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Gutter Sizing Calculator - Roof Drainage Gutter & Downspout Sizing for Residential & Commercial Buildings

Calculate gutter capacity, downspout count, and roof drainage area per the International Plumbing Code

Size roof gutters and downspouts based on roof drainage area, rainfall intensity, and gutter slope. Enter the roof plan area, local maximum rainfall rate (inches per hour for a 5-year, 5-minute storm), gutter length, and slope to determine the minimum gutter size and number of downspouts required. Supports standard K-style and half-round gutter profiles in 4-inch, 5-inch, 6-inch, and commercial box gutters. Calculates downspout sizing for round and rectangular profiles per IPC Table 1106.2. Includes overflow and secondary drainage considerations for code-compliant commercial roof drainage design.

Pro Tip: Gutter overflow from undersized downspouts causes more water damage than undersized gutters. A 5-inch K-style gutter can handle 1,200 square feet of roof area at a 2-inch-per-hour rainfall, but only if the downspouts can drain it fast enough. A single 2x3-inch rectangular downspout handles approximately 600 square feet of roof area. If your gutter run exceeds 600 square feet per downspout, the gutter backs up and overflows at the fascia even though the gutter cross-section is large enough. Size downspouts first, then verify gutter capacity.

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Roof Drainage & Gutter Sizing Calculator
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How It Works

  1. Calculate Roof Drainage Area

    Measure the horizontal roof plan area draining to each gutter in square feet. For hip and gable roofs, use the horizontal projection, not the sloped surface area. Add vertical wall area that drains onto the roof (half the wall area if the wall is adjacent to the roof).

  2. Determine Rainfall Intensity

    Find your local 5-year, 5-minute rainfall intensity from the building code or local weather data. Typical values range from 2 inches per hour in the Pacific Northwest to 8+ inches per hour in the Gulf Coast. The IPC provides rainfall maps by region.

  3. Set Gutter Parameters

    Enter the gutter length, slope (typically 1/16" to 1/4" per foot), and profile type (K-style or half-round). Steeper slope increases flow capacity but may create aesthetic issues on long runs. A minimum slope of 1/16" per foot prevents standing water.

  4. Review Gutter and Downspout Sizing

    See the recommended gutter size, number and size of downspouts, and the maximum roof area each downspout can serve. The calculator verifies that both gutter capacity and downspout capacity meet the design flow at the specified rainfall intensity.

  5. Check Secondary Drainage

    For commercial flat roofs, verify that secondary (emergency overflow) drainage is provided per IPC Section 1108. The calculator sizes overflow drains or scuppers that prevent roof ponding if the primary drains become blocked.

Built For

  • Roofing contractors selecting gutter sizes for residential re-roofing and gutter replacement projects
  • Home builders sizing gutters and downspouts for new residential construction
  • Architects specifying commercial gutter systems for flat and low-slope roof drainage
  • Property managers evaluating gutter capacity after building additions that increase roof area
  • DIY homeowners determining the correct gutter size for their home improvement project
  • Metal building contractors sizing eave gutters for pre-engineered steel buildings

Features & Capabilities

Gutter Profile Comparison

Compares K-style, half-round, and commercial box gutter capacities at the same slope. K-style gutters hold approximately 30% more water than half-round gutters of the same nominal width due to their flat-bottom cross section.

Downspout Sizing Tables

Sizes downspouts per IPC Table 1106.2 for round (3", 4", 5", 6") and rectangular (2x3", 3x4", 4x5") profiles. Shows the maximum roof area each downspout can serve at the design rainfall intensity.

Rainfall Intensity Lookup

Includes regional rainfall intensity data for major U.S. cities based on IPC Figure 1106.1. If your local intensity differs from the code base of 1 inch per hour, the calculator adjusts the allowable roof area proportionally.

Gutter Slope Calculator

Determines the optimal gutter slope for the run length and shows the total elevation drop from high end to low end. Balances flow capacity (steeper is better) against aesthetics and installation practicality.

Overflow Protection

For commercial applications, calculates secondary drain or scupper sizing per IPC 1108 to handle 100% of the primary drain capacity. Prevents structural roof damage from ponding water if primary drains clog.

Frequently Asked Questions

Calculate the horizontal roof area draining to each gutter run and look up your local rainfall intensity. For most of the U.S. with a 2-4 inch per hour 5-year rainfall rate, a standard 5-inch K-style gutter handles up to 1,200 square feet of roof area per gutter run. A 6-inch K-style gutter handles up to 2,000 square feet. If your roof area exceeds these limits, either upsize the gutter or add a downspout to divide the gutter into shorter runs. Most residential installations use 5-inch K-style gutters.
Each downspout can drain a limited roof area. A standard 2x3-inch rectangular downspout handles approximately 600 square feet at a 4 inch/hour rainfall intensity. A 3x4-inch downspout handles about 1,200 square feet. Space downspouts so that no gutter run exceeds the capacity of its downspout. Maximum recommended gutter run between downspouts is 40 feet for 5-inch gutters and 50 feet for 6-inch gutters. Place downspouts at low points and at corners where two gutter runs meet.
K-style gutters have a flat bottom and a decorative ogee front profile that resembles crown molding. They hold more water per inch of width than half-round gutters and are the standard for residential construction in North America. Half-round gutters are semicircular and are common on historic buildings, European-style architecture, and copper gutter installations. At the same nominal width, a K-style gutter has approximately 30% more cross-sectional area than a half-round. Half-round gutters are easier to clean because debris does not get trapped in corners.
Gutters should slope toward the downspout at a rate of 1/16 inch to 1/4 inch per foot of run. A 1/16 inch per foot slope is the minimum for positive drainage and is adequate for most residential installations. Steeper slopes (1/8" or 1/4" per foot) improve flow capacity and reduce standing water but create a more visible slope over long runs. For a 40-foot gutter run at 1/8" per foot, the total drop is 5 inches, which may be noticeable. For long runs, consider placing the downspout in the center and sloping both directions to reduce the total visible drop.
The most common causes of gutter overflow are: clogged downspouts (restricting flow out of the gutter), insufficient downspout count (gutter can hold the water but cannot drain fast enough), debris buildup in the gutter channel, improperly sloped gutters (flat spots cause backup), and end-to-end splash from steep roof valleys or sharp corners. During heavy rain, check whether the downspout is flowing at full capacity. If the downspout is clear but the gutter still overflows, you either need more downspouts or larger downspouts to increase drainage rate.
Secondary (emergency) overflow drainage prevents structural roof damage from water ponding if the primary roof drains become blocked. The International Building Code (IBC) and IPC Section 1108 require secondary drainage on commercial flat roofs. Secondary drains are typically set 2 inches above the primary drain level and must be sized to handle 100% of the design storm flow independently. Secondary drains can be interior roof drains, scuppers through parapet walls, or overflow roof edges. This requirement does not typically apply to residential steep-slope roofs.
Disclaimer: This calculator provides gutter and downspout sizing estimates based on IPC standards and manufacturer capacity data. Actual performance depends on installation quality, maintenance, gutter slope accuracy, and local rainfall patterns. Commercial roof drainage requires engineering design per the local building code. ToolGrit is not responsible for gutter sizing, water damage, or drainage system performance.

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