Is this precise gutter sizing?

No. It provides flow estimates. Use manufacturer tables and code requirements for gutter dimensions.

Does roof pitch matter?

This uses horizontal area. Pitch affects actual surface area but catchment is based on projected area; detailed hydraulics not modeled.

Should I apply safety factors?

Yes—consider higher intensity storms or partial blockages; the discharge factor can inflate flow as a safety margin.

Does this include downspout capacity?

It outputs flow; compare against downspout/gutter capacity to size both.

Yes, it converts roof area × rainfall (in/hr) to CFS in a simplified runoff formula.

construction calculator

Gutter Capacity Calculator

Estimate runoff flow from roof area and rainfall rate to size gutters/downspouts.

Results

Flow (CFS): 41.57
Flow (GPM): 18656.59

How to use this calculator

Enter roof area (sq ft).
Enter rainfall intensity (inches/hour) for your design storm.
Review flow in CFS and GPM; use manufacturer gutter/downspout capacity to size components.

Inputs explained

Rainfall intensity: Use local design storm intensity (e.g., 2–4 in/hr for many regions).
Discharge factor: Adjust flow up/down for slope/efficiency if desired; leave at 1 for baseline.

How it works

Flow (CFS) ≈ Roof area × Rainfall intensity ÷ 96.23 (simple runoff approximation).

Flow is then converted to GPM (1 CFS = 448.831 GPM).

Formula

Flow (CFS) ≈ (Roof area × Rainfall in/hr) ÷ 96.23
Flow (GPM) = CFS × 448.831

When to use it

Sizing gutters/downspouts for new construction.
Checking if existing gutters can handle local design storms.
Comparing flow for different roof areas or rainfall intensities.

Tips & cautions

Use local code or design manuals for target rainfall intensity and gutter sizing tables.
Consider debris guards and maintenance; capacity can drop if gutters clog.
Split large roof sections across multiple downspouts to reduce load per gutter run.
Simplified runoff estimate; does not include gutter slope, friction, or detailed hydraulics.
Check local codes and manufacturer capacity charts for final sizing.
Assumes 100% runoff; infiltration/ponding not modeled.

Worked examples

2,000 sq ft roof, 2 in/hr

Flow ≈ (2000 × 2) ÷ 96.23 ≈ 41.5 CFS
GPM ≈ 41.5 × 448.831 ≈ 18,600 GPM

1,200 sq ft roof, 3 in/hr

Flow ≈ (1200 × 3) ÷ 96.23 ≈ 37.4 CFS
GPM ≈ 16,800 GPM

Deep dive

Estimate gutter sizing flow by entering roof area and rainfall intensity to get CFS and GPM.

Use it alongside manufacturer capacity tables to choose gutter and downspout sizes.

FAQs

Is this precise gutter sizing?: No. It provides flow estimates. Use manufacturer tables and code requirements for gutter dimensions.
Does roof pitch matter?: This uses horizontal area. Pitch affects actual surface area but catchment is based on projected area; detailed hydraulics not modeled.
Should I apply safety factors?: Yes—consider higher intensity storms or partial blockages; the discharge factor can inflate flow as a safety margin.
Does this include downspout capacity?: It outputs flow; compare against downspout/gutter capacity to size both.
Is 96.23 a constant?: Yes, it converts roof area × rainfall (in/hr) to CFS in a simplified runoff formula.

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Simplified flow estimate. Verify gutter/downspout sizing with manufacturer data and local code requirements; consider debris, slope, and hydraulics in final design.