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Roof

Your roof catchment area and local climate

sq ft
in

Usage

How you plan to use the collected rainwater

sq ft
$/1000 gal

Annual Rainwater Collection

35,511 gal/year

Metal roof · 40 in rainfall

Annual Savings
$81
Recommended Tank
932 gal
Payback Period
11.5 years

Monthly Collection Estimate

Jan2,663 gal
Feb2,486 gal
Mar3,018 gal
Apr3,196 gal
May3,551 gal
Jun3,729 gal
Jul3,551 gal
Aug3,374 gal
Sep3,018 gal
Oct2,841 gal
Nov2,308 gal
Dec1,776 gal

Environmental Impact

Saves 16,230 gallons per year = 406 bathtubs worth of water

Tank Sizing Guide

Household SizeGarden OnlyGarden + ToiletsFull Non-Potable
1-2 People250-500 gal500-1,000 gal1,000-1,500 gal
3-4 People500-1,000 gal1,000-2,000 gal2,000-3,000 gal
5+ People1,000-1,500 gal2,000-3,000 gal3,000-5,000 gal

Installation Tips

  • 1.Install a first flush diverter to redirect the first 1-2 gallons of dirty runoff away from your tank.
  • 2.Place the tank on a level, compacted surface. A full 1,000-gallon tank weighs over 8,300 pounds.
  • 3.Use opaque (dark-colored) tanks to prevent algae growth. Never use translucent containers.
  • 4.Install a mesh screen on all inlets to keep mosquitoes, leaves, and debris out of the tank.
  • 5.Add an overflow pipe that directs excess water away from the foundation to prevent pooling.

Example Calculations

1Suburban Home with Metal Roof

Inputs

Roof Area1,500 sq ft
MaterialMetal
Annual Rainfall40 in
Garden Area500 sq ft
Toilet Flushes/Day10
Laundry Loads/Week4

Result

Annual Collection35,442 gal/year
Annual Savings$85
Recommended Tank739 gal
Payback Period8.7 years

A 1,500 sq ft metal roof (0.95 coefficient) with 40 inches of rain collects 35,442 gallons annually. Total demand from garden (6,230 gal), toilets (5,840 gal), and laundry (4,160 gal) is 16,230 gallons. At $5/1,000 gal, the savings offset a ~$739 tank cost in about 8.7 years.

2Rainy Climate with Asphalt Roof

Inputs

Roof Area2,000 sq ft
MaterialAsphalt Shingle
Annual Rainfall60 in
Garden Area800 sq ft
Toilet Flushes/Day15
Laundry Loads/Week5

Result

Annual Collection59,808 gal/year
Annual Savings$151
Recommended Tank1,246 gal
Payback Period6.2 years

A 2,000 sq ft asphalt roof (0.80 coefficient) in a 60-inch rainfall zone collects 59,808 gallons per year. Higher demand from a larger garden (9,968 gal), more flushes (8,760 gal), and laundry (5,200 gal) totals 23,928 gallons. At $5/1,000 gal that saves about $120, but higher collection enables a faster payback.

Frequently Asked Questions

Q

How much rainwater can I collect from my roof?

A 1,500 sq ft metal roof in an area with 40 inches of annual rainfall can collect about 35,500 gallons per year. The formula is roof area times rainfall in inches times 0.623 times your runoff coefficient. Metal roofs capture 95% of rainfall, while asphalt shingles capture about 80%.

  • 1,500 sq ft metal roof + 40" rain = 1,500 × 40 × 0.623 × 0.95 = 35,442 gal/year
  • Same roof with asphalt shingles (0.80) collects only 29,904 gal/year — 16% less
  • Pacific Northwest (55" rain): same metal roof yields 48,733 gal/year
  • Southwest desert (10" rain): same roof yields only 8,861 gal/year
  • Use only the roof area that drains into your gutters — exclude garage or porch overhangs without gutters
Q

What size rainwater tank do I need?

Tank size depends on your monthly collection rate. A good rule is to size your tank at 25% of your peak monthly collection. For a 1,500 sq ft roof with 40 inches of rain, that is roughly 740 gallons. Most residential systems use 500 to 2,500 gallon tanks.

  • Rule of thumb: tank = 25% of your peak monthly collection for garden use
  • 500–750 gallon tanks suit most suburban homes with modest gardens
  • 1,000–2,500 gallon tanks support full garden irrigation + non-potable indoor use
  • Underground cisterns (2,500–10,000 gal) are ideal for whole-house systems
  • In dry climates, size your tank for 2–3 months of supply to bridge dry spells
Tank TypeCapacityCost RangeFootprint
Slim-line barrel50–65 gal$50–$1202×1.5 ft
Polyethylene tank500–1,500 gal$300–$8004×4 ft
Above-ground cistern1,500–5,000 gal$800–$2,5006×6 ft
Underground cistern2,500–10,000 gal$2,000–$8,000Buried
Q

Is rainwater harvesting legal in all US states?

Rainwater harvesting is legal in most US states, though regulations vary. Colorado limited collection until 2016 but now allows up to two 55-gallon barrels. States like Texas and Oregon actively encourage it with tax incentives. Always check your local regulations before installing.

  • Colorado: allows up to two 55-gallon barrels (110 gal total) per household
  • Texas: offers property tax exemptions and sales tax exemptions for harvesting equipment
  • Oregon: allows up to 5,000 gallons without a permit; larger systems need permits
  • California: no restrictions on residential rainwater harvesting since 2012
  • Check county and HOA rules separately — some HOAs restrict visible barrels
Q

How much money does rainwater harvesting save?

Savings depend on your water price and usage. At the national average of $5 per 1,000 gallons, a system collecting 20,000 gallons per year saves about $100 annually. In areas with higher water costs like $10 per 1,000 gallons, savings double to $200 or more.

  • National average water rate: ~$5 per 1,000 gallons ($0.005/gal)
  • 20,000 gal/year collection at $5/1,000 gal = $100 annual savings
  • High-cost areas (San Francisco at $14/1,000 gal) save $280+ per year
  • Payback period for a $750 system: 3–8 years depending on local water rates
  • Savings increase every year as municipal water rates rise 3–5% annually
Q

What is a runoff coefficient for rainwater collection?

The runoff coefficient is the percentage of rain that actually flows off your roof into gutters. Metal roofs have a coefficient of 0.95 (95% capture), tile roofs 0.85, asphalt shingles 0.80, and flat roofs 0.60. Losses come from evaporation, splash, and absorption.

  • Coefficient of 0.95 means 95% of rainfall reaches your gutters, 5% lost
  • Losses come from evaporation (2–5%), splash-off at edges (1–3%), and absorption
  • New asphalt shingles absorb more water (0.80) than weathered shingles (0.85)
  • First-flush losses reduce effective coefficient by another 1–2% if using a diverter
  • Use a conservative coefficient for sizing — real-world performance is slightly lower than lab values

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Last Updated: Mar 11, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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