Max roof area per downspout by size

Typical published planning values — NOT a certified design. Actual gutter sizing depends on local rainfall intensity, roof geometry, valleys and debris; follow local code and the manufacturer’s data, and consult a pro for complex roofs. Structural roof-load, ice-dam / heat-cable and foundation/yard drainage are set by code and a professional — not engineered here.

A bigger downspout carries more roof area. The table below shows the labeled max roof area per downspout by size plus the 1-per-30–40 ft rule of thumb — planning values that drop in heavy-rain regions. Use it with the downspout count calculator and the downspout count & sizing calculator.

Downspout sizeMax roof areaSpacing rule
2×3 in rectangular600 ft²1 per 30–40 ft
3×4 in rectangular1,200 ft²1 per 30–40 ft
3 in round700 ft²1 per 30–40 ft
4 in round1,250 ft²1 per 30–40 ft

Labeled planning typicals — sizes to the roof area each outlet must carry at a design rain. Use the larger of the drainage-area count and the 1-per-30–40 ft rule; capacities drop in heavy-rain regions.

Related calculators

Downspout count calculatorHow many downspouts do you need? Runs both methods — roof area per outlet and the 1-per-30-to-40-ft rule — and recommends the larger, safer count.Downspout count & sizing calculatorHow many downspouts does a roof need? Size by drainage area and the 30–40 ft spacing rule, and pick a 2×3, 3×4 or round outlet. Free, with a worked example.