Rainwater Harvesting Calculator
Estimate annual rainwater capture, tank turnovers, storage supply days and water-cost savings from roof area and rainfall.
Default project preview
31,811 gallons
Tank supply
20 days
Usable annual water
18,250 gal
Annual water savings
$164.25
Simple payback guide
21.3 years
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Collection roof area
Enter this project assumption in ft². The current imperial default is 1800 ft².
Annual rainfall
Enter this project assumption in in. The current imperial default is 35 in.
Roof runoff coefficient
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 1. Maximum: 100.
First-flush and system losses
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Tank capacity
Enter this project assumption in gal. The current imperial default is 1000 gal.
Average non-potable demand
Enter this project assumption in gal/day. The current imperial default is 50 gal/day.
Water price
Enter this project assumption in per 1,000 gal. The current imperial default is 9 per 1,000 gal. Minimum: 0.
Installed collection-system cost
Enter this project assumption in a unitless value. The current imperial default is 3500. Minimum: 0.
How to read the outputs
Tank supply
at entered daily demand. Default preview: 20 days.
Usable annual water
limited by annual demand. Default preview: 18,250 gal.
Annual water savings
water charge only. Default preview: $164.25.
Simple payback guide
ignores maintenance and financing. Default preview: 21.3 years.
Runoff retained
combined collection efficiency. Default preview: 81%.
Capture volume
annual collected volume. Default preview: 120.4 m³.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Annual capture = roof area × annual rainfall depth × runoff coefficient × retained-system factor. Usable water is limited by entered annual demand.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use this calculator to estimate harvestable rainwater from roof catchment area, rainfall depth and an editable collection efficiency. It helps compare storage potential with demand but does not design overflow, treatment or potable-water systems.
Calculation steps
- 1Measure the roof plan area that drains to the tank.
- 2Multiply by rainfall depth and the unit conversion.
- 3Apply the collection-efficiency percentage.
- 4Compare the event or monthly yield with storage capacity and intended demand.
Worked numerical example
A 2,000 ft² roof receiving 1 inch of rain has a theoretical yield of about 1,247 gallons. At 85% collection efficiency, the usable estimate is roughly 1,060 gallons.
Purchasing and rounding
Tank capacity is selected in available whole sizes. Include first-flush, overflow, filtration, pump and access components separately.
Interpret the result
The calculated yield is a supply opportunity, not guaranteed stored water. A tank that is already full cannot capture the entire event.
Page-specific assumptions
- Rainfall is uniform across the catchment.
- Only connected roof area is included.
- Efficiency represents losses from first flush, splash and conveyance.
- Storage starts with enough free capacity.
Common mistakes
- Using roof slope area instead of horizontal catchment area without consistency.
- Assuming 100% collection.
- Ignoring tank overflow.
- Treating harvested water as potable without treatment approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Rainwater Harvesting Calculator questions
Why can annual capture exceed tank capacity?+
A tank can fill and empty many times during the year, so annual throughput can exceed one tank volume.
Does a larger tank always save more water?+
Not necessarily. Rainfall timing and demand determine how often storage overflows or sits empty.
Can harvested rainwater be used for drinking?+
Potable systems require treatment, testing and regulatory compliance beyond this calculator.