Gutter Calculator

It converts your roof’s drainage area and a design rainfall intensity into a required flow capacity, which you then match against a gutter (and downspout) size rated to carry that much water. In the USA: flow (GPM) = (effective roof area in sq ft × rainfall intensity in in/hr) ÷ 96.23. In metric regions: flow (L/min) = (effective roof area in m² × rainfall intensity in mm/hr) ÷ 60.

SMACNA’s Architectural Sheet Metal Manual is the standard US reference: it uses a 5-minute-duration, 10-year or 100-year design rainfall intensity (by city, from ASMM Table 1-2 or NOAA Atlas 14) times your effective roof area to get a required flow in GPM, then compares that against published gutter and downspout capacity tables. This calculator uses the same Q = (A × I) ÷ 96.23 formula and a small built-in K-style capacity table for a quick estimate — for a stamped commercial design, use SMACNA’s own calculator and tables directly.

For eaves gutters, yes — select Australia/NZ, enter your roof area and local rainfall intensity (check the Bureau of Meteorology for Australia or NIWA for New Zealand; intensities vary hugely, from around 139mm/hr in Albury to 250mm/hr+ in tropical Queensland), and compare the required flow against your chosen gutter profile’s rated capacity under AS/NZS 3500.3. Box gutters are different: they carry a critical overflow-safety requirement (failure floods the building interior, not just the ground outside), and AS/NZS 3500.3 expects a qualified hydraulic design for those — this calculator gives you the underlying flow-rate math, not a substitute for that design.

Yes — select UK for the BS EN 12056-3 method, which defaults to 75mm/hr (the standard design intensity for most of England, Wales, and lowland Scotland; western Scotland, the Lake District, Snowdonia, and exposed coastal sites need 100-150mm/hr — adjust the intensity field for your area). As a rough guide, a common 112mm half-round or 114mm square-line gutter profile suits roofs up to about 50-60m² of effective area at 75mm/hr; larger or higher-rainfall roofs need a deeper or high-capacity profile.

Select "Other / custom," then enter your own local rainfall intensity in mm/hr (tropical regions like Malaysia have very high short-duration intensities, often well above the UK/Australia defaults here — check your local meteorological or building authority for the design figure). The flow-rate formula and downspout guidance work the same way; only the built-in defaults are region-specific.

A common rule of thumb is one downspout per roughly 600-800 sq ft of effective roof area, or one every 30-40 feet of gutter run — this calculator uses 700 sq ft per downspout as a working default in Size My Gutters, and a 35 ft run-length default in Materials & Cost. Larger or more complex roofs may need more, positioned to avoid long, slow-draining runs.

No — this is an independent calculator built by Calculator Matters and is not affiliated with, endorsed by, or produced by The Home Depot, Lowe’s, Amerimax, or any other retailer or manufacturer. It uses the same general SMACNA-based sizing and materials-estimating approach that retailer tools typically offer, not any one company’s proprietary product catalog or pricing.

In the Materials & Cost mode, typical installed costs (before a local quote) run about $3-5/ft for vinyl, $4-9/ft for aluminum, $6-10/ft for steel, and $25-45/ft for copper — enter your own quote to replace these defaults. For a typical 150-200 ft home, that puts most aluminum jobs in the roughly $1,000-$1,900 range nationally, though local labor rates and roof complexity move this a lot.

Typical US pricing runs about $120-$230 per visit for an average home (roughly $0.50-$2.50 per linear foot), depending on stories, access, and how much debris has built up. The calculator’s Materials & Cost mode includes an adjustable per-foot cleaning-cost estimate for a rough planning figure alongside the installation cost.

If gutters are seamlessly formed on-site from a roll ("coil") of sheet metal using a gutter machine, rather than assembled from pre-cut sections, the amount of coil consumed is close to the total run length plus a small waste allowance. Turn on "Forming seamless gutters from coil stock on-site" in Materials & Cost to see that footage (a 5% waste allowance is the default).

No — "curb and gutter" in road and civil construction refers to the concrete curb-and-gutter profile poured along a street, sized by volume (cubic yards) for a linear run, which is a completely different calculation from roof rainwater drainage. This calculator only covers roof and eaves gutters; it does not estimate concrete curb-and-gutter volume for road construction.

A commonly cited minimum is about 1/16 inch of fall per foot of run, with 1/8 inch per foot a more comfortable practical target on residential K-style gutters, and up to 1/4 inch per foot on long runs; SMACNA’s manual recommends at least 1/8 inch per foot for built-in commercial gutters. The Size My Gutters mode’s advanced section turns a run length and a chosen slope into the total fall (drop) from the high end to the low end.

This is a free, online calculator — no app, signup, or download required. There’s no separate spreadsheet or PDF, but every formula is written out in full in the Formula section below, so you can rebuild the same sizing and materials math directly in a spreadsheet.