BYTETOOLS

Gutter Calculator

Total your gutter run from eave lengths, size 5 and 6 inch K-style or half-round against roof area and rainfall, and count downspouts, hangers, elbows and corners.

120.0
Total gutter run (ft)
12
Sections to buy
46
Hangers
3
Downspouts

Size check

1,800
Roof area (sq ft)
7,920
Adjusted drainage area
600
Sq ft per downspout

Adjusted area = roof area × pitch factor × rainfall intensity. It is compared against the maximum area each profile is rated for at 1 in/h. Every capacity below is editable — swap in your manufacturer's figure if you have one.

ProfileMax area at 1 in/h (sq ft)Verdict
5" K-style
 Undersized
6" K-style
 Adequate
5" half-round
 Undersized
6" half-round
 Undersized

Fittings and cost

$858.00
Gutter + hangers
$198.00
Downspouts
$110.00
Fittings
$1,166.00
Material total

Gutters are sized on adjusted drainage area, not on how long they are: roof area × pitch factor × your local rainfall intensity, compared against a profile's rated area at 1 in/h. Steeper roofs throw water further and drive rain hits them harder, which is what the pitch factor accounts for. Downspout count is roof area ÷ the square feet one spout can take — under-spouting is the usual reason a correctly sized gutter still overflows. Estimates only; a local building official will have the rainfall figure your jurisdiction expects.

What is the Gutter Calculator?

The ByteTools Gutter Calculator does the part most guides skip: the size check. Enter your roof area, the pitch factor and your local rainfall intensity and it works out an adjusted drainage area, then compares that against the rated capacity of 5-inch and 6-inch K-style and half-round profiles — every one of which you can edit if your manufacturer publishes different figures.

  • Eave-length list totalled into gutter run, sections and hanger count
  • Size check across 5" and 6" K-style and half-round, with editable capacities
  • Adjusted drainage area from roof area, pitch factor and rainfall intensity
  • Downspout count from square feet per spout, with the area each one serves
  • Elbows, inside and outside corner mitres, end caps and a full material cost
  • Copyable take-off; runs entirely in your browser with nothing uploaded

How to use the Gutter Calculator

  1. 1

    Pick your currency, the roof pitch factor and the hanger spacing you plan to use.

  2. 2

    Paste the eave lengths in feet, one number per run of gutter, separated by commas or spaces.

  3. 3

    Enter the roof area those gutters drain, your local rainfall intensity in inches per hour, the square feet each downspout can take, and the gutter section length.

  4. 4

    Check the size table — it marks each profile adequate or undersized against the adjusted drainage area, and every capacity figure is editable.

  5. 5

    Fill in downspout height, elbows per downspout, corners, end caps and unit prices to get the material total, then copy the take-off.

About the Gutter Calculator

The ByteTools Gutter Calculator does the part most guides skip: the size check. Enter your roof area, the pitch factor and your local rainfall intensity and it works out an adjusted drainage area, then compares that against the rated capacity of 5-inch and 6-inch K-style and half-round profiles — every one of which you can edit if your manufacturer publishes different figures.

The take-off side totals the gutter run from a list of eave lengths, splits it into sections, counts hangers at the spacing you pick, and works out how many downspouts the roof area needs. Elbows, corner mitres, end caps and the downspout drops are all priced so you get a material total rather than just a length.

Everything is computed in your browser and nothing is uploaded. Treat the sizing as a planning estimate: your local building department will have the rainfall intensity your jurisdiction expects, and under-spouting is the usual reason a correctly sized gutter still overflows.

Frequently asked questions

What size gutters do I need, 5 inch or 6 inch?

It depends on adjusted drainage area, not on the size of the house. Multiply roof area by the pitch factor and your local rainfall intensity, then compare against each profile's rated area. A steep roof in a heavy-rain region can push a modest house past 5-inch K-style.

How many downspouts do I need?

The common rule is one 2×3 downspout per 600 square feet of roof, or one 3×4 per 1,200 square feet. Enter whichever figure applies and the calculator divides your roof area by it. More, smaller-spaced downspouts almost always outperform fewer big ones.

What is the roof pitch factor for gutter sizing?

It is a multiplier that accounts for steep roofs catching more wind-driven rain and shedding it faster. Flat to 3-in-12 is 1.00, 4-in-12 to 5-in-12 is 1.05, 6-in-12 to 8-in-12 is 1.10, 9-in-12 to 11-in-12 is 1.20 and 12-in-12 or steeper is 1.30.

How far apart should gutter hangers be?

Thirty-two inches is typical, tightening to 24 inches where snow, ice or very heavy rain load the gutter, and 36 inches is usually the light-duty maximum. Pick a spacing in the dropdown and the hanger count updates.

Why does my gutter overflow even though it is the right size?

Usually because there are not enough downspouts, or one of them is blocked. A gutter fills from the far end towards the outlet, so a long run with a single spout at one end overflows at the middle long before the profile itself is the limit.

Do half-round gutters hold less water than K-style?

In the commonly published sizing tables they are rated for a smaller roof area than K-style of the same nominal size, largely because of how they are hung and drained. The capacities here are editable, so use your manufacturer's own figures if you have them.

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