Downspout Sizing Calculator

Enter roof area and rainfall intensity to select round or rectangular downspout sizes from IPC sizing tables.

Calculate Downspout Size

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Roof Drainage Area

Roof Area

Horizontal projection area draining to this downspout

Rainfall Intensity

in/hr

IPC 2021 §1106.1: 100-year, 1-hour rate for your site (Figures 1106.1(1)–(5)) or approved local data

Downspout Properties

Downspout Shape

Cross-sectional shape of the downspout

Material

Affects Manning's roughness coefficient

Diameter (optional)

Leave blank to size automatically, or enter an existing downspout to check it

Check a common size:

Downspout Length

For head loss calculation

Number of Elbows

90-degree elbows in the run

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.