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Rainwater & Drainage Hub

Gutter, Downspout & Drainage Calculators

Engineering-backed calculation utilities for gutter runs, 1/4" per 10-ft slope pitch, downspout cross-sectional capacity, and SMACNA storm sizing.

Specialized Gutter & Drainage Calculators

Select a calculator for gutter slopes, downspout counts, flow rates, and SMACNA sizing.

Eave Perimeter & Hardware Engine

Gutter Calculator

Calculate total linear footage, 10-ft/20-ft gutter sections, hidden hangers, corner miters, downspouts, and hardware.

Method: Sections = ⌈Run Length / Section Length⌉
Pitch & Elevation Drop Engine

Gutter Slope Calculator

Determine the exact drop in fractional inches, slope percentage, and intermediate chalk line marks for single or split gutter runs.

Method: Total Drop (inches) = Run Length (ft) × (Slope Rate per 10 ft / 10)
Drainage Capacity & Layout Engine

Downspout Calculator

Calculate the required number of downspouts, profile sizing (2x3" vs 3x4"), spacing analysis, and wall offset elbows.

Method: Downspouts = max(⌈Gutter Length / Max Spacing⌉, ⌈Roof Area / Max Drainage Capacity per Downspout⌉)
SMACNA Engineering Sizing Engine

Gutter Sizing Calculator

Calculate whether you need 5-inch, 6-inch, or 7-inch gutters based on roof catchment area, roof pitch factor, and rainfall intensity.

Method: Adjusted Area = Roof Area × Pitch Factor
Water Volume & Hydraulic Flow Engine

Gutter Volume Calculator

Calculate physical water capacity in US gallons and liters across gutter lengths, plus peak storm inflow rates in GPM.

Method: Volume (Gallons) = (Cross Section Area in sq ft × Gutter Length in ft) × 7

Gutter Slope & Drainage Guides

Learn exact slope drops per 10 feet, downspout spacing formulas, and SMACNA catchment sizing.

6 min read

How to Calculate Gutter Slope & Pitch (The 1/4" Per 10-Foot Rule)

Detailed instructions on measuring gutter pitch, calculating elevation drop in fractional inches, and installing slope across long eave runs.

7 min read

How Many Downspouts Do I Need? Sizing, Spacing & Layout Guide

Understand the engineering rules behind downspout spacing, 2x3 vs 3x4 cross-sectional capacity, and proper foundation discharge distances.