Gutter Length Calculator
Gutters run along the eaves of a roof and carry rainwater away from the foundation, so ordering the right number of linear feet starts with the roof footprint, not the slope. This gutter length calculator takes the length and width of the building's roof footprint and returns the full eave perimeter, which is the total run of gutter you need if you are guttering all four sides. Many roofs only need gutters on the two long eaves, where the roof actually drains, so the calculator also shows the length for the two long sides alone. Measure the horizontal footprint of the roof at the eaves, including any overhang, rather than the sloped roof surface, because gutters follow the level eave line. Every figure here is computed deterministically from your dimensions, so the same roof always returns the same gutter length. Enter your measurements below to size a gutter order before you shop, decide between guttering all sides or only the draining eaves, or check an existing run, with a worked example that reconciles exactly to the calculator defaults so you can follow each step. Add a little extra length for corner joints, end caps, and offcuts when you place your order.
Full eave gutter equals the roof footprint perimeter, twice the length plus twice the width. A 40 ft by 30 ft roof footprint has a 140.00 ft perimeter, or 80.00 ft along the two long eaves.
Gutter length formula
full perimeter = (2 x length) + (2 x width)
two long eaves = 2 x length
length and width are the roof footprint at the eaves, in feet
Guttering all four sides needs the full footprint perimeter. Guttering only the two long draining eaves needs twice the length. Measure the level eave footprint, not the sloped roof.
Worked example
Take a roof with a footprint 40 feet long and 30 feet wide at the eaves.
- Two long eaves: 2 x 40 = 80.00 ft
- Full perimeter: (2 x 40) + (2 x 30) = 80 + 60 = 140.00 ft
The full eave perimeter is 140.00 feet, or 80.00 feet for the two long sides alone, matching the calculator's default inputs exactly. Add extra for joints and offcuts.
Gutter Length Calculator: frequently asked questions
Do I need gutters on all four sides?
Not always. On a simple gable roof, water drains off the two long eaves only, so many homes gutter just those sides. A hip roof drains on all four sides and usually needs gutters all the way around. Use the figure that matches how your roof actually sheds water.
Should I measure the roof slope or the footprint?
Measure the level footprint at the eaves, including any overhang, not the sloped roof surface. Gutters run horizontally along the eave line, so the slope length does not matter for gutter quantity.
How much extra should I order?
Add roughly 10 percent for corner miters, joins, end caps, and offcuts, and round up to the lengths your supplier sells. Gutter sections come in standard lengths, so a little waste at each cut is normal.
What about an irregular roof shape?
For an L-shaped or complex roof, break the eaves into straight runs, measure each one, and add them together. The simple rectangle perimeter here is best for straightforward gable and hip roofs.
Does this tell me how many downspouts I need?
No, gutter length and downspout count are separate questions. Use a downspout count calculator, which sizes the number of downspouts from the roof drainage area rather than the eave length.
Official sources
- Measurement, units and dimensional standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.