# Gutter & Downspout Calculator — Size, Count & Cost > Free gutter and downspout calculator: size K-style gutters and downspouts from roof area, pitch and rainfall, count how many downspouts you need, and get a full accessory takeoff — hangers, elbows, outlets, end caps and cost — metric or imperial, in your browser. Size K-style gutters and downspouts, count the drops and materials, and read the layout — run by run, entirely in your browser. - **HTML page:** https://siteometry.com/en/tools/gutter-downspout-calculator - **Category:** [Roofing & Siding](https://siteometry.com/en/tools/category/roofing-siding) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## How this calculator works 1. **Choose units & currency** — Pick metric or imperial and your currency; every field and result follows. 2. **Set the rainfall intensity** — Pick a preset (moderate 1 in/hr is typical) or enter a custom design rate from NOAA Atlas 14 or local code. 3. **Add your gutter runs** — Enter each eave as a run — length × roof depth or a direct plan area — one card per side of the house. 4. **Set pitch & ends** — Give each run its roof pitch and how its ends terminate (open, or into a corner) so end caps count correctly. 5. **Tune profile & accessories** — Keep the recommended profile or override it, set hanger spacing, drop height, corners and optional prices. 6. **Read the takeoff & layout** — The rail shows the profile, downspout count and full material list; the diagram shows where each drop lands. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#howto_ ## Gutter & downspout sizing, explained A gutter system has one job: catch the rain that runs off a roof and carry it to downspouts before it can spill over, pool at the foundation, or overwhelm a run. Sizing it well is a matter of matching the roof's catchment area and the local storm intensity to enough downspout cross-section and enough drops along the run. This calculator works that chain in order — projected roof area, a roof-pitch factor, a design rainfall rate, and the well-corroborated downspout-capacity rule — to recommend a K-style gutter and downspout profile, count the downspouts, and lay out a full accessory takeoff. Everything runs in your browser; nothing you type leaves your device. Treat the results as a planning reference, not stamped engineering. The numbers use industry rules of thumb and convenient rainfall tiers; a permitted job should be checked against your local code and a real design rainfall for your location. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#gutter-guide-intro_ ## Parts of a gutter & downspout system Before sizing, it helps to name the parts. The gutter is the horizontal channel along the eave; on most homes it is a K-style (ogee) profile in a 5-inch or 6-inch nominal width. It is held to the fascia by hangers spaced along the run, and closed at any open end by an end cap. Water leaves the gutter through an outlet into a downspout — the vertical pipe, typically 2×3 in or 3×4 in rectangular (or round). A pair of offset elbows at the top steps the downspout back to the wall, and a kick-out elbow at the bottom throws water away from the foundation. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#gutter-anatomy_ ## How gutter sizing works The sizing chain converts a roof plane into a downspout count. Start from the plan (footprint) area each gutter run drains, multiply by a roof-pitch factor to get the effective drainage area, then divide by how much area one downspout can carry at your rainfall rate. The gutter profile follows from the downspout it is paired with. The downspout-capacity rule is the sturdiest number in the whole method — one square inch of downspout drains about 100 square feet of roof at a 1 in/hr storm — so the tool sizes from the downspout and lets the gutter size follow, rather than leaning on gutter-only capacity figures that sources disagree on. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#sizing-method_ ## Roof pitch and drainage area Enter the roof's projected (plan or footprint) area, not the sloped surface area. Rainfall is measured as a vertical depth, so the horizontal area a roof covers is what actually catches the rain — using the longer sloped area over-sizes the system. In length × depth mode, the depth is the horizontal run from eave to ridge, not up the slope. Steeper roofs catch a little more wind-driven rain, so the plan area is multiplied by a pitch factor from 1.00 (low slope) up to 1.30 (12-in-12 and steeper). A 1,000 ft² plan at 6-in-12 becomes 1,100 ft² of effective area. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#pitch-and-area_ ## Rainfall intensity and your region Gutter sizing uses a short-duration storm rate in inches per hour — not annual rainfall. The presets below are convenient tiers; the moderate 1 in/hr default reproduces the familiar residential rule that one 2×3 downspout drains about 600 ft². Heavier rates shrink that figure fast and drive up the downspout count. For a real design rate, look up your location in NOAA Atlas 14 or follow your local plumbing code's rainfall map. Note too that gutter sizing (a short 5-to-15-minute burst) and storm-drain sizing (a 1-hour rate under the plumbing code) use different durations, so the two conventions genuinely differ — a source of variation you may notice between calculators. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#rainfall-intensity_ ## How this calculator works Each gutter run is one card; project settings hold the rainfall rate, profile, hangers and accessories. The tool sizes every run and aggregates the takeoff live as you type. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#how-calculator-works_ ## How many downspouts, and where The downspout count is the largest of three requirements: enough capacity for the effective area, at least one drop, and at least one drop per 40 ft of run. Whichever demands the most downspouts wins, so a long low-slope run can be spacing-driven while a compact steep run is area-driven. Drops are distributed evenly, including one at each end of a two-or-more-downspout run. The example below is a 40 ft gable eave at moderate rainfall — right at the 40 ft threshold — which lands two downspouts, one at each end, with the gutter sloping down to each from a center high point. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#downspouts-and-layout_ ## Hangers, elbows and fall Hangers space about every 24 in on center, tightening to 18 in in snow and ice country, with the first hanger set roughly 6 in in from each end. Each downspout takes about three elbows (two offset elbows up top plus a kick-out) and one outlet; each open gutter end takes an end cap. Sealant is estimated from the caps, outlets and corners. Slope the gutter about ¼ in per 10 ft of run toward each downspout. A run longer than 40 ft should not fall in one direction across its whole length — slope it from a center high point down to a downspout at each end instead. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#accessories-and-fall_ ## Worked examples Two runs through the method, from roof to downspout count. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#examples_ ## Common mistakes & a planning-only reminder The usual errors are sizing from sloped area instead of plan area (over-sizing), ignoring local rainfall (a moderate-climate layout under-drains a storm belt), and running one downspout on a long eave where the 40 ft rule wants two. The pitch factor, hanger spacing, slope, elbow counts and rainfall tiers are industry rules of thumb, not code tables — so verify the plan before you buy or build. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#mistakes_ ## Gutter & downspout FAQ Common questions about sizing gutters, counting downspouts and reading the takeoff. ### How many downspouts do I need? As a rule of thumb, one downspout drains about 600 sq ft of roof at moderate rainfall, and you want at least one every 40 ft of gutter run. Heavier rain or larger roofs need more. This tool sizes the count from your roof area, pitch and rainfall intensity, taking whichever of capacity or spacing demands more drops. ### What size gutter do I need — 5" or 6"? Most homes use 5" K-style gutters with 2×3 downspouts. Step up to 6" K-style with 3×4 downspouts on large or steep roofs, or in heavy-rain climates, where a 5"/2×3 system would need downspouts closer together than about 20 ft. ### How far apart should gutter hangers be? Space hangers about every 24 in on center in mild climates, tightening to 18 in where snow and ice load the gutter. Set the first hanger about 6 in in from each end of the run. ### Why does rainfall intensity change the result? Downspout capacity is the roof area a drop can carry, and it falls as the storm rate rises — a 2×3 drains about 600 ft² at 1 in/hr but only 300 ft² at 2 in/hr. A rainier region therefore needs more or larger downspouts for the same roof, which is why the preset (or a NOAA Atlas 14 rate) drives the count. ### Why do gutter and storm-drain sizes differ? They use different storm durations. Gutter sizing uses a short 5-to-15-minute burst intensity, while storm-drain sizing under the plumbing code uses a 1-hour rate. Both are valid for their purpose, so a gutter sized here can differ from a roof-drain figure — this tool sizes gutters and downspouts, not internal storm drains. _On the page: https://siteometry.com/en/tools/gutter-downspout-calculator#faq_ ## Related calculators - [Roofing Calculator — Pitch, Area & Materials](https://siteometry.com/en/tools/roofing) - [Roof Truss Calculator — How Many Trusses & Cost](https://siteometry.com/en/tools/roof-truss-calculator) - [Siding Calculator — Squares, Panels, Boxes & Trim](https://siteometry.com/en/tools/siding-calculator) - [Roof Pitch Calculator — Slope to Degrees, Percent & X-in-12](https://siteometry.com/en/tools/roof-pitch-calculator)