Rainfall Calculator User Guide

What It Does:
Calculate water volume from rainfall on any surface area, compare rainfall between cities, and access rainfall data for planning water harvesting or drainage systems.

How to Use:

💧 Volume Calculator

Calculate total water volume from rainfall on a specific area.

1. Enter Area Dimensions:
   • Length – Surface area length in feet
   • Width – Surface area width in feet
   • (Can be roof, lawn, patio, parking lot, etc.)
2. Input Rainfall Amount:
   • Enter rainfall depth in inches
   • Can be from a single storm or period total
3. Select Time Period:
   • Single Storm Event – One rain event
   • Daily Average – Typical daily amount
   • Monthly Total – Entire month’s rainfall
   • Annual Total – Yearly total
4. Calculate:
   • Click “Calculate Volume”
   • Results show:
     • Total gallons and liters
     • Cubic feet and meters
     • Water weight in pounds
     • Rain barrel capacity needed

📊 City Comparison

Compare annual rainfall collection potential between two cities.

1. Select First City:
   • Choose from 10 major U.S. cities
   • Includes Seattle, Miami, Phoenix, NYC, etc.
2. Select Second City:
   • Choose another city for comparison
3. Enter Roof/Area Dimensions:
   • Length and width in feet
   • Same area used for both cities
4. Compare:
   • Click “Compare Cities”
   • Results display:
     • Annual rainfall for each city
     • Total gallons collectable per city
     • Difference in gallons and percentage
     • Barrel requirements for each location

Example: See how much more water a Seattle roof collects vs. Phoenix roof of same size!

📖 Rainfall Data Tab

Reference guide for 10 major U.S. cities with detailed rainfall information.

Information Provided:

• Annual total rainfall (inches)
• Wettest and driest months
• Rainy season patterns
• Regional rainfall characteristics

Cities Included:

• Seattle, WA (38″)
• Portland, OR (36″)
• New York, NY (46″)
• Miami, FL (62″)
• Phoenix, AZ (8″)
• Chicago, IL (37″)
• Houston, TX (50″)
• Los Angeles, CA (15″)
• Denver, CO (17″)
• Atlanta, GA (50″)

Rainfall Formula:

Basic Calculation:

textGallons = Area (sq ft) × Rainfall (inches) × 0.623

Example:

• 1,000 sq ft roof
• 1 inch of rain
• Result: 623 gallons

Quick Reference:

• 1 inch rain on 100 sq ft = 62 gallons
• 1 inch rain on 500 sq ft = 312 gallons
• 1 inch rain on 1,000 sq ft = 623 gallons
• 1 inch rain on 2,000 sq ft = 1,246 gallons

Practical Applications:

Water Harvesting:

• Size rain barrels for roof catchment
• Plan cistern capacity
• Estimate annual collection potential

Drainage Planning:

• Calculate stormwater runoff volume
• Size drainage systems
• Plan retention ponds

Garden Irrigation:

• Estimate free water availability
• Reduce municipal water dependency
• Plan supplemental watering needs

Cost Savings:

• Average garden use: 50 gal/week
• At $0.01/gallon: $26/year savings
• 1,000 gallons collected = $10 saved

Quick Tips:

✅ 1,000 sq ft roof = 623 gallons per inch of rain
✅ Use annual rainfall data for capacity planning
✅ Account for 20% loss from evaporation/overflow
✅ Check wettest months for peak collection
✅ Multiply by 0.8 for actual roof runoff efficiency

Pro Tip: A typical 1,500 sq ft roof in Seattle (38″ annual rain) can collect about 35,500 gallons per year – that’s enough to fill 355 standard rain barrels or irrigate a large garden all season without using municipal water!

Angelina Everly

Angelina Everly leads the editorial desk at Live Green Gardens, blending practical plant care, hands-on product testing, and approachable outdoor styling. She focuses on step-by-step how-tos, buyer’s guides, and small-space makeovers that work in real life and real budgets. When she’s not comparing pruning shears or setting up a drip kit, you’ll find her creating cozy corners with planters, solar lights, and pollinator-friendly picks—always with clear pros/cons and safety notes so you can buy once and garden happy.