Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Recommended gutter size (in)
5
3 in downspouts needed
4
4 in downspouts needed
2
Pitch-adjusted roof area
1980
Design drainage area
1980
Peak flow (gal/min)
103.0
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter roof footprint area (sq ft).
- 2Enter roof pitch (rise per 12) (in).
- 3Enter peak rainfall intensity (in/hr).
- 4Read your recommended gutter size (in) on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Gutter size follows the adjusted roof area: drainage area times rainfall intensity. A single 3 in downspout drains roughly 600 sq ft of roof in heavy rain.
Worked example
Using the values the calculator loads with:
Inputs
- Roof footprint area: 1800 sq ft
- Roof pitch (rise per 12): 6 in
- Peak rainfall intensity: 5 in/hr
Results
- Recommended gutter size (in): 5
- 3 in downspouts needed: 4
- 4 in downspouts needed: 2
- Pitch-adjusted roof area: 1,980
- Design drainage area: 1,980
- Peak flow (gal/min): 103
What each field means
Inputs
- Roof footprint area (sq ft)
- The roof footprint area used in the calculation, measured in sq ft. Starts at 1800 sq ft so you have a working example on load.
- Roof pitch (rise per 12) (in)
- The roof pitch (rise per 12) used in the calculation, measured in in. Starts at 6 in so you have a working example on load. Accepted range: 0.5–24 in.
- Peak rainfall intensity (in/hr)
- The peak rainfall intensity used in the calculation, measured in in/hr. Starts at 5 in/hr so you have a working example on load. Accepted range: 1–12 in/hr.
Results
- Recommended gutter size (in)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 3 in downspouts needed
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 4 in downspouts needed
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Pitch-adjusted roof area
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Design drainage area
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Peak flow (gal/min)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the gutter & downspout sizing calculator work?
Gutter size follows the adjusted roof area: drainage area times rainfall intensity. A single 3 in downspout drains roughly 600 sq ft of roof in heavy rain. The underlying maths is: Adjusted area = roof area × pitch factor; downspouts = area ÷ capacity per spout.
What do I need to enter?
3 values: roof footprint area, roof pitch (rise per 12), and peak rainfall intensity. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the recommended gutter size (in) result mean?
It is returned as a whole number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Gutter Sizing Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/gutter-sizing
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/gutter-sizing" target="_blank" rel="noopener">Gutter Sizing Calculator — RevenueLab</a> (2026).</p>
Source: [Gutter Sizing Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/gutter-sizing) (2026).