What This Solves
Calculates the flow capacity and hydraulic properties of roof gutters (K-style, half-round, box) using Manning's equation.
Best Used When
- You are selecting a gutter size and profile for a residential or commercial roof
- You need to verify that an existing gutter can handle the design rainfall without overtopping
- You want to compare capacity differences between K-style, half-round, and box gutter profiles
Do NOT Use When
- You need to size downspouts that carry water from the gutter to grade — Use Downspout Sizing Calculator
- You are sizing flat roof drains rather than edge-mounted gutters — Use Roof Drain Sizing Calculator
Key Assumptions
- Flow is calculated using Manning's equation for open channel flow in the gutter cross-section
- Gutter slope is uniform along its length
- No debris or ice accumulation reduces the effective gutter cross-section
- Gutter outlets (downspouts) have adequate capacity to accept the gutter discharge
Input Quality Notes
Gutter capacity is sensitive to slope — even small slope changes significantly affect flow. Use the actual installed slope, not the nominal roof pitch. Manning's n for metal gutters is typically 0.011-0.013.
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Gutter Flow Calculator
Calculate gutter flow capacity using Manning's equation for open channel flow. Enter your gutter properties to determine flow capacity, velocity, and hydraulic characteristics. Supports K-style, half-round, box, and custom gutter shapes.
Calculate Gutter Flow Capacity
For educational purposes only. Not a substitute for professional engineering judgment.
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 Type | Size | Approx. Capacity | Typical Use |
|---|---|---|---|
| K-Style | 5" | 5-10 gpm | Residential |
| K-Style | 6" | 10-18 gpm | Large Residential |
| Half-Round | 6" | 15-25 gpm | Historic/Premium |
| Box | 8" | 40-70 gpm | Commercial |
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
| Material | Manning's n | Notes |
|---|---|---|
| Aluminum | 0.011 | Standard |
| Galvanized Steel | 0.012 | Standard |
| Copper | 0.011 | Standard |
| Vinyl/PVC | 0.009 | Smoothest surface |
| Painted Steel | 0.012 | Standard |
| Stainless Steel | 0.010 | Standard |
| Cast Iron | 0.013 | Roughest surface |
Source: FHWA HEC-22 (2009), Table 4-1
About Gutter Flow Analysis
Gutters are open channels that collect and convey rainwater from roof surfaces to downspouts and the storm drainage system. Proper sizing ensures gutters can handle design rainfall without overflowing.
Manning's Equation for Open Channel Flow
Gutter flow capacity is calculated using Manning's equation:
V = (k/n) × R2/3 × S1/2
Where:
- V = Flow velocity (fps or m/s)
- k = Unit conversion factor (1.49 for US customary, 1.0 for SI)
- n = Manning's roughness coefficient
- R = Hydraulic radius (A/P)
- S = Channel slope (ft/ft or m/m)
Gutter Shape Selection
- K-Style (Ogee) - Most common residential style. Provides good capacity with decorative appearance. Available in 4", 5", and 6" sizes.
- Half-Round - Traditional style with semicircular cross-section. Self-cleaning and efficient. Common on historic buildings.
- Box Gutters - Rectangular cross-section for maximum capacity. Used in commercial and industrial applications.
- Fascia Gutters - Integrated with the fascia board for a clean appearance.
Design Considerations
- Slope - Minimum 1/16" per foot (0.5%) recommended; 1/8" per foot (1%) ideal
- Fill Ratio - Design for 75-85% full to allow for freeboard
- Downspout Spacing - Typically 20-40 ft maximum between downspouts
- Flow Regime - Subcritical flow (Fr < 1) provides stable, tranquil conditions
- Material - Affects Manning's n and durability
Froude Number and Flow Regime
The Froude number (Fr) indicates the flow regime:
- Subcritical (Fr < 1) - Tranquil, stable flow. Preferred for gutters.
- Critical (Fr = 1) - Transitional flow. Unstable conditions.
- Supercritical (Fr > 1) - Fast, shooting flow. May cause splashing at corners.
Resources
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Last verified: February 2026