Downspout Sizing Calculator

Calculate required downspout size based on roof area and rainfall intensity. Uses IPC tables for round and rectangular downspout sizing. Free professional-grade drainage calculator.

Calculate Downspout Size

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

Input Parameters

Roof Drainage Area

sf

Horizontal projection area draining to this downspout

in/hr

Design rainfall intensity for your location

Downspout Properties

Cross-sectional shape of the downspout

Affects Manning's roughness coefficient

in

Leave blank to size automatically

ft

For head loss calculation

90-degree elbows in the run

years

Design storm frequency (years)

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 - Uses code tables relating roof area, rainfall, and downspout size
  • Effective Area - Roof area adjusted for design rainfall intensity
  • Safety Factor - Always verify capacity ratio is greater than 1.0

Round Downspout Capacity (IPC Table 1106.2)

Diameter (in)Max Roof Area (sf)Flow Capacity (gpm)
254423
31,61067
43,460144
56,280261
610,200425

Based on 1 in/hr rainfall rate. Adjust for actual design intensity.

Rectangular Downspout Capacity (IPC Table 1106.3)

Size (W x D)Max Roof Area (sf)Flow Capacity (gpm)
2" x 3"1,02042
2" x 4"1,76073
3" x 4"3,400142
3" x 5"5,000208
4" x 5"7,960332
4" x 6"10,750448

Based on 1 in/hr rainfall rate. Source: IPC 2021, Table 1106.3
Powered by DrainageCalculators.com — free drainage & stormwater calculators