Gutter Flow Calculator

Enter gutter shape, dimensions, slope, and roughness to calculate flow capacity, velocity, hydraulic radius, and Froude number with Manning's equation.

Calculate Gutter Flow Capacity

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

Input Parameters

Gutter Properties

Gutter Shape

Cross-sectional shape of the gutter

Material

Gutter material affects surface roughness

Gutter Size

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

Slope

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

Fill Ratio

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

Manning's n override (optional)

Replace the material screening default with project or manufacturer data

Roof Drainage (Optional)

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

Rainfall Intensity

in/hr

Design rainfall intensity for your region

Roof Area

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

What this capacity does not determine

This is a uniform-flow check at one depth and slope. Roof runoff accumulates along a gutter run, while bends, outlets, seams, debris, and local backwater can reduce actual performance. The result therefore does not establish a maximum gutter length or downspout spacing.

Use the optional roof-area comparison at the downstream design section, then verify outlet locations, overflow provisions, and allowable contributing area against the locally adopted roof-drainage requirements and manufacturer data.

Manning's n for Gutter Materials

Material	Manning's n	Notes
Aluminum	0.011	Screening default
Galvanized Steel	0.012	Screening default
Copper	0.011	Screening default
Vinyl/PVC	0.009	Smooth-channel screening default
Painted Steel	0.012	Screening default
Stainless Steel	0.010	Screening default
Cast Iron	0.013	Rougher screening default

Generic screening defaults only. Manning's n is empirical; use project or manufacturer data when available. Open-channel method reference: Chow (1959).