Rainwater Harvesting Calculator — Collection, Savings & Tank Size

Roof

Your roof catchment area and local climate

Roof Area

Roof Material

Annual Rainfall

Usage

How you plan to use the collected rainwater

Garden Area

Toilet Flushes per Day

Laundry Loads per Week

Water Price

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 Size	Garden Only	Garden + Toilets	Full Non-Potable
1-2 People	250-500 gal	500-1,000 gal	1,000-1,500 gal
3-4 People	500-1,000 gal	1,000-2,000 gal	2,000-3,000 gal
5+ People	1,000-1,500 gal	2,000-3,000 gal	3,000-5,000 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.

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Frequently Asked Questions

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

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 Type	Capacity	Cost Range	Footprint
Slim-line barrel	50–65 gal	$50–$120	2×1.5 ft
Polyethylene tank	500–1,500 gal	$300–$800	4×4 ft
Above-ground cistern	1,500–5,000 gal	$800–$2,500	6×6 ft
Underground cistern	2,500–10,000 gal	$2,000–$8,000	Buried

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

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

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

Roof