Gutter Calculator
Estimate the linear feet of gutter from your eaves perimeter and the number of downspouts your roof area needs — sized by 2×3 or 3×4 downspout capacity so the system actually keeps up with the runoff.
How to Use
- Enter the eaves perimeter in feet — the total length of roof edge that will carry gutter.
- Enter the roof area in square feet, which drives how much water the gutters have to move.
- Pick a downspout size: 2×3 handles about 800 ft² of roof each, 3×4 about 1,200 ft².
- Read the gutter length and the number of downspouts the roof needs.
- Check the Show Work below, then add elbows, end caps, and hangers to your parts list.
Show Work
Notes
About the Gutter Calculator
Whether you are at a desk or on your phone, the Gutter Calculator makes building, materials estimation and site work easy — and it is completely free. Estimate the linear feet of gutter from your eaves perimeter and the number of downspouts your roof area needs — sized by 2×3 or 3×4 downspout capacity so the system actually keeps up with the runoff.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
How many downspouts do I need?
A common rule is one downspout per 800–1,200 square feet of roof drained, depending on the downspout size and your local rainfall. Bigger 3×4 spouts carry more, so you need fewer of them. This tool divides your roof area by the chosen downspout capacity and rounds up so heavy rain has somewhere to go.
How much should gutters slope?
Aim for about 1/4 inch of fall per 10 feet of run toward the downspout. Too little and water pools and overflows; too much and it looks crooked and races past the outlet. On long runs, slope from a high point in the middle down toward a downspout at each end.
What size gutter and downspout should I use?
Most homes use 5-inch K-style gutter with 2×3 downspouts, which is fine for average roofs and rainfall. Large roofs, steep pitches, or heavy-rain regions step up to 6-inch gutter with 3×4 downspouts to avoid overflow. Match the downspout size in this tool to what you plan to install.
How far apart should gutter hangers be?
Space hangers about every 24 inches in snow or ice country so the gutter can carry the extra weight, and up to every 36 inches in milder climates. Add an extra hanger near each corner and downspout outlet where stress concentrates. Closer spacing is cheap insurance against sagging.
Does roof pitch affect gutter sizing?
Yes — a steeper roof sheds water faster and concentrates it, effectively increasing the load on the gutter compared to its flat footprint. For very steep roofs, bump up a downspout size or add an extra outlet. This calculator uses your roof area as the baseline, so add capacity if your pitch is aggressive.
How do I use the Gutter Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Re-guttering a house
Turn the eaves perimeter into total gutter length and the right downspout count for a full replacement.
Sizing downspouts for a big roof
Check whether 2×3 spouts keep up or you should step up to 3×4 to prevent overflow.
Quoting a gutter job
Get linear feet and outlet count up front to price materials before ordering.
Adding gutters to a garage or shed
Size a small run and figure how many downspouts a low roof area actually needs.
Planning rainwater drainage
Know your downspout count and placement so you can route runoff away from the foundation.
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