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Calculate Downspout Size
For educational purposes only. Not a substitute for professional engineering judgment.
Downspout Sizing Overview
Downspouts convey collected rainwater from gutters to ground level or the storm drainage system. Proper sizing ensures the downspout can handle peak rainfall without backing up into the gutter system.
- IPC Method - Converts roof runoff to gpm and checks listed leaders in 2021 IPC Table 1106.3
- Design Storm - IPC 2021 §1106.1 uses the 100-year, 1-hour rainfall rate, not a 10-year storm
- Effective Area - Roof area adjusted for design rainfall intensity
- Safety Factor - Always verify capacity ratio is greater than 1.0
Round Leader Capacity (2021 IPC Table 1106.3)
| Diameter (in) | Derived Area @ 1 in/hr (sf) | Flow Capacity (gpm) |
|---|---|---|
| 2 | 2,886.9 | 30 |
| 2.5 | 5,196.42 | 54 |
| 3 | 8,853.16 | 92 |
| 4 | 18,476.16 | 192 |
| 5 | 34,642.8 | 360 |
| 6 | 54,177.49 | 563 |
| 8 | 116,245.84 | 1208 |
Gpm capacities: 2021 IPC Table 1106.3. Areas are derived at 1 in/hr, not copied code values.
Rectangular and Square Leaders (2021 IPC Table 1106.3)
| Size (W x D) | Derived Area @ 1 in/hr (sf) | Flow Capacity (gpm) |
|---|---|---|
| 2" x 2" | 2,886.9 | 30 |
| 1.5" x 2.5" | 2,886.9 | 30 |
| 2.5" x 2.5" | 5,196.42 | 54 |
| 2" x 4" | 8,853.16 | 92 |
| 2.5" x 3" | 8,853.16 | 92 |
| 3" x 4.25" | 18,476.16 | 192 |
| 3.5" x 4" | 18,476.16 | 192 |
| 4" x 5" | 34,642.8 | 360 |
| 4.5" x 4.5" | 34,642.8 | 360 |
| 5" x 6" | 54,177.49 | 563 |
| 5.5" x 5.5" | 54,177.49 | 563 |
| 6" x 8" | 116,245.84 | 1208 |
Gpm capacities: 2021 IPC Table 1106.3. Areas are derived at 1 in/hr.
Standard downspout sizes and capacities
Common US downspouts are 2 × 3 in and 3 × 4 in rectangular, and 2 to 6 in round. The table rates each against IPC 2021 Table 1106.3 (vertical leaders) and converts that flow to the horizontally projected roof area one downspout can drain: area (sq ft) = gpm × 96.23 ÷ rainfall (in/hr).
| Downspout size | Table 1106.3 row | Capacity (gpm) | Roof area at 1 in/hr (sq ft) | 2 in/hr | 3 in/hr | 4 in/hr |
|---|---|---|---|---|---|---|
| 2 × 3 in rectangular (nominal) | Rated as 2 × 2 in | 30 | 2,887 | 1,443 | 962 | 722 |
| 3 × 4 in rectangular (nominal) | Rated as 2 × 4 in | 92 | 8,853 | 4,427 | 2,951 | 2,213 |
| 2 in round | Listed | 30 | 2,887 | 1,443 | 962 | 722 |
| 3 in round | Listed | 92 | 8,853 | 4,427 | 2,951 | 2,213 |
| 4 in round | Listed | 192 | 18,476 | 9,238 | 6,159 | 4,619 |
| 5 in round | Listed | 360 | 34,643 | 17,321 | 11,548 | 8,661 |
| 6 in round | Listed | 563 | 54,177 | 27,089 | 18,059 | 13,544 |
Listed sizes appear in Table 1106.3. Rated as marks a size that does not, rated at the listed row shown. ≈ marks a size within 2% of a listed row.
2 × 3 and 3 × 4 are not listed in Table 1106.3. The calculator rates an unlisted size at the largest listed leader that fits inside it (2 × 2 in for a 2 × 3; 2 × 4 in for a 3 × 4), which is conservative. The next listed sizes that enclose them are 2½ × 3 in (92 gpm) and 3 × 4¼ in (192 gpm). For code work use a listed size or the manufacturer's rated capacity.
These are leader capacities only. The gutter and its outlet can limit flow first, so check the gutter with the Gutter Flow Calculator.
Metric downpipe sizes
The same rating for common metric downpipes, entered in millimetres. Area (m²) = flow (L/min) × 60 ÷ rainfall (mm/h).
| Downpipe size | Table 1106.3 row | Capacity (L/s) | Roof area at 25 mm/h (m²) | 50 mm/h | 75 mm/h | 100 mm/h |
|---|---|---|---|---|---|---|
| 68 mm round (UK standard) | Rated as 2.5 in round | 3.4 | 491 | 245 | 164 | 123 |
| 75 mm round | ≈ 3 in round | 5.8 | 836 | 418 | 279 | 209 |
| 80 mm round | Rated as 3 in round | 5.8 | 836 | 418 | 279 | 209 |
| 100 mm round | ≈ 4 in round | 12.1 | 1,744 | 872 | 581 | 436 |
| 65 mm square (UK) | Rated as 2.5 × 2.5 in | 3.4 | 491 | 245 | 164 | 123 |
| 100 × 75 mm rectangular | Rated as 2.5 × 3 in | 5.8 | 836 | 418 | 279 | 209 |
Metric sizes within 2% of a listed inch size (for example 75 mm ≈ 3 in, 100 mm ≈ 4 in) use that row. UK downpipes are designed to BS EN 12056-3 through Approved Document H (section H3), which sizes the gutter, its outlet and the downpipe together; see the Part H drainage guide. Source: ICC, 2021 International Plumbing Code, §1106 and Table 1106.3 (1 gpm = 3.785 L/min).
Worked example: sizing one downspout
A 1,200 sq ft roof section (horizontal projection) drains to one downspout. The 100-year, 1-hour rainfall rate for the site, read from IPC Figure 1106.1 or approved local data, is 3 in/hr.
- Peak flow. Q = A × i ÷ 96.23 = 1,200 × 3 ÷ 96.23 = 37.4 gpm.
- Check a 2 × 3 in downspout. Rated 30 gpm (not listed; rated as the 2 × 2 in row). 30 ÷ 37.4 = 0.80, so one 2 × 3 is too small.
- Check a 3 × 4 in downspout. Rated 92 gpm. 92 ÷ 37.4 = 2.46, so one 3 × 4 is adequate.
- Or split the roof. Two 2 × 3 downspouts draining 600 sq ft each carry 18.7 gpm apiece, under their 30 gpm rating.
- Smallest listed sizes that work: 2½ in round or 2½ × 2½ in square (54 gpm), or 2 × 4 in rectangular (92 gpm).
Open this example in the calculator to change the roof area, rainfall or downspout size.
How many downspouts do I need?
Divide the total peak flow by the capacity of the downspout you plan to use, and round up: number of downspouts = (roof area × rainfall ÷ 96.23) ÷ downspout gpm. A 2,400 sq ft roof at 3 in/hr produces 74.8 gpm, so it needs three 2 × 3 downspouts (30 gpm each) or one 3 × 4 (92 gpm).
Then check the layout: each downspout should drain no more roof than its share, measured as the horizontal projection plus half of any wall that sheds rain onto the roof (IPC 1106.4). Long gutter runs and where the roof valleys discharge also affect how many outlets you need, so size the gutter itself with the Gutter Flow Calculator.
How to measure a downspout
- Rectangular: measure the two outside faces of a straight section, not the crimped end or an elbow. Enter the two sides in either order (2 × 3 is the same as 3 × 2). US 2 × 3 and 3 × 4 are nominal sizes and the bodies often measure a little less, around 1¾ × 2¾ in and 2¾ × 3¾ in; either set of numbers gives the same rating here.
- Round: measure the outside diameter, or wrap a tape around the pipe and divide the circumference by 3.14.
- Metric: switch the calculator to SI units and enter millimetres (for example 68 mm round or 65 mm square downpipe).
- Below ground: the drain line a downspout feeds is usually 3 in or 4 in pipe. Size it as a sloped pipe with the Manning's pipe calculator, not as a leader.
About Downspout Sizing
Downspouts (also called leaders or conductors, and downpipes in the UK and Australia) convey rainwater from gutters to ground level or the storm drainage system. Proper sizing ensures the downspout can handle peak rainfall without causing gutter overflow or backup.
IPC Sizing Method
The 2021 International Plumbing Code sizes vertical leaders by flow: convert the horizontal projected roof area and the design rainfall rate to gpm, then choose a leader whose Table 1106.3 capacity is at least that flow. The table lists:
- Round leaders - 2, 2½, 3, 4, 5, 6 and 8 in diameter (30 to 1,208 gpm)
- Rectangular and square leaders - 2×2, 1½×2½, 2½×2½, 2×4, 2½×3, 3×4¼, 3½×4, 4×5, 4½×4½, 5×6, 5½×5½ and 6×8 in
- Unlisted sizes - the common 2×3 and 3×4 in are not in the table; this calculator rates them at the largest listed leader that fits inside (30 and 92 gpm)
In US houses the rectangular 2×3 and 3×4 in sizes are the usual stock with K-style gutters; round downspouts are common with half-round gutters, on commercial and historic buildings, and as metric downpipes.
Key Design Parameters
- Roof Area - Horizontal projection of the roof draining to this downspout, plus half the area of any vertical wall that diverts rain onto it (IPC 1106.4)
- Rainfall Intensity - IPC 2021 §1106.1: the 100-year, 1-hour rate from Figures 1106.1(1)–(5), or approved local weather data
- Effective Area - Roof area adjusted for rainfall intensity relative to the reference 1 in/hr rate
Flow Calculation
Required flow rate is calculated from the roof area and rainfall intensity:
Q = (A × i) / 96.23
Where:
- Q = Flow rate (gpm)
- A = Roof area (sf)
- i = Rainfall intensity (in/hr)
- 96.23 = Conversion factor
Design Considerations
- Minimum Size - The smallest leaders in Table 1106.3 are 2 in round, 2×2 in and 1½×2½ in (30 gpm)
- Elbows - The optional friction estimate adds 2 ft (round) or 2.5 ft (rectangular) of equivalent pipe length per 90-degree elbow; this is a screening assumption, not an IPC value
- Outlets - Ensure adequate capacity at discharge point
- Multiple Downspouts - Consider when roof area exceeds largest size capacity
- Material - Affects friction (Manning's n) but primarily aesthetic choice
Frequently asked questions
What size downspout do I need?
Work out the peak flow, then pick a downspout that carries it. Flow (gpm) = roof area (sq ft) × rainfall rate (in/hr) ÷ 96.23, using the 100-year, 1-hour rate the International Plumbing Code requires. Compare that flow with the leader capacities in IPC 2021 Table 1106.3: 2 in round 30 gpm, 3 in round 92 gpm, 4 in round 192 gpm. This calculator rates a 2×3 in rectangular downspout at 30 gpm and a 3×4 in at 92 gpm. For example, 1,200 sq ft at 3 in/hr is 37.4 gpm, so use one 3×4 or split the roof between two 2×3 downspouts.
Are downspouts 2x3 or 3x4?
Both are standard US sizes. 2×3 in is the usual size on 5 in K-style gutters; 3×4 in is used with 6 in gutters, on larger or steeper roof sections, and where fewer downspouts are wanted. Neither size is listed in IPC 2021 Table 1106.3, so this calculator rates each at the largest listed leader that fits inside it: 30 gpm for a 2×3 and 92 gpm for a 3×4, about three times as much.
How much roof area can one downspout drain?
Roof area served (sq ft) = downspout capacity (gpm) × 96.23 ÷ rainfall rate (in/hr), measured as the horizontal projection of the roof. At 3 in/hr a 2×3 downspout (30 gpm) drains about 960 sq ft, a 3×4 (92 gpm) about 2,950 sq ft and a 4 in round (192 gpm) about 6,160 sq ft. At 4 in/hr those fall to about 720, 2,210 and 4,620 sq ft. Add half the area of any wall that sheds rain onto the roof (IPC 1106.4).
What rainfall intensity should I use to size downspouts?
IPC 2021 §1106.1 sizes vertical leaders for the 100-year, 1-hour rainfall rate shown in Figures 1106.1(1) through 1106.1(5), or for rainfall rates from approved local weather data. A 10-year storm is not the code basis. A 1-hour rainfall depth in inches is the same number as the average rate in in/hr, so where your building official accepts NOAA Atlas 14, its 100-year, 60-minute depth can be entered directly.
What size pipe should a downspout drain into?
Adapters commonly connect a 2×3 downspout to 3 in pipe and a 3×4 downspout to 4 in pipe. Check the buried line as a sloped pipe, not as a leader: at 1% slope a smooth-wall 3 in pipe flowing full carries roughly 47 to 52 gpm and a 4 in pipe roughly 100 to 110 gpm (Manning, n = 0.010 to 0.011), and corrugated pipe carries less. A 4 in line is the safer choice for a 3×4 downspout or for several downspouts joined together; use the Manning's pipe calculator for your actual slope and material.
What is the difference between a downspout, a leader and a downpipe?
They are the same thing: the vertical pipe that carries roof water down from the gutter. The IPC calls an exterior one a leader and an interior one a conductor; downpipe and rainwater pipe are the UK, Irish and Australian terms. In the UK, downpipes are sized to BS EN 12056-3 through Approved Document H (section H3) rather than the IPC, so treat the metric IPC ratings on this page as a comparison.
Reference & standards
Rainfall Intensity Data
Look up design rainfall intensity for your location.
IPC Drainage Tables
Selected 2021 IPC tables (pipe, round leader, gutter) in gpm.
Roof Drainage Design Guide
Learn the fundamentals of roof drainage system design.
Downspout Disconnection Guide
Disconnect downspouts from storm drains and send roof runoff away from the foundation.
What This Solves
Sizes downspouts for roof drainage based on roof area, rainfall intensity, and IPC tables for round and rectangular sections.
Best Used When
- You are sizing downspouts for a residential or commercial roof drainage system
- You need to determine the number and size of downspouts for a given roof area
- You want to verify an existing downspout system has adequate capacity for the design rainfall
Do NOT Use When
- You need rainfall demand for roof drains or scuppers on a flat or low-slope roof — Use Roof Rainfall Demand Calculator
- You need to size gutters that collect water before it enters the downspouts — Use Gutter Flow Calculator
Key Assumptions
- Round and rectangular leaders: 2021 IPC Table 1106.3 capacities (gpm); unlisted sizes such as 2x3 and 3x4 in are rated at the largest listed leader that fits inside them
- Rainfall intensity is uniform across the entire roof area
- Elbows and offsets enter only the optional friction estimate (2-2.5 ft equivalent length each), not the Table 1106.3 capacity
- The downspout discharges freely at the bottom (no backpressure)
Input Quality Notes
IPC 2021 §1106.1: use the 100-year, 1-hour rainfall rate (Figures 1106.1(1)-(5)) or approved local data, not a 10-year storm. Add half the area of any vertical wall that diverts rainwater onto the roof (IPC 1106.4).
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Last verified: February 2026