Try a Common Scenario
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Which kind of gutter are you sizing?
Roof, eaves, or box gutter
You are in the right calculator. Enter the gutter shape, fall, material, and fill depth to estimate its open-channel capacity.
Street, curb, or roadway gutter
Use the FHWA HEC-22 roadway-gutter calculator for triangular spread T, curb depth, and depressed or composite sections.
For the rest of a roof-drainage system, use the downspout calculator, roof-drain calculator, or the roof and gutter design guide.
Roof-gutter flow capacity from Manning’s equation
A roof gutter behaves as a partially full open channel. This tool derives its flow area and wetted perimeter from the selected profile, then computes hydraulic radius, velocity, and discharge at the entered gutter fall. If rainfall intensity and roof area are supplied, it also compares capacity with the required roof runoff.
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
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).
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 - Modeled as a simplified trapezoid using nominal dimensions. Verify the actual manufacturer profile before final design.
- 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.
- Custom Rectangular - Uses the entered clear width and depth; it does not represent ogee, fascia, or other proprietary profiles.
Design Considerations
- Slope - Enter the installed longitudinal fall and verify minimum fall against local requirements and manufacturer instructions
- Fill Ratio - Design for 75-85% full to allow for freeboard
- Downspout Spacing - Must be checked from cumulative contributing roof area, outlet capacity, and local requirements; this uniform-flow calculation does not determine it
- Flow Regime - Subcritical flow (Fr < 1) provides stable, tranquil conditions
- Material - Affects Manning's n and durability
Gutter fall, slope, and regional terminology
Gutter fall and gutter slope describe the same longitudinal grade. Box gutter and eaves-gutter rules differ by jurisdiction, especially in Australia, New Zealand, and the United Kingdom. Use this calculator for a hydraulic capacity check, then verify minimum fall, overflow provision, downpipe spacing, and freeboard against the locally adopted plumbing or roof-drainage standard, including AS/NZS 3500.3 where it applies.
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.
Reference & standards
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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Last verified: February 2026