Gutter Calculator
Work out exactly how much gutter you need — linear feet, downspouts, hangers, elbows, end caps and miters — plus a sized-for-your-rainfall drainage check, U.S. material pricing, a material comparison, and a downloadable PDF.
Uses 2024-2025 U.S. average prices and standard drainage rules of thumb. Downspout counts assume a 4 in/hr reference rainfall intensity. Confirm sizing against your local code. For planning purposes only.
| Material | Price / ft | Gutter Cost | Total Estimate | Typical Life |
|---|
For your 80 ft of gutter, with the same downspouts, hardware and labor. Copper and zinc cost the most up front but outlast everything else.
How to use the gutter calculator
Four quick steps turn your roof size, pitch, and rainfall into a gutter plan — the size, the downspout count, and the linear feet — with a size comparison and a downloadable PDF.
Enter the roof area
The footprint that drains to the gutters, or your house dimensions.
Set pitch & rainfall
Pick the roof slope and your local peak rainfall intensity in inches per hour.
Choose gutter & downspout
Select 5″ or 6″ gutters and a downspout size to size the system.
Read results & save a PDF
Get the recommended size, downspout count, linear feet, and a downloadable plan.
How the gutter estimate is calculated
Gutters are sized by drainage load. The calculator turns your roof footprint into an effective catchment area using a pitch factor, weighs it against rainfall intensity, and matches that to gutter and downspout capacity. Here is the exact math.
Worked example — 2,000 sq ft footprint, 6/12 pitch, heavy rain
Catchment: 2,000 × 1.10 (pitch) = 2,200 sq ft
At 5 in/hr: one 3×4 downspout per ~720 sq ft
Downspouts: 2,200 ÷ 720 ≈ 4 downspouts
Gutter: 6″ K-style for the load and pitch
≈ 6″ gutters with ~4 downspouts for a steep 2,000 sq ft roof in heavy rain.
Gutter sizing charts
Handy lookups for the questions people ask most — gutter capacity, roof pitch factors, downspout coverage, and spacing. Figures are typical guidance; local rainfall intensity and code drive the final size.
| Gutter | Relative capacity | Best for |
|---|---|---|
| 5″ K-style | Baseline | Most homes, moderate roofs |
| 6″ K-style | ~40% more | Large or steep roofs, heavy rain |
| 5″ half-round | ~Less than 5″ K | Traditional / historic homes |
| 6″ half-round | ~Similar to 5″ K | Larger traditional roofs |
K-style holds more water than half-round of the same width. A 6″ gutter also pairs with bigger 3×4″ downspouts, adding drainage capacity.
| Roof pitch | Pitch factor |
|---|---|
| Flat to 3/12 | ×1.00 |
| 4/12 to 5/12 | ×1.05 |
| 6/12 to 8/12 | ×1.10 |
| 9/12 to 11/12 | ×1.20 |
| 12/12 and steeper | ×1.30 |
Steeper roofs catch more wind-driven rain and shed it faster, so the effective area is larger. Multiply the footprint by the factor before sizing.
| Downspout | Roof area drained |
|---|---|
| 2×3 in rectangular | ~600 sq ft |
| 3 in round | ~700 sq ft |
| 3×4 in rectangular | ~1,200 sq ft |
| 4 in round | ~1,250 sq ft |
Approximate at moderate rainfall. Higher rainfall intensity reduces the area each downspout can handle, so add downspouts in wet regions.
| Gutter run | Downspouts |
|---|---|
| Up to 30 ft | 1–2 |
| 30–50 ft | 2–3 |
| 50–80 ft | 3–4 |
| 80–120 ft | 4–6 |
Rule of thumb: one downspout per 20 to 35 feet of run, toward the closer end in heavy-rain regions. Corners and long runs need extra.
footprint × pitch
5″ or 6″
count & spacing
Sizing gutters is a chain: work out the catchment, pick a gutter size to carry it, then add enough downspouts to drain it.
- Catchment — the roof footprint times a pitch factor for the slope.
- Gutter size — 5″ for most roofs, 6″ for large, steep, or heavy-rain roofs.
- Downspouts — enough to drain the area, spaced along each run.
Everything the calculator works out
One roof gives you a complete gutter plan — the size, the downspouts, the length, and the cost.
Key figures behind gutter sizing
Built for any gutter job
From replacing overflowing gutters to speccing a new roof, the same calculator sizes the system to the roof and the rain.
Their gutters overflow and they want the right size and enough downspouts to fix it for good.
- Measure the eaves length
- Size up if they overflow now
- Add downspouts on long runs
Speccing gutters on a re-roof and needs the size, downspout count, and linear feet to quote.
- Apply the local rainfall intensity
- Factor in the roof pitch
- Save the PDF for the estimate
Designing drainage for a new home and wants the gutter and downspout sizing to meet the rainfall load.
- Total catchment across roof planes
- Choose 5″ vs 6″ by load
- Plan downspout locations early
7 tips for sizing gutters right
Small habits keep your gutters from overflowing and your foundation dry, whether you DIY or hire out.
Gutter calculator FAQ
The sizing, downspout, pitch, and rainfall questions people ask most before buying gutters.
Most homes use 5-inch K-style gutters, but larger roofs, steep pitches, or heavy-rain regions often need 6-inch. Work out your effective catchment area, which is the roof footprint times a pitch factor, then match it to the gutter’s capacity and your local rainfall intensity.
As a rough guide, a 5-inch gutter handles a moderate roof, while a 6-inch gutter carries about 40% more water and suits large or steep roofs and intense rainfall.
Start with the roof’s footprint area that drains to the gutter, multiply by a pitch factor for the roof’s steepness, and factor in your area’s peak rainfall intensity in inches per hour. Compare that adjusted area to the capacity of a 5-inch versus 6-inch gutter.
Steeper roofs and higher rainfall push you toward the larger size. The calculator combines these so you don’t have to look up the tables by hand.
A common rule is one downspout for every 20 to 35 feet of gutter run, with more in heavy-rain areas. By area, a 3×4 inch downspout drains roughly 1,200 square feet of roof at moderate rainfall, and a smaller 2×3 inch handles about 600.
Divide your effective catchment area by the per-downspout coverage, and check the spacing along each gutter run. Long runs and intense rain need downspouts closer together.
A 6-inch K-style gutter holds noticeably more water than a 5-inch, roughly 40% more capacity, and pairs with larger 3×4 inch downspouts. That makes it better for big roofs, steep pitches, and regions with heavy downpours.
Five-inch gutters are the standard on most homes and cost less, while 6-inch reduce overflow risk and clog less on demanding roofs. Match the size to your roof area, pitch, and rainfall rather than defaulting to the smaller one.
A steeper roof effectively catches more wind-driven rain and sheds it faster, so gutter sizing multiplies the footprint area by a pitch factor greater than one. A low-slope roof uses a factor near 1.0, while a steep roof can use 1.2 or more.
This raises the effective area the gutters must handle, which can push you to a larger gutter or more downspouts. The calculator applies the pitch factor automatically once you enter the slope.
Gutter sizing uses peak rainfall intensity in inches per hour for a short, intense storm, not the annual average. This figure varies widely by region and is published in local rainfall intensity data, often for a 5-minute or 10-minute storm.
Higher intensity means more water hitting the roof at once, so gutters and downspouts must be larger or more numerous. Use your local design intensity for an accurate result rather than a national average.
Measure the total length of all the eaves, the lower roof edges where water runs off, since that’s where gutters go. On a simple gable roof, that’s usually the two long sides; on a hip roof, it’s all four edges.
Add up those lengths in feet to get your linear footage, then add a little for waste and corners. The calculator can turn your house dimensions and roof type into a gutter length estimate.
Undersized gutters or too few downspouts overflow in heavy rain, sending water down the walls and foundation, which can cause erosion, basement leaks, and fascia rot. Overflow also defeats the purpose of having gutters at all.
Sizing to your roof area, pitch, and local rainfall avoids this. If your existing gutters overflow, the fix is often a larger size, more downspouts, or both, rather than just cleaning them more often.
General Estimating Notice: This calculator estimates gutter size, downspout count, and gutter length from your roof area, pitch, rainfall intensity, and the sizes you select, using typical capacity and pitch-factor figures. Actual requirements depend on your specific roof geometry, local rainfall intensity data, and building code, and manufacturer capacities vary. Confirm sizing against local code and product specifications, and have gutter and roof work done safely by a qualified professional. For planning purposes only.