Stormwater Runoff Calculator

Rational method Q = C×i×A: runoff coefficient by material (metal 0.95, shingle 0.90, tile 0.85, green roof 0.35) × rainfall intensity × area in acres — output in CFS and GPM

Quick Estimate

Roof Area?

sqft

Roof Material (Runoff Coefficient)?

Rainfall Intensity (i)?

in/hr

Peak Runoff (Q = CiA)

0.165

Roof Area in Acres0.0459

Peak Runoff74

Runoff Coefficient (C)0.90

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Need more accuracy?

Extended mode adds scenarios, charts, and detailed breakdowns

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How to Use the Stormwater Runoff Calculator

Undersized gutters overflow during heavy rain, causing fascia rot and foundation damage. The Stormwater Runoff Calculator matches gutter and downspout size to your roof's drainage load.

Measure the horizontal projection area of the roof that drains into each gutter run.
Find the design rainfall intensity for your area (inches per hour for a 10-year storm).
Enter both values into the calculator.
The result shows the minimum gutter width and number of downspouts needed.
Add one downspout for every 40 linear feet of gutter as a general rule.

Tip: Start with the Quick calculator for a fast estimate. Switch to Extended mode for scenario comparison and charts, or Professional mode for complete parameter control.

Formula & Methodology

The Stormwater Runoff Calculator uses the following established formulas:

Flow Rate (GPM) = Roof Area (sq ft) × Rainfall (in/hr) ÷ 96.25 K-factor method: Flow = 0.433 × A × i (A in sq ft, i in in/hr) 5" K-style handles ~5,520 sq ft at 1 in/hr 6" K-style handles ~7,960 sq ft at 1 in/hr

Gutter capacity also depends on slope. A 1/16" per foot slope is minimum; 1/8" per foot is recommended.

Frequently Asked Questions

What size gutters do I need for my roof?

5-inch K-style gutters handle most residential roofs under 1,500 sq ft of drainage area. Larger roofs, steep pitches, or high-rainfall areas often need 6-inch gutters. Get our Stormwater Runoff Calculator to calculate the exact requirement for your situation.

How many downspouts do I need?

As a rule of thumb, one downspout for every 30–40 linear feet of gutter. Each downspout should handle no more than 600–800 sq ft of roof area. More downspouts mean faster drainage and less overflow risk.

What is the best gutter material?

Aluminum is the most popular — lightweight, rust-proof, and affordable at $4–$9 per linear foot installed. Copper is premium and very long-lasting. Vinyl is cheapest but warps in extreme temperatures. Steel is durable but can rust.

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Sources & References

The formulas, code requirements, and reference data used in this calculator are sourced from the following authoritative organizations and publications. Local codes vary — always verify with your jurisdiction's building department.

International Residential Code (IRC) 2024 — Chapter 9: Roof Assemblies — ICC
NRCA Roofing Manual (Steep-Slope & Low-Slope) — National Roofing Contractors Association
Asphalt Shingle Installation Manual — ARMA (Asphalt Roofing Manufacturers Association)
International Plumbing Code (IPC) Chapter 11 — Storm Drainage — ICC
SMACNA Architectural Sheet Metal Manual — SMACNA
Asphalt Roofing Manufacturers Association — Drainage Guidelines — ARMA

Disclaimer: Calculator results are estimates intended for planning and educational purposes only. They do not replace professional engineering analysis, building permit review, or contractor estimates. Always consult a licensed roofing contractor or structural engineer for final specifications.