Gutter size calculator (5" vs 6")

Find the smallest gutter that drains your roof — from footprint, pitch and rainfall.

Typical published planning values — NOT a certified design. Actual gutter sizing depends on local rainfall intensity, roof geometry, valleys and debris; follow local code and the manufacturer’s data, and consult a pro for complex roofs. Structural roof-load, ice-dam / heat-cable and foundation/yard drainage are set by code and a professional — not engineered here.

Calculator

ft²
in/hr
Local 5-min / 100-yr design storm
Result
Recommended gutter size5" K-style
Effective roof area2,108 ft² (2,000 × 1.054)
Its max drainage area at 6 in/hr2,500 ft²
Rainfall intensity used6 in/hr

A 2,000 ft² footprint at a 4/12 pitch is about 2,108 ft² of effective roof area; in a 6 in/hr rain that drains within 5" K-style. The drainage-area-per-size figures are LABELED published planning values that scale inversely with rainfall intensity — confirm your local rainfall and follow local code and the manufacturer’s data for complex roofs, valleys and long runs.

Gutter sizing is a flow problem. The roof feeds water into the gutter; the gutter has to carry it away faster than the storm delivers it. Two things drive the load: how much roof effective area drains into that run, and how hard it rains.

This is the site’s signature engine. It takes your footprint, multiplies by the pitch factor to get the effective drainage area, then picks the smallest size whose capacity clears that area at your rainfall intensity — using the labeled drainage table below.

Formula

effective_area = footprint × pitch_factor
pitch_factor = √(1 + (rise/run)²)
max_drainage_area(size) = base_at_6in/hr × 6 ÷ rainfall_in/hr
size = smallest size where max_drainage_area ≥ effective_area

Steeper roofs catch more wind-driven rain, so the pitch factor bumps the plan-view footprint up. Harder rain shrinks every size’s capacity — the same gutter that copes in Seattle can overflow on the Gulf Coast.

Worked example

A. 2,000 ft² footprint at 4/12 pitch → 2,000 × 1.054 = 2,108 ft² effective. At 6 in/hr, a 5" K-style clears 2,500 ft² ≥ 2,108 → 5" K-style is enough.

B. Same 2,108 ft², but a hard 8 in/hr storm. Now 5" K-style clears only 2,500 × 6 ÷ 8 = 1,875 ft² < 2,108. Step up: 6" K-style clears 2,880 ft² → 6" K-style.

C. 3,000 ft² effective at 6 in/hr. 5" (2,500) is short → 6" K-style (3,840) → 6" K-style.

The lesson: 5-inch handles most moderate-climate homes; big roofs, steep pitches and heavy-rain regions push you to 6-inch.

When to size up

  • Half-round holds less. A 5" half-round carries about what a 5" K-style does minus a chunk — it usually needs one size larger for the same roof. Switch the profile above to see it.
  • Long runs and valleys. A valley dumps two roof planes into one spot. Size up, add a downspout, or split the run — the table assumes even loading.
  • Debris. Pine needles and shingle grit shrink real-world capacity. In heavy tree cover, size up or plan on guards.
  • Confirm local rainfall. Use your area’s 5-minute / 100-year intensity — see rainfall by region.

Then set the outlets with the downspout count calculator and check the profile trade-off in K-style vs half-round.

Reference table

Gutter size2 in/hr4 in/hr6 in/hr8 in/hr
5" K-style7,5003,7502,5001,875
6" K-style11,5205,7603,8402,880
7" K-style16,5608,2805,5204,140
5" half-round5,7602,8801,9201,440
6" half-round7,5003,7502,5001,875

Max roof area (ft²) a size can drain, by rainfall intensity. Labeled SMACNA-style planning values that scale inversely with intensity (base at 6 in/hr × 6 ÷ intensity). See the full dataset.

Frequently asked questions

What size gutters do I need for a 2,000 sq ft house?
At a typical 4/12 pitch and 6 in/hr rain, a 2,000 ft² footprint (~2,108 ft² effective) drains fine with 5-inch K-style. In an 8 in/hr downpour, step up to 6-inch.
Are 6-inch gutters worth it?
A 6-inch K-style carries roughly 50% more roof area than a 5-inch and clogs less. It is worth it for large roofs, steep pitches, heavy rain or valley-fed runs. For a modest roof in a mild climate, 5-inch is plenty.
Does roof pitch really change the gutter size?
Yes. A steeper roof catches more wind-driven rain, so the effective drainage area is larger than the flat footprint. The pitch factor √(1+(rise/run)²) captures that — a 12/12 roof adds ~41%.
Where do I find my rainfall intensity?
Use the local 5-minute, 100-year rainfall intensity in inches per hour. Our rainfall-by-region table gives planning bands, but confirm with your county or code office for the exact value.
Is this a certified drainage design?
No. These are labeled published planning values. Actual sizing depends on local rainfall, roof geometry, valleys and debris — follow local code and the manufacturer’s data, and consult a pro for complex roofs.

Related calculators

Roof-pitch drainage-area calculatorConvert a roof footprint into the effective drainage area for gutter sizing. Enter footprint and pitch to apply the SMACNA pitch multiplier.Downspout count calculatorHow many downspouts do you need? Runs both methods — roof area per outlet and the 1-per-30-to-40-ft rule — and recommends the larger, safer count.5" vs 6" gutter capacityCompare 5-inch and 6-inch K-style gutter capacity at your rainfall intensity and see the extra cost of stepping up to 6-inch over your linear feet.