Gutter Calculator

The big number is the feet of gutter to buy. It is the sum of your runs, plus 1 inch for each capped end, plus the extra you chose to order, rounded up to 10 foot sections. The list below it gives the fittings that go with that gutter.

• Downspouts, outlets and elbows come from the run lengths and your feet-per-downspout setting.
• End caps and corners come from the corner count you enter.
• The per run table shows downspouts, hangers and slope drop for each roof edge.
• Peak flow and the size line appear only when you fill in the roof area.

Measure the roof area with the square footage calculator if you do not have it.

The calculator treats each roof edge as one run. It adds the runs, counts the parts that each run needs, and checks the roof area against a flow estimate. Every rule comes from a named source, and none of the numbers is a local code value.

Linear feet, sections and joints

Gutter to buy = (sum of runs + 1 inch per end cap) x (1 + extra). Home Depot adds 1 inch at each end and suggests 10 percent extra. Sections are 10 feet, the usual length Home Depot gives. Joints equal the pieces in a run minus one.

Downspout count and length

Plan about one downspout for every 20 to 30 feet of gutter. This Old House says 25 to 30 feet and Home Depot says 20 to 30. Family Handyman puts a downspout at each end of a run of 40 feet or more. Length is the height from the fascia to the ground plus at least 4 feet for the extension.

Slope drop per run

Pitch the gutter about 1/4 inch for every 10 feet toward the downspout. A 30 foot run drops 0.75 inch. A 40 foot run slopes from the middle, so it drops 0.50 inch on each side. This Old House also mentions 1/2 inch per 10 feet in a tip. Three sources agree on 1/4.

Peak flow from roof area and rainfall

One inch of rain on one square foot is 144 cubic inches, which is 0.623 gallons. Peak flow in gallons per minute is the roof footprint in square feet x rainfall in inches per hour x 0.623 / 60. Rain falls on the footprint, so a sloped area is divided by the square root of 1 + (pitch / 12) squared. Find your local rate in the NOAA Atlas 14 data server. The result is before losses. Check the pitch with the roof pitch calculator.

A hip roof has four edges of 40, 30, 40 and 30 feet and four outside corners. With 30 feet per downspout, 24 inch hangers and 10 percent extra, it needs 154 feet of gutter, 6 downspouts and 74 hangers. The roof footprint is 44 by 34 feet, or 1,496 square feet.

The 4 inch per hour rainfall is an example input, not a design value. Per downspout the flow is 10.4 gpm. A gable roof with two 60 foot runs and all four ends capped needs 132.4 feet, 4 end caps and 4 downspouts.

Most houses use 5-inch or 6-inch gutters. This Old House lists 5-inch for roofs up to 5,520 square feet and 6-inch for roofs up to 7,960. Home Depot says steeper roofs need wider gutters. The calculator uses these limits as a first screen only.

Two sources do not agree on these limits. A Google AI Overview reads the same 5,520 and 7,960 figures as gallons per hour, and puts a 5-inch gutter at 2,000 to 2,200 square feet. Treat the screen as a starting point. Professionals size gutters with SMACNA's Architectural Sheet Metal Manual, and this page has not opened that table.

The standard downspout is 2x3 inch. A 3x4 inch downspout is the oversized choice for large roof areas or heavy rain, from Family Handyman. The calculator suggests 3x4 when the area passes the 5-inch screen. That link is a CalcSummit rule.

A gutter run needs sections, connectors, end caps, corners, outlets, elbows, downspouts and hangers. The calculator counts the ones that have a count rule in the sources. It lists the rest, because no source gave a rule for them.

Corner pieces are often called miters. The calculator gives you the corner count and does not add a separate miter number.

The usual errors are measuring the wrong edge, counting corners twice and skipping the extension. Each one changes the parts list, so check them before you order.

• Measuring the footprint of the house instead of the eave line. Gutter follows the fascia, which sits past the wall.
• Counting a corner as two end caps. A corner joins two run ends, so each corner removes two end caps.
• Adding gutter to gable ends. Gable ends usually do not get gutter, but the sloped edges on each side do.
• Setting downspouts by house side instead of run length. A long side needs more than one.
• Forgetting the extension. Add at least 4 feet to each downspout.
• Hanging a run dead level. Pitch it toward the downspout.

Ask a gutter installer or an architect when the roof area is above the 6-inch screen, when a roof drains onto a lower roof, or when local rules set a rainfall rate. This page is a planning aid and does not replace a drainage design.

Flat roofs, box gutters, scuppers and roof drains are outside this calculator. For roof area and squares, use the roofing calculator.

Count corners before you shop. Each corner uses up two run ends, so a house with four runs and four corners needs no end caps. Measure each downspout from the fascia to the ground and add 4 feet for the extension, as This Old House advises. Then add the extra you chose to order.

Published November 2026. Last updated November 2026. Next review May 2027. This tool is a planning aid, not a drainage design or a quote.

David Chen, RA LEED APRA LEED AP

Reviews: area calculators · 29 calculators reviewed

David Chen is a Registered Architect (RA) and LEED Accredited Professional BD+C with 16 years of architectural practice. He holds California architect license #A-35207 and LEED AP BD+C credential #10294751. Previously at Gensler, he co-authored two RCI whitepapers on roof assembly performance. At CalcSummit, he writes and verifies all building envelope calculators — roofing, siding, windows, gutters, and waterproofing — against IBC 2021 and NRCA standards.

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