Enter roof area and pitch. Get recommended gutter size, downspout count and spacing, and installed cost — no address or lead form.
Optional: overrides
Was this gutter calculator helpful?
How this calculator works
Gutter sizing follows the SMACNA Architectural Sheet Metal Manual: a gutter has to handle not just roof area but effective roof area — footprint adjusted for pitch (steeper roofs shed water faster) and for local rainfall intensity (a 6 in/hr storm dumps twice the water of a 3 in/hr storm).1 Downspouts are sized separately, because a correctly sized gutter can still overflow if the water cannot exit fast enough.2
Effective area = Roof area × Pitch factor × Rain factor Gutter size = smallest size whose capacity ≥ Effective area Downspouts = ⌈Effective area ÷ Downspout capacity⌉ (min 2)
Most homes do not need the calculation — a 5″ K-style gutter with 2×3 downspouts handles standard residential roofs in standard rainfall. The math starts to matter at roughly 1,500 sq ft of effective area, which is where a 5″ gutter stops keeping up and a 6″ gutter becomes the better call.3
Roof area, adjusted for pitch and rainfall, gives the effective area the gutter must handle. The calculator applies both factors automatically.
Gutter size and downspout capacity — the reference
These are the standard SMACNA capacities used in the calculator. All figures assume standard 4 in/hr rainfall and are adjusted in the calculator for the rainfall factor you select.
Gutter style
Max effective area
Typical price / LF installed
Best for
5" K-style aluminum
~1,500 sq ft
$7–$10
Most residential roofs under 1,500 effective sq ft
6" K-style aluminum
~2,500 sq ft
$10–$14
Large or steep roofs; new construction standard
5" half-round aluminum
~1,200 sq ft
$12–$18
Historic homes, exposed rafter tails
6" half-round aluminum
~2,200 sq ft
$15–$22
Premium residential, large historic homes
6" copper K-style
~2,200 sq ft
$25–$40
Premium builds, coastal, 50+ year lifespan
Downspout size
Max tributary area
Typical price / LF installed
Notes
2×3 rectangular
~1,500 sq ft
$6–$10
Standard for 5" K-style
3×4 rectangular
~3,000 sq ft
$8–$12
Standard for 6" K-style
3" round
~2,000 sq ft
$7–$11
Half-round systems
4" round
~4,000 sq ft
$10–$16
Large half-round, commercial
Downspout spacing — where most installs fail
Downspouts are usually spaced every 30–40 linear feet of gutter on center, with a minimum of two per continuous run (one at each end).4 Wider spacing risks mid-run overflow on any heavy storm — the gutter fills faster than it can drain to the outlets.
The most common installation mistake is one downspout per long run. On a 60 ft run with a single downspout at one end, the far end of the gutter overflows first. Even at standard rainfall, the slope of the gutter can carry water only so far before it backs up.4
Both layouts look fine on paper. The one on the left drains under heavy rain; the one on the right has a mid-run overflow every time it pours.
Cost per foot and total estimate (2026)
Installed prices for aluminum K-style gutters and downspouts, national averages with region and material factored in. Copper and half-round run higher.
Material
Gutter $/LF
Downspout $/LF
100 LF + 3 downspouts
5" K-style aluminum
$7–$10
$6–$10
$900–$1,200
6" K-style aluminum
$10–$14
$8–$12
$1,200–$1,700
5" half-round aluminum
$12–$18
$7–$11
$1,450–$2,000
6" half-round aluminum
$15–$22
$10–$16
$1,850–$2,600
6" copper K-style
$25–$40
$20–$35
$2,900–$4,600
Field note from the estimating desk: the most expensive gutter mistake is sizing by roof area instead of effective area. A homeowner with a 6/12 roof in a 4 in/hr rainfall zone reads "2,200 sq ft" from the roof plans, installs a 5″ gutter based on the 1,500 sq ft capacity rule, and gets overflow at the first storm. The effective area was 2,530 — 30% above what the gutter could handle. The fix is a 6″ gutter, which costs about 30% more upfront but eliminates the overflow. Second mistake: one downspout on a 60 LF run. Even a 6″ gutter with a single outlet will back up at the far end because the slope cannot move water fast enough.
Frequently asked questions
What size gutters do I need?
For most homes under 1,500 sq ft of effective roof area, 5″ K-style is sufficient. Above that, 6″ K-style is the standard. Effective area is roof area × pitch factor (1.15 at 6/12) × rain factor (1.0 at 4 in/hr) — so a 2,200 sq ft roof at 6/12 in standard rain needs 6″. In high-rainfall zones (Gulf Coast, Southeast), the switchover happens at a smaller roof.
How many downspouts do I need?
Minimum two per continuous run (one at each end), with downspouts spaced no more than 35–40 LF apart.4 A 40 LF run gets 2 downspouts; a 60 LF run needs 3; a 100 LF run needs 3–4. Downspout capacity also matters — a 2×3 downspout can handle about 1,500 sq ft of effective area, so if your gutter holds more than that, you need more downspouts.
What is effective roof area?
Effective roof area is roof area × pitch factor × rainfall factor. It is the number gutter sizing is based on, not the raw roof area. A steeper roof sheds water faster, so it needs a larger gutter; higher local rainfall does the same. A 2,200 sq ft roof at 6/12 in standard rainfall has an effective area of about 2,530 sq ft.1
5" or 6" K-style gutter — which is better?
5″ is the standard for most homes and is fine up to about 1,500 sq ft of effective area. 6″ is better for large roofs, steep pitches, high-rainfall areas, and new construction. Many builders now default to 6″ because the cost difference is small (about 30% more per LF) and the capacity headroom eliminates overflow risk.
How much do gutters cost per linear foot?
5″ K-style aluminum: $7–$10 per LF installed. 6″ K-style: $10–$14 per LF. 5″ half-round: $12–$18. Copper: $25–$40 per LF. Downspouts add $6–$16 per LF depending on size and material.3
How often should gutters slope?
Standard slope is 1/4 inch per 10 feet of run, toward the downspout. This is enough to move water without making the slope visible. A run that slopes less than this pools water; a run that slopes more looks uneven against the fascia. At 40 LF run, that is 1 inch of total drop end-to-end.5
How many hangers do I need per foot of gutter?
One hanger every 24 inches (2 per LF) is standard, or every 36 inches in areas without heavy snow. Hidden hangers are standard on aluminum K-style; spike-and-ferrule on older installs. Use more in snow country — the weight of ice can pull gutters off if hangers are sparse.
Can I install gutters myself?
Yes — aluminum K-style is one of the more DIY-friendly exterior projects, but you need a gutter machine or pre-cut sections, a helper to hold lengths, and reasonable ladder safety. DIY saves roughly 40–50% of installed cost. Seamless (continuous) gutters require a machine and are almost always worth hiring a pro for.
What is the difference between K-style and half-round gutter?
K-style has a flat face and a decorative front, is cheaper, and is what most homes have. Half-round has a curved profile, is more expensive, and is used on historic homes and premium builds. Half-round carries slightly less water per LF than K-style at the same nominal width.
Do gutters need to be replaced with the roof?
Not necessarily, but often at the same time. If the roof is being torn off, the gutters have to come down — and older gutters are usually at the end of their life anyway. Aluminum gutters last 20–30 years; copper 50–100. If the gutters are 15+ years old when the roof is replaced, replacing them at the same time is often cheaper than doing it separately.
Standards and sources
SMACNA — Architectural Sheet Metal Manual, 7th Edition. Gutter sizing tables, effective area formula, and pitch adjustment factors. smacna.org
International Code Council — IRC R903.4 Roof Drainage. Gutter and downspout sizing requirements and overflow provisions. codes.iccsafe.org
HomeGuide — Gutter Installation Cost (2026). Installed per-linear-foot pricing by material and system. homeguide.com
Gutter Supply — Downspout Spacing and Placement Guide. 2 minimum downspouts per run, 35–40 LF spacing guidance. guttersupply.com
National Roofing Contractors Association — Roof Drainage and Gutters. 1/4 inch per 10 ft slope standard and hanger spacing. nrca.net
BM
Builders Measures estimating team
Reviewed against SMACNA sizing tables and NRCA drainage guidance. Effective area factors and capacities are industry-standard — always confirm gutter size and downspout capacity with your supplier for local rainfall data. Last reviewed October 2026.