# Downspout Sizing Guide: 2×3 vs. 3×4 for Kansas City Homes

*Premier Gutters KC — premiergutterskc.com — 816-469-9563 — Published 2026-07-11*

Most Kansas City homes were built with the smallest downspouts money could buy. Here's the actual math on what size your roof needs — and the 3x capacity upgrade that costs almost nothing at install time.

## The three sizes
- 2x3 rectangular (builder standard, 5" gutters): ~30-40 GPM
- 3x4 rectangular (KC seamless standard): ~110-145 GPM — about 3x a 2x3
- Round 3"-4": historic/copper systems; 4" round outcarries a 3x4

## KC sizing math
- Use the roof's HORIZONTAL FOOTPRINT, not sloped surface area.
- GPM ≈ roof sq ft x rainfall intensity (in/hr) x 0.0104. KC 10-yr, 5-min burst ≈ 7 in/hr (verify per NOAA Atlas 14).
- Example: 700 sq ft per downspout x 7 x 0.0104 ≈ 51 GPM → exceeds a 2x3 (30-40 GPM); a 3x4 handles it.

## Capacity per downspout (KC 10-yr burst ~7 in/hr | conservative 100-yr ~9.5 in/hr)
- 2x3 rectangular: ~450-550 sq ft | ~300-400 sq ft
- 3" round: ~1,300 sq ft | ~900-1,000 sq ft
- 3x4 rectangular: ~1,600 sq ft | ~1,100-1,200 sq ft
- 4" round: ~2,600 sq ft | ~1,800-2,000 sq ft

## Placement rules
- One downspout per 30-40 ft of gutter run; outlets near valleys; discharge 6-10 ft from the foundation (KC clay).

## Signs yours are undersized
Overflow mid-run when clean; water overshooting at valleys; gulping/backup at the elbow; tiger striping; mulch blowouts; basement damp after intense-but-short rains.

## Upsizing
Cheapest at install/replacement time (3x4 standard on 6" systems). Problem runs can take a larger outlet + 3x4 retrofit. Match buried drain lines to the new flow.

Free written quote: https://premiergutterskc.com/schedule/ — or call/text 816-469-9563.