Free Gutter Sizing Tool
Gutter Size Calculator
Enter your roof area, pitch, and eave length to see whether you need 5 or 6 inch gutters, how many downspouts, and every part on the list, sized for Denver rainfall. Every assumption is shown. No signup, no email.
The roof surface from a roof report or insurance estimate. Don't know it? Measure your roof by address.
Add up every roof edge that gets a gutter. Pacing it off along the foundation and adding about 2 ft per corner for the overhang is close enough.
Roof pitch
We turn your roof area back into its footprint, then apply the ×factor, SMACNA's allowance for wind-driven rain on steeper roofs.
Stories to the eave
About 10 ft of downspout per story.
Optional: corners
Outside corners wrap around the house. Inside corners tuck into an L or a bump-out.
Gutter size
We pick the size for you. Tap the other one to compare downspouts and materials.
Recommended gutter
Enter your roof area and eaves above
Medium slope · 2 stories
- Gutter runadd length
- Downspouts
- Water to outlets
Enter your roof area and eave length to see your gutter size, downspouts, and a full materials list.
What Size Gutters Do I Need?
Gutter sizing follows the method in the SMACNA Architectural Sheet Metal Manual, the reference most sheet metal and gutter pros work from. It boils down to how much water reaches each downspout in a hard, short burst of rain:
- Find the footprint. Rain falls on the roof's footprint, so a sloped roof area is divided back down by its slope.
- Adjust for pitch. Wind drives rain into steeper roofs, so SMACNA multiplies the footprint by 1.00 up to 3/12, 1.05 at 4/12 to 5/12, 1.10 at 6/12 to 8/12, 1.20 at 9/12 to 11/12, and 1.30 at 12/12. The calculator's low-slope option uses 1.05 to stay on the safe side.
- Multiply by local rainfall. Denver's design storm is 5.7 inches per hour, the 5-minute burst expected about once every 10 years. The 100-year burst is 9.1 in/hr.
- Check each outlet's share. Water flows along the gutter to the nearest downspout, so each outlet's share of the roof has to fit the gutter's rating.
- Count downspouts. At least one every 35 ft of gutter, and enough that none carries more than it can drain.
| Gutter | Rated (adjusted sq ft) | Roof per downspout |
|---|---|---|
| 5" K-style | 5,520 | 968 sq ft |
| 6" K-style | 7,960 | 1,396 sq ft |
| 6" half-round | 3,840 | 674 sq ft |
Worked Example
A two-story Denver home with 2,400 sq ft of roof at a 6/12 to 8/12 pitch, 140 ft of eaves, 4 outside corners, and 2 inside corners:
- 2,400 sq ft ÷ 1.12 slope = 2,143 sq ft footprint, × 1.10 pitch factor = 2,357 sq ft design area
- 140 ft of gutter ÷ 35 ft = 4 downspouts, each draining about 589 sq ft
- 589 sq ft × 5.7 in/hr = 3,359 adjusted sq ft. That's under the 5,520 a 5" K-style is rated for, so 5" K-style gutters with 2x3 downspouts, with room to spare for a 100-year storm
- 140 LF of gutter, 80 LF of downspout (4 × 20 ft), 4 outlets, and 12 elbows
- 4 end caps, 6 corner miters, and 140 ft × 12 ÷ 16 in. = 105 hidden hangers
Same house, but 3,000 sq ft of roof draining onto only 100 ft of eaves: each of 3 outlets would drain 982 sq ft, past what a 5" gutter can carry, so it calls for 6" K-style with 3x4 downspouts.
How Many Downspouts Do I Need?
Downspouts are usually what limits a gutter system, not the gutter itself. Two rules set the count, and the higher one wins:
- Spacing: one downspout for every 30 to 40 ft of gutter. We use 35 ft. Runs longer than about 40 ft slope from the middle to a downspout at each end.
- Capacity: SMACNA rates a downspout at 1,200 sq ft of roof per square inch of opening in 1 in/hr of rain. At Denver's 5.7 in/hr that is about 1,263 sq ft for a 2x3 (6 sq in) and 2,526 sq ft for a 3x4 (12 sq in).
Each downspout needs a drop outlet and about three elbows: two at the top to bring it back to the wall and one at the bottom to throw water away from the foundation. Add an extension or splash block so it lands 4 to 6 ft out.
Gutters For Colorado Hail, Snow, And Pine Needles
A calculator gets the size right. A gutter system that lasts on the Front Range also has to stand up to weather the rainfall tables don't cover:
- Hail. Hail dents gutters and downspouts along with the shingles, which is why they are so often replaced together after a storm. Heavier .032 in. aluminum shrugs off dents that mark thinner .027 in. stock. If a storm just came through, start with our hail damage restoration and storm damage guides.
- Snow and ice dams. Ice dams start with heat leaking into the attic, not with the gutters, but a gutter full of ice is heavy. Hangers every 16 in. instead of 24 in. keep it from pulling away from the fascia. The real cure is good attic ventilation.
- Seamless aluminum. Seamless gutters are formed from a coil on site to the exact length of each run, so the only joints are at corners and end caps, where sectional gutters leak first.
- Pine needles and gutter guards. Needles slip through wide screens and pack into elbows. Fine micro-mesh guards and 3x4 downspouts keep a system flowing under pines and spruces, though no guard is maintenance free.
- Foundations. Much of the Front Range sits on clay soil that swells when it gets wet, so every downspout should discharge well away from the house.
New gutters go up best with a new roof, when the drip edge and gutters can be set as one system. See what goes into a full roof replacement, browse our roofing services, or size the rest of the job with the shingle calculator.
Frequently asked
Questions
Every home and homeowner is different, which is why clear communication is at the heart of what we do. To help you better understand our process, products, and what to expect when working with us, we've gathered a list of frequently asked questions and straightforward answers here.
SCHEDULE AN ESTIMATE →Replacing Your Roof After Hail?
Have the gutters checked at the same time. A free, in-person estimate takes about 30 minutes: we measure every eave and downspout drop, check the gutters and fascia for hail damage, and put the price in writing.