Gutter Calculator

Why the US formula divides by 96.23 and the metric one divides by 60

Every number this page produces starts from a single conversion: a drainage area multiplied by a design rainfall intensity, turned into a flow rate you can hold against a gutter and downspout rated to carry it. The two divisors look arbitrary and are not. 96.23 is the unit constant that takes square feet times inches per hour into US gallons per minute. 60 is doing nothing more exotic than turning an hour into minutes, because 1mm of rain landing on 1m² is exactly 1 litre — so area in m² times intensity in mm/hr is already litres per hour before you touch it.

There is no app to install, no signup, and no spreadsheet or PDF to download — but every step is written out above, so you can rebuild the same sizing and materials arithmetic in a spreadsheet of your own and check this page against it.

The 4 in/hr default is a placeholder — SMACNA wants your city’s 5-minute storm

The intensity field is the one number you should not leave alone. A US design rainfall intensity is a 5-minute-duration figure for a 10-year or a 100-year storm at your specific location, published in SMACNA’s ASMM Table 1-2 or in NOAA Atlas 14. The 4 in/hr sitting in the field is a simplified planning default only, and because flow is directly proportional to intensity, a site whose true design figure is 6 in/hr is being under-sized by exactly half again.

SMACNA’s Architectural Sheet Metal Manual is the standard US reference and it works the way this page does: design intensity times effective roof area gives a required flow in GPM, which you then compare against published gutter and downspout capacity tables. The size recommendation here compares your flow against 5" and 6" K-style capacities taken from that manual — 10.2 and 19.6 GPM at a 1/16 in/ft reference slope. The 4" and 7" rows are approximate extrapolations and the result flags them as such. For a stamped commercial design, carry the flow figure from here across to SMACNA’s own calculator and tables.

139mm/hr in Albury, 250mm/hr in Queensland, 75mm/hr across most of England

Australia and New Zealand (AS/NZS 3500.3). Intensity here is a 5-minute-duration figure at a 5% annual exceedance probability — a 1-in-20-year event — and it varies more than any other input on this page: around 139mm/hr in Albury against 250mm/hr or more in tropical Queensland. That is close to a factor of two, which is why no single national default is offered and none would be safe. Look your location up through the Bureau of Meteorology in Australia or NIWA in New Zealand, enter it directly, and compare the resulting flow against your chosen eaves-gutter profile’s rated capacity.

The UK (BS EN 12056-3). Selecting UK sets 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, so move the intensity field rather than trusting the default. At 75mm/hr a common 112mm half-round or 114mm square-line profile suits roofs up to roughly 50-60m² of effective area; take the intensity to 150mm/hr and the same profile covers about half that roof, which is the entire argument for a deeper or high-capacity section on a wet-coast house.

Anywhere else. Choose "Other / custom" and type your own intensity in mm/hr. Tropical regions — Malaysia is the usual example — run short-duration intensities well above both the UK and Australian defaults built in here, so check your local meteorological or building authority for the design figure before sizing anything. Only the defaults are region-specific: the flow formula and the downspout guidance behave identically whatever number you enter.

A 1,000 sq ft roof asks for 47.8 GPM, and no K-style size here carries it

Take a 1,000 sq ft roof section at a 1.15 pitch factor, so 1,150 sq ft effective, at the 4 in/hr default: Flow = (1,150 × 4) ÷ 96.23 ≈ 47.8 GPM. Hold that against the built-in table — 10.2 GPM for 5" K-style, 19.6 GPM for 6" — and the honest answer is that one run of residential K-style gutter does not carry it. The calculator returns the largest listed size, 7" and approximate, and flags that multiple gutters and downspouts or a deeper commercial profile are likely needed. That is exactly the result where a stamped SMACNA calculation or a roofer’s judgement should take over from a quick estimate.

That pitch factor is the one roofers use; how to calculate roofing squares explains how pitch changes the area you measure.

Splitting the flow is what downspouts are for. The working rule is one downspout per roughly 600-800 sq ft of effective roof area, or one every 30-40 feet of run; Size My Gutters uses 700 sq ft per downspout and Materials & Cost uses a 35 ft spacing. The 1,150 sq ft example above therefore wants two, and a more complex roof wants them positioned to break up long, slow-draining legs rather than simply counted off an area.

Slope decides whether the water reaches them. A commonly cited minimum is 1/16 inch of fall per foot of run, 1/8 inch per foot is the more comfortable residential target, and long runs go to 1/4 inch per foot; SMACNA’s manual asks for at least 1/8 inch per foot on built-in commercial gutters. The advanced panel in Size My Gutters turns a run length and a chosen slope into the total drop from the high end to the low end — on a 40 ft run at 1/8 inch per foot that is 5 inches of fall, and 5 inches is the figure to check against your fascia before you commit to the slope.

$975 installed for 150 ft of aluminum, $3,750 to $6,750 for the same run in copper

Switch to Materials & Cost, and a 150 ft run split into two legs, aluminum at $6.50/ft installed, resolves to 15 ten-foot sections, 75 hangers at 24" spacing, 5 downspouts at 35 ft spacing, 4 end caps, and an estimated $975 installed.

Material, not length, is the lever that moves that total. Typical installed rates 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 — so the same 150 ft of geometry is roughly $450-$750 in vinyl and $3,750-$6,750 in copper, a spread of about eight times for an identical run. Across a typical 150-200 ft home most aluminum jobs land near $1,000-$1,900 nationally, and local labor rates and roof complexity move that a long way. Replace every default with your own quote the moment you have one.

Two smaller figures sit alongside it. Cleaning is priced per visit — typically $120-$230 for an average home, or roughly $0.50-$2.50 per linear foot depending on stories, access, and how much debris has built up — and the mode carries an adjustable per-foot cleaning cost so the recurring number sits next to the one-off one. And if the gutters are being formed on site from a roll of sheet metal by a gutter machine rather than assembled from pre-cut lengths, turn on the coil-stock option: seamless forming consumes close to the total run length plus waste, and the default allowance is 5%.

Box gutters, half-round profiles and road curbs: three jobs this page refuses

Box gutters are the refusal that matters most. An eaves gutter that overflows spills onto the ground; a box, valley, or parapet gutter that overflows spills into the building. AS/NZS 3500.3 expects a qualified hydraulic design for those, carrying its own critical overflow provision, and this calculator does not provide one — it gives you the underlying flow-rate arithmetic and nothing beyond it.

“Curb and gutter” is a different trade entirely. In road and civil construction the phrase means the concrete curb-and-gutter profile poured along a street, sized by volume in cubic yards for a linear run. This page cannot estimate that; it covers roof and eaves rainwater drainage only.

And it is not a retailer’s tool. This is an independent calculator built by Calculator Matters, not affiliated with, endorsed by, or produced by The Home Depot, Lowe’s, Amerimax, or any manufacturer. It follows the same general SMACNA-based sizing and materials-estimating approach those tools typically offer, without any one company’s proprietary product catalog or pricing behind it.

The remaining limitations are the assumptions you inherit by leaving the defaults where they are:

• Rainfall-intensity defaults are simplified planning starting points, not your exact local design value — look that up for anything beyond a rough estimate.
• The built-in US gutter-size table only covers K-style at one reference slope; half-round and other profiles, and other slopes, have different capacities.
• Box gutters, valley gutters, and other internal/parapet drainage are not covered — these carry overflow-safety requirements that need a qualified design.
• Materials and cost figures are general starting points; local labor rates, roof complexity, and material availability move the real number a lot.

Sources and methodology

US sizing uses SMACNA’s Architectural Sheet Metal Manual formula and capacity figures; Australia/NZ and UK sizing follow AS/NZS 3500.3 and BS EN 12056-3 respectively. Country defaults are general, sourced starting points, not a live data feed or a substitute for local code.

• SMACNA — Downspout & Gutter Sizing Calculator (USA)
• NCC — Gutters and downpipes for housing, AS/NZS 3500.3 (Australia)
• MGMA / NBS — Rainwater Drainage Design, BS EN 12056-3 (UK)

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