Gutter Calculator — Size, Downspouts and Hangers
Gutters are usually sized by habit rather than arithmetic, which is why so many overflow. Enter the roof they drain and this works out the size, the number of downspouts and the parts to buy.
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How to work it out
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Measure the roof, not the slope
Use the footprint — the area the roof covers seen from above. Rain falls vertically, so a steep roof catches no more water than a flat one of the same plan area. The pitch matters, but not the way you might think.
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Adjust for pitch and rainfall
A steep roof throws water into the gutter faster, so it has to cope with a higher instantaneous flow. That is what the pitch factor is for — 1.1 for a typical 6-in-12, up to 1.3 for anything 12-in-12 or steeper. Rainfall intensity scales it again for where you live.
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Pick the size
A 5 inch K-style gutter handles about 5,520 sq ft of effective drainage area; a 6 inch handles 7,960. Most houses are comfortably inside the 5 inch figure, which is why 5 inch is standard — but a large roof, a steep one, or a wet climate can push you over.
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Space the downspouts
The rule is one square inch of downspout cross-section for every 100 sq ft of roof. A 3 × 4 in. downspout is 12 sq in., so it drains 1,200 sq ft. Installers also cap the run each one serves at around 40 feet, because water has to get to the outlet before it can go down it.
Gutter and downspout capacity
Effective drainage area each size can handle at moderate rainfall.
| Part | Size | Cross-section | Handles |
|---|---|---|---|
| Gutter | 5 in. K-style | — | 5,520 sq ft |
| Gutter | 6 in. K-style | — | 7,960 sq ft |
| Downspout | 2 × 3 in. | 6 sq in. | 600 sq ft |
| Downspout | 3 in. round | 7.1 sq in. | 706 sq ft |
| Downspout | 3 × 4 in. | 12 sq in. | 1,200 sq ft |
| Downspout | 4 in. round | 12.6 sq in. | 1,255 sq ft |
One square inch of downspout per 100 sq ft of roof is the working rule.
Pitch multipliers
A steeper roof delivers the same water faster.
| Pitch | Multiplier | A 1,600 sq ft roof becomes |
|---|---|---|
| Flat to 3-in-12 | 1.00 | 1,600 sq ft |
| 4-in-12 to 5-in-12 | 1.05 | 1,680 sq ft |
| 6-in-12 to 8-in-12 | 1.10 | 1,760 sq ft |
| 9-in-12 to 11-in-12 | 1.20 | 1,920 sq ft |
| 12-in-12 or steeper | 1.30 | 2,080 sq ft |
This is not more rain. It is the same rain arriving at the gutter in less time.
Do I need 5 inch or 6 inch gutters?
Five inch handles about 5,520 square feet of effective drainage area, six inch about 7,960. Work out your effective area — footprint times pitch factor times rainfall factor — and see which side of 5,520 it lands on.
Most single-family houses sit well inside five inch. You get pushed to six by a large roof draining to one side, a steep pitch, heavy regional rainfall, or a valley dumping a big area into a short length of gutter. That last one catches people out: the size has to suit the worst run, not the average.
Measure the footprint, not the slope
Rain falls straight down. A 12-in-12 roof and a flat roof covering the same ground catch exactly the same volume of water, even though the steep one has far more surface. So the number that matters is the plan area — what the roof covers as seen from above.
Pitch still matters, but for a different reason: a steep roof sheds its water quickly rather than letting it soak and trickle, so it hits the gutter in a shorter, sharper burst. That is what the multiplier accounts for.
Why gutters overflow when they are the right size
Nearly always the downspouts, not the gutter. A gutter is a holding trough; it is the downspout that empties it. Too few, too small, or too far apart and the trough fills faster than it drains no matter how big it is.
The other usual culprit is fall. A gutter needs about a quarter inch of drop for every ten feet toward the outlet. Dead level looks better from the ground and holds standing water, which is how you get overflow at one end and sediment at the other.
Hanger spacing and the ice question
Thirty-six inches is about the widest anyone recommends, thirty-two is typical, and twenty-four is what you want anywhere that gets snow or ice. A gutter full of ice is extraordinarily heavy, and hangers pulling out is the usual result.
Hidden hangers screwed into the fascia are far better than the old spike-and-ferrule. Spikes work loose as the fascia moves through a few seasons, and once one goes the next one carries its load too.
Where the water goes after the downspout
Not next to the foundation, which is where most downspouts put it. A thousand square feet of roof sheds about 620 gallons in an inch of rain, and dumping that at the base of the wall is how basements get wet and how footings settle.
Get it at least four to six feet out, further on clay, and make sure the ground falls away from the house. An extension and a splash block cost almost nothing compared with what they prevent.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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effective area = roof footprint × pitch factor × rainfall factorPlan area, adjusted for how fast it arrives and how hard it rains locally. -
gutter size: 5 in. up to 5,520 sq ft, 6 in. up to 7,960 sq ftPublished K-style capacities at moderate rainfall intensity. -
downspouts = effective area ÷ (cross-section sq in. × 100)One square inch of downspout drains about 100 sq ft of roof. -
also: downspouts ≥ run ÷ 40 ftWater has to reach the outlet. Long runs need more of them regardless of area. -
hangers = (run in inches ÷ spacing) + 1Plus one for the far end.
Common questions
What size gutters do I need?
Five inch K-style for most houses. Move to six if your effective drainage area — roof footprint times pitch factor times rainfall factor — comes out above about 5,520 sq ft, or if one run is taking a disproportionate share of the roof.
How many downspouts do I need?
One per 1,200 sq ft of roof for a 3 × 4 in. downspout, or one per 600 for a 2 × 3. Whichever that gives you, also add enough that no single one is serving more than about 40 feet of gutter.
Should I measure the roof surface or the footprint?
The footprint. Rain falls vertically, so a steep roof catches no more water than a flat one covering the same ground. The extra surface area of a slope is irrelevant to how much water arrives.
How much fall should a gutter have?
About a quarter inch per ten feet toward the downspout. On a long run, split it and pitch from a high point in the middle toward outlets at both ends rather than running forty feet of steadily dropping gutter along one fascia.
Are gutter guards worth it?
Under trees, often. Elsewhere they mostly solve a problem you do not have, and the fine-mesh types can shed water straight over the front in heavy rain because the sheet of water never breaks through the mesh. They reduce cleaning; they do not eliminate it.
How many feet of gutter do I need?
Measure the eaves that will carry gutter — usually the two long sides on a simple gable roof, all four on a hip. Gutter comes in 10 ft sections for DIY, or seamless in one continuous run if you have it rolled on site.
Can I put 6 inch gutters on with 2 × 3 downspouts?
You can, and people do, and it defeats the point. A bigger trough draining through the same small outlet just holds more water before it overflows. If you are going to six inch gutter, go to 3 × 4 downspouts with it.
How much water comes off a roof?
About 620 gallons per 1,000 square feet per inch of rain. A 1,600 sq ft roof in a one inch storm sheds roughly a thousand gallons — which is worth picturing before you decide where the downspout empties.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- SMACNA Architectural Sheet Metal Manual — gutter and downspout sizing method
- This Old House gutter sizing guide — K-style capacities and pitch multipliers
- Manufacturer product data — section lengths, hanger spacing and downspout sizes
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 15, 2026.