Gutter Size Calculator
This free gutter size calculator sizes gutters and downspouts from the roof area draining to the gutter and your local rainfall intensity, not a rule of thumb. It reads the answer straight from the plumbing code: 2021 UPC Table 1103.3, which lists the roof area each gutter size drains at a given slope and rainfall, or 2021 IPC Table 1106.6, which lists gutter capacity in gallons per minute. It is different from a materials take-off: this tells you what size to buy, not how many feet.
It also sizes the downspouts from UPC Table 1103.1 or IPC Table 1106.3, because an undersized downspout is what actually makes a gutter overflow. A standard 2×3 downspout is credited with 30 gallons per minute, a 3×4 with 92 — so at 3 inches per hour of rain a 2×3 drains about 960 square feet of roof and a 3×4 about 2,950.
Pick your city from the 217 in IPC Appendix B for its 100-year, 1-hour rainfall rate, or enter your own. Two things to know: both code tables are written for half-round gutters, so a K-style gutter is sized as a half-round of the same width; and the IPC table credits a gutter with roughly twice the roof area the UPC table does, so choose the code your area adopted. Sizing only — no pricing, no signup.
Gutter Size Calculator
Find the right gutter size (5", 6", or larger) and how many downspouts you need, from your roof drainage area and local rainfall — straight from the UPC or IPC gutter and leader tables. Free, no signup.
Roof draining to the gutter
The roof area that sheds into THIS gutter run — usually one side of the roof, not the whole footprint.
Use the horizontal (footprint) area, not the sloped surface — both codes size from projected roof area, with no pitch factor. Add any vertical wall that sheds onto this roof (half its area counts). The UPC and IPC tables give different answers; pick the code your area adopted.
Rainfall
Local rain intensity sets how much roof a gutter can drain.
Gutter & downspouts
Slope and profile change gutter capacity; pick a downspout size to size the count.
Steeper slope adds capacity. Neither code has a K-style table, so K-style gutters are sized as a half-round of the same width. Enter the run length to limit each downspout to 50 ft of gutter (Copper Development Association handbook; neither code sets a spacing).
Size it to the roof, then split the run
The gutter's cross-section sets how much roof it can drain. A 6" K-style carries far more than a 5" — so a bigger roof or a wetter climate steps you up a size. Divide the capacity below by your local rainfall intensity to get the real roof area it handles.
Gutters aren't hung level — they pitch to the downspouts. A long run crowns at a high point in the middle and falls both ways to a downspout at each end, with no downspout draining more than 50 feet of gutter (Copper Development Association handbook).
Calculation Formulas
Both codes size from the horizontally projected roof area draining to the gutter, plus half of a single vertical wall that sheds onto that roof (UPC 1103.4; IPC 1106.4). Neither code applies a roof-pitch factor.
Example:
1,000 sq ft of roof + ½ × 200 sq ft of wall = 1,100 sq ft.
R is the design rainfall in inches per hour and A the drainage area in square feet. The IPC's gutter and leader tables are in gallons per minute.
Example:
1,100 sq ft × 3.0 in/hr × 0.0104 ≈ 34 gpm.
UPC Table 1103.3 lists the roof area each semicircular gutter drains at 2–6 in/h for slopes of 1/16" to 1/2" per foot. The 1 in/h value is double the 2 in/h value; divide by your rainfall.
Example:
5" at 1/8" per foot: 1,760 × 2 = 3,520 ÷ 3.0 in/hr = 1,173 sq ft.
IPC Table 1106.6 lists half-round gutters at 1/8" per foot (4", 5", 6", 8", 10") and 1/16" per foot (8", 10" only). The calculator never credits a steeper slope than the table lists.
Example:
5" at 1/8" per foot: 74 gpm ÷ (3.0 × 0.0104) = 2,372 sq ft.
Leader capacity comes from UPC Table 1103.1 or IPC Table 1106.3 (same values). A rectangular downspout is credited with the largest listed leader it fully contains.
Example:
3×4 = 92 gpm → 2,949 sq ft at 3.0 in/hr, so 1,100 sq ft needs 1.
Neither code sets a spacing limit. The Copper Development Association handbook limits each downspout to 50 ft of gutter; applied only when you enter a run length.
Example:
A 60 ft run gets at least 2 downspouts.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| 5" half-round, UPC | 2,500 / 3,520 / 5,000 / 7,080 sq ft | At 1 in/h for 1/16" / 1/8" / 1/4" / 1/2" per foot (UPC Table 1103.3). Divide by your rainfall. |
| 6" half-round, UPC | 3,840 / 5,440 / 7,680 / 11,080 sq ft | Same slopes at 1 in/h (UPC Table 1103.3). |
| 5" / 6" half-round, IPC | 74 / 110 gpm | At 1/8" per foot (IPC Table 1106.6) — about twice the UPC area for the same gutter. |
| 2×3 downspout | 30 gpm | Credited as a 2" round leader (UPC Table 1103.1 / IPC Table 1106.3): 2,880 sq ft at 1 in/hr, 960 sq ft at 3 in/hr. |
| 3×4 downspout | 92 gpm | Credited as a 3" round leader (it is smaller than a 4" round): 8,800 sq ft at 1 in/hr. |
| Area to flow | 0.0104 gpm per sq ft per in/hr | IPC Equation 11-1. |
| Rainfall basis | 100-year, 1-hour | IPC 1106.1; city values from IPC Appendix B Table B101 (informative). |
| Wall allowance | 50% of one wall | UPC 1103.4 and IPC 1106.4. UPC gives other percentages for two or three walls. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
Uniform Plumbing Code 2021 — Storm Drainage(UPC Tables 1103.1 & 1103.3; Section 1103.4)
View StandardTable 1103.3 sizes semicircular gutters by roof area at 1/16", 1/8", 1/4" and 1/2" per foot slope; Table 1103.1 sizes leaders; 1103.4 adds wall area.
Key Requirements:
- •Size gutters on horizontal projected roof area
- •Table values are for semicircular gutters
- •Rectangular leaders need at least the area of the equivalent round leader, sides no more than 3:1
- •Add 50% of one wall draining onto the roof
International Plumbing Code 2021 — Storm Drainage(IPC Tables 1106.3 & 1106.6; Equation 11-1; Section 1106.4; Appendix B)
View StandardConverts rainfall to flow with GPM = R × A × 0.0104, then limits gutters (Table 1106.6) and vertical leaders (Table 1106.3) by gpm. Design rainfall is the 100-year, 1-hour rate.
Key Requirements:
- •Flow through gutters shall not exceed Table 1106.6
- •Flow through vertical leaders shall not exceed Table 1106.3
- •Add one-half of vertical wall area diverting to the roof
- •100-year, 1-hour rainfall from Figures 1106.1(1)–(5) or approved local data
International Residential Code 2021(IRC R903.4)
View StandardRequires roof drainage for one- and two-family dwellings but does not size gutters numerically, so local officials often defer to the plumbing code tables above.
Key Requirements:
- •Roofs shall be drained
- •Gutter size not specified
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Local Rainfall Intensity
The biggest single driver
Design rainfall is the 100-year, 1-hour rate. IPC Appendix B lists 217 cities, from 0.6 in/hr in Juneau to 6.2 in/hr in Hilo.
Regional Examples:
UPC vs. IPC Tables
Same roof, different answer
The IPC's 2021 gallon-per-minute table credits a half-round gutter with roughly twice the roof area that the UPC's area table does. A 1,000 sq ft roof at 3.0 in/hr and 1/8" slope gets a 5" gutter under the UPC and a 4" gutter under the IPC.
Regional Examples:
K-style Gutters
Not in either code table
Both codes tabulate semicircular gutters. K-style is the most common residential profile, but no code table lists it, so it is sized here as a half-round of the same nominal width.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
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How to Use This Calculator
- Enter the roof area that drains to the gutter run you're sizing, measured as its horizontal footprint — usually one side of the roof (or enter that roof plane's length × width).
- Pick your plumbing code (UPC or IPC) and add any vertical wall that sheds onto the roof — half its area counts.
- Choose your city for its 100-year, 1-hour rainfall rate from IPC Appendix B, or enter your own rate from NOAA Atlas 14.
- Pick your gutter slope, profile (K-style or half-round) and downspout size.
- Optionally enter the total gutter run length to limit each downspout to 50 ft of gutter (Copper Development Association handbook).
- Click Calculate for the recommended gutter size, the code-table capacity of each size at your conditions, and the downspout count.
Why Roof Area and Rainfall Decide the Size
A gutter has a fixed capacity, and the two things that fill it are how much roof drains into it and how hard it rains. A 1,000-square-foot roof in New Orleans (4.8 inches per hour in IPC Appendix B) sends about 50 gallons per minute into its gutter; the same roof in Seattle (1.4 inches per hour) sends about 15. In 2021 UPC Table 1103.3, a 5-inch half-round gutter at 1/8 inch per foot drains 3,520 square feet at 1 inch per hour, and a 6-inch drains 5,440 — divide by your rainfall for the real limit. Slope helps: the same 5-inch gutter drains 5,000 square feet at 1/4 inch per foot. Neither code uses a roof-pitch factor; both size from the horizontal footprint plus half of any wall that sheds onto the roof. The most common mistake is defaulting to 5-inch gutters and 2×3 downspouts on every house regardless of roof area or rainfall.
Frequently Asked Questions
What size gutters do I need?
It depends on the roof area draining to that gutter and your local rainfall, not the size of the house. Take the horizontal footprint of the roof feeding the gutter (usually one side), add half of any wall that sheds onto it, and compare with the code table at your rainfall. In 2021 UPC Table 1103.3 at 1/8 inch per foot, a 5-inch gutter drains 3,520 square feet at 1 inch per hour — so about 1,170 square feet in Chicago (3.0 inches per hour) and about 730 in New Orleans (4.8). Pick the smallest size that covers your area.
5-inch vs 6-inch gutters — which do I need?
Compare your drainage area with each size's capacity at your rainfall. Under 2021 UPC Table 1103.3 at 1/8 inch per foot and 3 inches per hour of rain, a 5-inch gutter handles about 1,170 square feet and a 6-inch about 1,810 — roughly 55% more. Under 2021 IPC Table 1106.6 the same gutters carry 74 and 110 gallons per minute, about 2,370 and 3,530 square feet. If your roof area falls between the two, the larger gutter or an extra downspout splitting the run is the fix.
How many downspouts do I need?
Divide the drainage area by what one downspout can carry at your rainfall, and round up. UPC Table 1103.1 and IPC Table 1106.3 give the same leader capacities: a 2×3 downspout is credited as a 2-inch round leader (30 gallons per minute) and a 3×4 as a 3-inch round leader (92 gallons per minute). At 3 inches per hour that is about 960 and 2,950 square feet. Neither code limits spacing; the Copper Development Association handbook limits each downspout to 50 feet of gutter, so take the larger of the two counts.
How do I figure the roof area for gutter sizing?
Use the horizontally projected (footprint) area that drains to the gutter you're sizing — usually one slope of the roof, not the whole building — and don't use the sloped surface area. Both the UPC and IPC size from projected area and neither applies a roof-pitch factor. If a vertical wall sheds rain onto that roof, add half of the wall's area (UPC 1103.4; IPC 1106.4).
What rainfall rate should I use for gutter sizing?
The 100-year, 1-hour rainfall rate for your location (IPC 1106.1). IPC Appendix B lists 217 cities — 4.8 inches per hour in New Orleans, 3.0 in Chicago, 2.4 in Denver, 1.4 in Seattle, up to 6.2 in Hilo. If your town isn't listed, use the NOAA Atlas 14 precipitation frequency tool (PFDS) or the value your building department requires.
Does gutter slope affect what size I need?
Yes — a steeper gutter carries more water. In 2021 UPC Table 1103.3 a 5-inch gutter drains 2,500 square feet at 1 inch per hour when sloped 1/16 inch per foot, 3,520 at 1/8, 5,000 at 1/4 and 7,080 at 1/2. The 2021 IPC table lists half-round gutters only at 1/8 inch per foot (and 1/16 for 8- and 10-inch), so the calculator never credits a steeper slope than the IPC table shows.
Why do the UPC and IPC give different gutter sizes?
The UPC still uses an area-based table, while the 2021 IPC table is in gallons per minute and credits a half-round gutter with roughly twice the roof area. A 1,000-square-foot roof at 3 inches per hour and 1/8 inch per foot gets a 5-inch gutter under UPC Table 1103.3 and a 4-inch gutter under IPC Table 1106.6. Use the code your jurisdiction adopted; one- and two-family dwellings under the IRC (R903.4) aren't given a size, so ask your building department which table it accepts.
Is this the same as the gutters materials calculator?
No. This gutter size calculator tells you what size gutter and how many downspouts to use, based on your roof and rainfall — a sizing question. The separate gutters calculator is a materials take-off: once you know the size, it estimates the linear feet, hangers, elbows, end caps and sealant to buy.