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Yard Drainage Fixes

Downspout Calculator: How Many Do I Need, and What Size?

Plan one downspout for every 600 square feet of roof with 2×3-inch downspouts, or every 1,200 square feet with 3×4-inch, and never more than 40 feet of gutter between them. Heavy-rain regions like eastern North Carolina need about a third more. This calculator does that math for your roof, pitch, and storm — and, because this is a drainage site, it also tells you how much water lands at each outlet and where it should go. For how much water the whole roof sheds, see the roof runoff calculator.

What do you want to know?
Roof pitch
Design storm (rainfall intensity)A 5-minute burst, not the day's total. Eastern North Carolina sits in the heavy band.
Gutter size
Downspout size

Recommended

Plan on 6 downspouts

One 2×3 in rectangular downspout drains about 400 sq ft of roof at 6 in/hr, so 2,200 sq ft of pitch-adjusted roof needs 6. 120 ft of 5-inch gutter at no more than 40 ft per outlet needs 3. Switching to 3×4 downspouts would bring it to 3.

By capacity

6

2,200 sq ft (pitch-adjusted) ÷ 400 sq ft each

By gutter run

3

120 ft at ≤ 40 ft per outlet

Per outlet, 1-in storm

195 gal

lands at each of 6 downspouts — that is the drainage problem

The count is set by whichever rule needs more. Capacity says how much roof one outlet can drain in a burst; the gutter-run rule stops water travelling so far along the gutter that it overtops before it reaches an outlet. Where each one should discharge →

Where they go

Downspout placement and dischargePlan view of a rectangular roof with 6 downspouts spread along the two eaves, each with an extension carrying water several feet away from the house.ridgeguttereach outlet extended 4–6 ft from the wall, or piped to a drain
Spread outlets along both eaves, keep them off the corners that are hardest to drain, and give every one somewhere to go — an extension, a buried line, or a dry well.

How the calculator works

Two rules, and the count is whichever needs more:

  • Capacity. Each square inch of downspout cross-section drains about 100 square feet of roof at a 4 in/hr burst. A 2×3 is 6 square inches (600 sq ft), a 3×4 is 12 (1,200 sq ft), a 4-inch round is about 12.6. Capacity falls in proportion to rainfall intensity, and steeper roofs deliver water faster, so the roof area is multiplied by a pitch factor of 1.0 (flat) to 1.3 (12/12).
  • Gutter run. No more than 40 feet of 5-inch gutter, or 50 feet of 6-inch, should drain to one outlet, regardless of roof area. Long runs overtop at the far end before the water reaches the downspout.

The rainfall presets are 5-minute intensities, roughly a 10-year storm, from NOAA Atlas 14. Eastern North Carolina sits in the heavy band at about 6 to 7 in/hr, which is why the default is not the 4 in/hr most gutter guides quietly assume.

Downspout sizing chart

Square feet of roof one downspout can drain, by size and storm intensity:

DownspoutCross-section3 in/hr4 in/hr6 in/hr (NC)8 in/hr
2×3 in rectangular6 sq in800 sq ft600 sq ft400 sq ft300 sq ft
3×4 in rectangular12 sq in1,600 sq ft1,200 sq ft800 sq ft600 sq ft
4 in round12.57 sq in1,676 sq ft1,257 sq ft838 sq ft629 sq ft
Gutter run limit40 ft per outlet on 5-inch gutters · 50 ft on 6-inch

Where each downspout should discharge

Gutter guides stop at the count. For drainage, the count is where the problem starts: each outlet concentrates hundreds of gallons at one point in a storm. Three rules for where that water goes:

  • Away from the wall. Extend every outlet 4 to 6 feet onto ground that slopes away. A downspout ending at the foundation is the single most common cause of water pooling near the foundation and a wet crawl space.
  • Not all to one corner. Spread outlets along both eaves. Routing several downspouts to the same low corner creates the pond you were trying to avoid.
  • Somewhere that can take it. A splash block on flat clay is not a plan. A buried line to daylight, a catch basin, or a dry well sized for the outlet's gallons is. The per-outlet figure above feeds the dry well calculator directly.

What this calculator does not tell you

It assumes the gutters are clean, correctly sloped, and hung below the roof edge so water actually enters them. It does not size the gutter itself, account for valleys that dump onto one section of gutter, or handle roofs that drain onto lower roofs. And it says nothing about whether the ground can absorb what the outlets deliver — that is the yard question the rest of this site exists for.

Frequently asked questions

How many downspouts do I need?
Plan one 2×3-inch downspout for every 600 square feet of roof, or one 3×4-inch for every 1,200, at a moderate 4 in/hr storm — and never more than 40 feet of gutter draining to one outlet. Heavy-rain regions like eastern North Carolina need about a third more. A 2,000 sq ft roof therefore needs 4 to 5 standard downspouts or 3 oversized ones.
How many downspouts per foot of gutter?
One outlet for every 30 to 40 feet of gutter run, and never more than 40 feet on a 5-inch gutter (50 feet on a 6-inch). Beyond that, water travels so far along the gutter in a burst that it overtops at the far end before it reaches the downspout — which is why corners overflow even when the roof area math says you have enough.
What size downspout for a 5-inch gutter?
A 2×3-inch downspout is the standard match for a 5-inch K-style gutter and drains about 600 square feet of roof in a moderate storm. Upsizing to 3×4 on a 5-inch gutter is common in heavy-rain areas: it doubles the capacity of each outlet and clogs less, at almost no extra cost.
What size downspout for a 6-inch gutter?
A 3×4-inch downspout, which drains about 1,200 square feet of roof at 4 in/hr. A 6-inch gutter also allows up to 50 feet of run per outlet. Pairing a 6-inch gutter with 2×3 downspouts wastes the gutter's capacity — the outlet becomes the bottleneck.
Can a gutter have too few downspouts?
Yes, and it is the most common gutter problem. The signs are water sheeting over the front edge in heavy rain, overflow at the corners, and erosion or pooling under one spot. The calculator's 'check my current downspouts' mode shows how far over capacity the existing set runs.
Do I need more downspouts in a heavy rain area?
Yes. The 600-square-feet rule assumes a 4 in/hr burst. Eastern North Carolina sees 5-minute intensities around 6 to 7 in/hr in a 10-year storm, which cuts each downspout's capacity by a third or more. Either add outlets or move to 3×4 downspouts, which is usually the cheaper fix.
How far should a downspout drain from the house?
At least 4 to 6 feet, and onto ground that slopes away. Each outlet on a 2,000 sq ft roof delivers around 300 gallons in a one-inch storm; dumping that at the foundation is the leading cause of a wet yard and a wet crawl space. Extensions, buried lines to daylight, a catch basin, or a dry well all work — the calculator's per-outlet gallons figure sizes those.

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