exact footprint; a 4/12 roof multiplies by 1.054; a 6/12 by 1.118; a 12/12 by 1.414.
Worked example. A 2,000 ft² footprint at 4/12 pitch → 2,000 × 1.054 = 2,108 ft² of effective roof area. That is the number that sizes your gutter. Do it in the roof-pitch drainage-area calculator, and if the roof has several planes draining to one gutter run, add the effective areas of the planes that feed that run.
Step 2 — rainfall intensity
Gutter capacity scales inversely with rainfall intensity. The harder it can pour where you live, the less roof a given gutter drains. The design figure is the US 5-minute / 100-year intensity in inches per hour: roughly 4–6 in/hr across the northern states, 5–7 in the Midwest and Northeast, and 7–9 along the Gulf and Southeast. Check yours against the rainfall intensity by region table. When in doubt, size up.
Step 3 — read the size off the drainage table
The gutter-size drainage-area reference lists the max roof area each size drains at a 6 in/hr design rain, and scales it to any intensity with max area(intensity) = max area at 6 in/hr × 6 ÷ intensity. At 6 in/hr: a 5-inch K-style handles ~2,500 ft², a 6-inch K-style ~3,840 ft², a 5-inch half-round ~1,920 ft².
Back to the example. Effective area 2,108 ft² at 6 in/hr: a 5-inch K-style (2,500) clears it → 5-inch is fine. Now the same roof in an 8 in/hr downpour: the 5-inch drops to 2,500 × 6 ÷ 8 = 1,875 ft², which is below 2,108 → step up to 6-inch K-style (3,840 × 6 ÷ 8 = 2,880 ft², clears it). Feed those numbers into the gutter size calculator and it picks the size for you.
The edge cases most guides skip
- Long runs and one downspout. The table sizes the gutter for the roof it carries. A very long run draining to a single outlet can overflow even at the "right" size — split it with a second downspout or a mid-run high point instead of upsizing the whole gutter.
- Valleys. A roof valley dumps a concentrated stream into one spot. That local load is worse than the average, so add capacity (a 6-inch gutter or an extra downspout) under a big valley.
- Half-round. At the same nominal size it holds less than K-style, so it usually needs one size larger for the same roof — see K-style vs half-round.
- Debris. Pine needles and shingle grit shrink real capacity. If the table says you are right at the edge, size up rather than betting on a perfectly clean gutter.
What to measure first
Get the roof footprint (length × width of the area draining to each gutter run), the pitch (rise in inches over 12 of run — read it off a rafter or measure it), and your local rainfall intensity. With those three, the size is deterministic. Everything else — material, seamless vs sectional, guards — is a separate decision that does not change the size.
Bottom line: 5-inch handles most roofs up to ~2,500 ft² of effective area at a moderate rain; go 6-inch for bigger roofs, steep pitches, heavy-rain regions, big valleys or long single-outlet runs. This is a labeled planning value, not a certified design — for complex roofs, follow local code and the manufacturer’s data, and ask a pro.