Gutter Flow Calculator

Calculate gutter flow capacity using Manning's equation. Analyze K-style, half-round, and box gutters. Determine flow velocity, hydraulic radius, and Froude number. Free professional-grade roof drainage calculator.

Calculate Gutter Flow Capacity

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

Input Parameters

Gutter Properties

Cross-sectional shape of the gutter

Gutter material affects surface roughness

in

Nominal width (K-style/box) or diameter (half-round)

Longitudinal slope of the gutter (ft/ft or m/m)

How full the gutter runs (0-1, typically 0.75-0.85)

Roof Drainage (Optional)

Enter rainfall intensity and roof area to compare gutter capacity to required flow.

in/hr

Design rainfall intensity for your region

sf

Horizontal projected area draining to this gutter run

Gutter Flow Design Overview

Gutter flow capacity is calculated using Manning's equation for open channel flow. The calculator determines how much water a gutter can convey based on its shape, size, material, and slope.

  • K-Style Gutters - Most common residential style, trapezoidal cross-section
  • Half-Round - Traditional style, semicircular cross-section
  • Box Gutters - Commercial/industrial, rectangular cross-section
  • Fill Ratio - Typically 75-85% full to prevent overflow

Typical Gutter Flow Capacities

Gutter TypeSizeApprox. CapacityTypical Use
K-Style5"5-10 gpmResidential
K-Style6"10-18 gpmLarge Residential
Half-Round6"15-25 gpmHistoric/Premium
Box8"40-70 gpmCommercial

Note: Actual capacity depends on slope, fill ratio, and material. Values shown are approximate at 1/16" per foot slope.

Manning's n for Gutter Materials

MaterialManning's nNotes
Aluminum0.011Standard
Galvanized Steel0.012Standard
Copper0.011Standard
Vinyl/PVC0.009Smoothest surface
Painted Steel0.012Standard
Stainless Steel0.010Standard
Cast Iron0.013Roughest surface

Source: FHWA HEC-22 (2009), Table 4-1
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