Gutter Calculator is a free suite of drainage calculators for the four questions every gutter project asks: will the gutter carry the storm, how many downspouts does the run really need, what fittings and fasteners complete the system, and what exactly should go in the truck. We built it around one belief: a gutter size is only trustworthy when you can see where it came from — the effective area, the pitch factor, the design rainfall, and the published capacity it is compared against.
Every roof sheds a numberPhoto: Unsplash4
Linked tools: sizing, BOM, hydraulics, shopping list
36
US city rainfall presets built in
2
Unit systems, converting in place
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Accounts, uploads, or saved inputs
The idea for this site came from a familiar frustration. Gutter advice is rarely written where you need it: it lives in a manufacturer’s capacity brochure that assumes one rainfall, or in a contractor’s habit that assumes another, or in a forum thread where “5-inch is standard everywhere” is treated as physics — and every generic online calculator returns an answer with no visible method behind it. Chasing the number is easy; knowing whether to believe it is the hard part.
The strange thing is that the arithmetic is old, public, and short. The effective-area formula, the pitch correction, and the capacity-per-inch tables have been in sheet-metal and drainage references for generations: area times rainfall intensity gives the flow, the profile’s rated area gives the capacity, and the downspouts are simply the gutter’s exits. What is scarce is not the math — it is the presentation. Calculators that show the formula, let you pick the rainfall that matches your city, and refuse to pretend that a single size fits every roof.
So we built the calculator we wanted to use. The flagship tool takes the roof footprint, pitch, run geometry, and design rainfall as explicit inputs rather than hidden defaults, and answers with a sized system — the smallest standard profile that clears the storm, the downspout count and spacing that keep each run drained, and a near-limit flag when your load lands within 10% of a profile’s ceiling. The BOM, hydraulic check, and shopping list follow the same pattern: printed method, published data, honest limits.
And the site stays the way it started: free, without accounts, with every calculation running locally in your own browser. We have no interest in your roof data — architecturally, we never receive it. The advertising that keeps the lights on sits clearly apart from the tools, and nothing about the answer is gated behind a subscription.
The method, visiblePhoto: UnsplashThe one-sentence version
We put the sizing formula, the published capacity data, and the rainfall assumption on the same screen — so the number you buy against is one you can defend.
A gutter project raises four practical questions — how big the gutter must be, how the downspouts drain it, what fittings and fasteners complete the system, and how the plan turns into a purchase. Each calculator takes one of those and answers it completely, and the answers chain: sizing produces the runs, the runs produce the spouts, the geometry produces the fittings, and the whole system becomes a checklist you can carry to the counter.
The flagship tool takes your roof footprint, pitch, and local design rainfall, then returns the smallest standard gutter profile that clears the storm — 4", 5", 6", K-style or half-round — along with the downspout count, size, and spacing that keep each run drained. Run splitting is built in: a hip roof with four runs is sized run by run, not as one giant number, which is why a big hip roof can still fit 5" gutters on every side.
The calculator applies the pitch factor on top of the horizontal footprint — about 11.8% at 6:12 and 20.2% at 8:12 — because steep roofs shed water faster and collect wind-driven rain. Enter the sloped surface by mistake and the tool catches the double-count; enter an upper roof or a tall adjacent wall and the vertical-wall area folds into the load exactly the way IPC 1106 treats it.
Beyond the metal: elbows, outlet tubes, end caps, inside and outside miters, hidden hangers, and downspout straps — counted from your corners, eave height, wall offset, and ground extension. Hanger and strap spacing tightens automatically for your climate zone: 24" on center in standard regions, down to 12" in extreme snow country and 16" with 4-ft strapping in the Florida HVHZ.
A SMACNA-style verification pass checks storm flow in gpm against the gutter's cross-section and the downspouts' total open area — the same ~100 sq ft per in² rule the capacity tables use, applied to your real numbers. Then everything converts to a checkable purchase list in standard 10-ft sections, elbows, and hangers, with a waste factor applied and a one-tap PDF export for the supplier or contractor.
The same arithmetic serves very different decisions — a purchase, a bid, an inspection note, a lesson. These are the people we had in mind while building it; if you recognize yourself in one of them, start with the tool that answers your question.
The most common visitor arrives with a quote in one hand and a suspicion in the other. Enter the roof and the local rainfall, get the size, the downspout count, and a realistic parts list — then read any contractor's bid with a defensible number of your own beside it.
Weekend warriors hanging their own sectional aluminum get the full picture: which profile clears the storm, where each downspout lands, how far apart the hangers go in their climate, and how many 10-ft sections, elbows, and miters to load into the truck — with the waste factor already counted.
Bid preparation is arithmetic under time pressure. The calculator turns a roof sketch and a rainfall figure into a sized system and a complete BOM in minutes — useful for double-checking a supplier's takeoff, pricing a seamless run, or explaining to a homeowner why 6" is the honest answer on that one long run.
Roofline changes ripple: a new addition adds catchment, a steeper pitch moves the numbers, a removed downspout overloads its neighbors. Re-run the sizing on the modified geometry before the old assumptions get built in — the run-splitting view shows exactly which eave now carries more than it did.
Undersized gutters, missing spouts, and hangers spaced for a milder climate all leave signatures. The calculator's capacity tables and climate-zone spacing rules give inspectors and insurance adjusters a published basis for what a system should have been — useful language for a report or a claim file.
Every formula is printed on the page, every capacity constant is in a visible table, and the worked example can be re-derived by hand. Change the design rainfall and watch the downspout count jump — drainage stops being an abstraction and becomes a decision you can defend.
Six commitments that shape every page on the site — the same commitments we would want from a tool we used on the eave.
Every number the tools produce can be re-derived on paper. The effective-area formula, the pitch factor, the ~100 sq ft per in² downspout rule, and the flow check in gpm are written out on the pages where they are used — a calculator that hides its method is a black box, and black boxes do not belong on a roof.
Capacity planning values follow the published 1 in/hr tables for standard K-style and half-round profiles, downspout capacity follows the same basis, vertical high-wall area is treated the way IPC 1106 treats it, and hanger spacing follows IRC and FBC climate-zone practice. Design rainfall points to NOAA Atlas 14, not to a guess.
There is no account, no server round-trip, and no saved history on our side. Every calculation runs locally on each keystroke — which is also why results appear instantly and why the tools keep working offline once the page has loaded. Close the tab and the inputs are gone.
Rainfall intensity varies inside a single metro area, and a gutter sized for a 2 in/hr storm will overflow in the one that arrives at 4. Rather than presenting one number as truth, the tool flags near-limit sizing — when your load lands within 10% of a profile's ceiling — so the uncertainty is part of the answer instead of a footnote.
No pro tier, no paywall in front of the real answer, no sign-up step between you and the calculation. The site is funded by clearly separated advertising; the calculators themselves are not the product being sold, and no result is ever gated behind an upsell.
Defaults that match common practice, a unit toggle that converts every field together, a scaled roofline tracer that measures the eaves for you, and a worked example beside the method — the tools are written to be understood on first read, not decoded.
Every constant and table the calculators read from traces to a published source. These are the sources, what each one governs, and where it shows up in the tools.
The point rainfall values behind every in/hr figure. The tool ships presets for 36 US cities and tells you where to verify the exact value for your own address — because intensity varies within a metro area.
The published planning tables at 1 in/hr of rainfall — a 5" K-style gutter handles roughly 5,520 sq ft, a 3×4" downspout about 1,200 sq ft — that drive the sizing engine and the hydraulic check.
The treatment of tall adjacent walls and upper roofs that drain onto a lower catchment — the classic way an 'adequate' gutter ends up undersized. The vertical-wall input exists for exactly this case.
Standard 24" on-center hanger spacing, tightened to 12" in extreme snow country — because the fastening fails before the gutter does when roof loads pile up.
16" hanger spacing with 4-ft strap reinforcement in Florida's High-Velocity Hurricane Zone — selected automatically when you pick a hurricane climate, because 140 mph gusts do not negotiate with 24" spacing.
Gutter Calculator is not affiliated with, endorsed by, or sponsored by any standards body, government agency, or gutter manufacturer. Standards are referenced for identification and educational purposes — for work governed by a permit or a specification, consult the current edition of the standard, the local building department, and the manufacturer’s published data.
Three honest comparisons — against the calculators that hide their method, against the contractor habits every trade relies on, and against the folklore that fills the gap between them.
Plenty of online calculators return a number and nothing else. Here, the sizing formula sits beside the inputs, the capacity constants are in a visible table, and the worked example can be followed line by line. When you can see the method, you can audit it — and an auditable answer is the only kind worth trusting on a roof that has to shed the next storm.
A good installer carries years of rule-of-thumb — one downspout per 30 feet, 5" K-style on most homes — and for most houses that habit is right. The calculator's job is different: show you which assumption moves the number. When a quote disagrees with a calculation, compare the design rainfall and the run geometry first; the arithmetic is almost never the culprit.
Forum advice — '6-inch everything', 'gutters are all the same', 'one spout is fine' — ignores that drainage is a chain: the roof sheds, the gutter carries, the downspouts discharge, the grade moves it away. A bigger gutter cannot rescue too few spouts, and perfect sizing cannot rescue a clogged one. Numbers with their assumptions attached beat anecdotes every time.
The toolset grows from what users ask for at the estimate bench. Three directions are active right now.
Long-form guides on rainfall intensity, K-style vs. half-round, downspout routing for complex rooflines, and climate-zone fastening are on the workbench — each one written to end at a number you can enter somewhere.
A leaf-guard capacity helper, ice-dam exposure screening, and a drainage extension planner are the requests that come in most often from the estimate bench.
More city presets, regional intensity notes, and clearer guidance on picking a design storm for the roof you actually have — 36 cities today, growing from what users ask for.
Have a request? The fastest way to see a tool built is to describe the problem you were trying to solve — the contact page routes requests straight to the work queue.
The calculators live on the homepage as one connected workflow — sizing feeds the BOM, the BOM feeds the list. These are the doors into it.
A note on responsibility
The calculators are planning aids for sizing, comparing, and learning — not a substitute for a site-specific design, a permit review, or a qualified installer’s judgment. For work governed by local code, verify the design rainfall and the fastening requirements where you build. See our full disclaimer for details.
Questions about the site, the data, or the method? Write to us — every message is read.