Downspouts & Drainage
The downspout & drainage cluster: a downspout count & sizing calculator, a downspout extension / discharge-distance calculator, an elbows & offsets quantity, a rainwater-volume calculator, and a downspout cost. Get roof water 4–6 ft from the foundation; beyond the discharge point, grading, French drains and dry wells are a foundation/yard-drainage job (basementcalcs / landscapingcalcs / a pro).
Measure your eave runs and confirm capacity, spacing and coverage against the exact product you buy. Allow extra for corners, waste and slope (~5–10%). Sizes, capacities, spacings and panel lengths vary by product and brand — read the label and the manufacturer’s data.
Planning estimate: this is a planning estimate from the numbers you enter — not a bid or a contract. Gutter pricing depends on material, size, linear feet, guards, fascia condition, removal, height/access and local labor. Get itemized written quotes from licensed, insured gutter contractors before you commit.
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.Downspout extension calculatorCount the extension pieces that discharge roof water 4–6 ft from the foundation. Free downspout calculator with the formula and a worked example.Downspout elbows & offsets calculatorCount downspout elbows and offset fittings: 2 top plus 1 bottom per downspout, and 2 per soffit offset. Free calculator with the formula and a worked example.Rainwater volume calculatorHow many gallons does a roof shed per inch of rain? Footprint × 0.6234 × runoff. Free rainwater calculator with the formula and a worked example.Downspout costEstimate downspout cost from your own prices: pipe per foot plus elbows and fittings. Free calculator with the formula, a worked example and a labeled band.
Every Downspouts & Drainage tool is calculated server-side — visible immediately — with its formula, a worked example and a reference table, and works on the measurements you take and the prices you enter from your own quotes. Background reading is in the guides.