Gutter Capacity Calculator

Plan gutters using real hydraulics, not rough guesses anymore. Switch units, shapes, slopes, and materials in seconds easily. See capacity, peak runoff, and maximum roof area instantly. Download clean CSV and polished PDF of results. Confident designs begin here, whatever the storm brings today.

Inputs

White theme Hydraulics Open-channel flow
Use >1.0 for wind-driven rain or steep pitches.
×
Set internal dimensions of waterway.
Set internal diameter for half-round profile.
Capacity computed at partial depth; gutters rarely run brim-full.
Count
Size mm Ø
W×H × mm
Cd discharge coefficient
Downspout modeled as orifice: Q = CdA√(2gh) with head ≈ water depth.

Results

  • Required flow (peak): – L/s  (– gpm)
  • Gutter capacity (Manning, adj.): – L/s  (– gpm)
  • Utilization (required / capacity): –
  • Max roof area this gutter supports: –
  • Downspout total capacity: – L/s  (– gpm)
  • Recommended max downspout spacing: –

Design advisory

Example data

Illustrative scenarios; adapt to your local code, storms, and manufacturer specs.

# Units Intensity Area C Slope n Shape Dims Fill% Req L/s Cap L/s Util

Formulas used

Peak runoff, Qreq = i · A · C · (1 + SF) · Fp
i = rainfall intensity (mm/hr or in/hr) converted to m/s,
A = roof plan area (m²), C = runoff coefficient, SF = safety factor (decimal), Fp=exposure/pitch factor.

Gutter capacity (open channel, Manning): Q = (1/n) · A · R2/3 · S1/2
n = Manning roughness, A = flow area (m²), R = hydraulic radius = A/P where P is wetted perimeter (m), S = slope (m/m).
Rectangular partial fill: A = b·y, P = b + 2y.
Half-round partial fill (radius R, depth y ≤ R): let θ = arccos(1 - y/R). Then A = R²(θ − sinθ·cosθ), P = 2Rθ.

Adjusted capacity: Qadj = Q · (1 − debris%) accounting for screens/debris/margins.

Downspout check (orifice): Q = Cd · A · √(2 g h) with Cd user-specified, g=9.81 m/s², head h≈operating water depth.
Round: A = πD²/4. Rectangular: A = W·H.

Recommended spacing: For uniform inflow q' = Qreq/L, spacing smax ≈ 2 Qds,one / q'.

All capacities are idealized hydraulics; verify with local codes and manufacturer data.

How to use this calculator

  1. Select units, then enter rainfall intensity and roof area.
  2. Pick a runoff coefficient and set safety factor and Fp.
  3. Choose gutter shape, dimensions, slope, and roughness.
  4. Set operating fill depth and debris allowance for realism.
  5. Configure downspouts: shape, size, count, and discharge coefficient.
  6. Optional: enter gutter length to get recommended downspout spacing.
  7. Click Calculate; export CSV or PDF for your record.

FAQs

Use your jurisdiction’s design storm, often the 5-, 10-, or 100-year event with a specified duration (e.g., 5-minute). Consult local drainage manuals or national standards.

Enter the plan (projected) area draining to this gutter run, including any contributing upper roofs or valleys. Split complex roofs into sections where sensible.

Yes, gutters are open channels. The equation provides a reasonable first estimate, but final sizing should follow local codes and manufacturer capacity charts.

Insufficient downspout capacity can back up flow and cause overflow. This tool gives an orifice-based check; always compare with product literature and code rules of thumb.

A modest factor accounts for debris, partial blockages, and construction tolerances. Many designers apply 10–25% unless a governing standard specifies otherwise.

Both can work. Half-rounds often self-clean better; rectangular profiles offer larger wetted perimeters for a given width. Choose based on appearance and manufacturer data.

Quick tips

  • Intensity drives sizing more than any other input.
  • Increasing slope or reducing roughness boosts capacity.
  • Use realistic partial fill and debris allowances.
  • Round downspouts often flow better than sharp corners.
  • Verify with local building codes and product charts.

About this tool

Single-file calculator using standard open-channel hydraulics. Exportable outputs help with submittals and recordkeeping.

Reference: Typical Manning Roughness (n)

Material / Conditionn (typical)Notes
PVC / very smooth plastic0.011New, clean surfaces
Cast iron (smooth)0.013Light scale acceptable
Aluminum / galvanized steel0.015Common sectional gutters
Copper0.017Natural patina increases roughness
Corrugated / ribbed interiors0.019–0.022Use higher n for ribs

Reference: Runoff Coefficient (C) for Roofs

Roof surfaceTypical CRemarks
Metal sheet (standing seam)0.95–1.00Very smooth; minimal storage
Clay/concrete tile0.85–0.95Minor texture; high runoff
Asphalt shingle0.80–0.95Depends on slope and condition
Bitumen / membrane0.85–0.95Flat roofs often have high C
Green roof (extensive)0.30–0.70Varies with soil depth and saturation

Reference: Common Gutter & Downspout Sizes

Areas are approximate; verify with manufacturer catalogs. Rectangular areas shown at 90% fill depth; half-round areas shown at brim-full (semicircle).

TypeNominal sizeInternal dimsApprox flow areaNotes
Rectangular / K-style100×75 mm100×75 mm~67.5 cm² @ 90% depthLight-duty, small sheds
Rectangular / K-style125×80 mm125×80 mm~90 cm² @ 90% depthCommon residential run
Rectangular / K-style150×100 mm150×100 mm~135 cm² @ 90% depthLarger roof sections
Half-roundØ100 mmDiameter 100 mm~39.3 cm² brim-fullTraditional appearance
Half-roundØ125 mmDiameter 125 mm~61.3 cm² brim-fullBetter self-cleaning
Downspout (round)Ø75 mm—~44.2 cm² section areaMatch spacing to inflow
Downspout (round)Ø100 mm—~78.5 cm² section areaHigher capacity
Downspout (rect)60×80 mm60×80 mm48.0 cm² section areaCheck corner losses

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.