AREA × RAINFALL × EFFICIENCY
How much rainwater can I collect?
Enter a simplified rectangular catchment or an area you already know, then add rainfall depth. The calculator first finds the theoretical volume and applies your collection-efficiency assumption.
- 01
Enter catchment dimensions or a known area.
- 02
Enter rainfall for the event or period you are estimating.
- 03
Choose a collection-efficiency assumption.
- 04
Optionally compare the result with container capacity.
How the rainwater calculation works
The calculator converts area and rainfall to compatible units, calculates the theoretical maximum with full precision, then applies the efficiency fraction. Rounding happens only when results are displayed.
Rainfall and catchment area explained
Catchment area is the surface contributing water to the system. Length × width is a simplified rectangle; roof pitch, overhangs and multiple sections can require a more detailed measured area. Rainfall depth describes how deep the rain would be if spread evenly across a flat surface.
Worked example
1 1,000 × 1 × 0.6233766 ≈ 623 gallons.
2 At 90% selected efficiency ≈ 561 gallons.
3 The adjusted equivalent is ≈ 2,124 liters.
4 This is a volume estimate, not guaranteed capture.
1 40 × 25 = 1,000 liters theoretical.
2 At 90% selected efficiency = 900 liters.
What does collection efficiency mean?
Efficiency accounts for water that may not reach storage because of splash, first flush, overflow, gutter losses and other system conditions. The 100% default is the theoretical maximum. Actual efficiency depends on the roof and collection system, so no lower percentage is prescribed as universal.
How much rain does one inch produce?
One inch of rainfall over one square foot is approximately 0.6233766 US gallons. A 1,000-square-foot catchment therefore receives about 623 gallons per inch before applying collection losses.
How much rain does one millimeter produce?
One millimeter of rain over one square meter equals one liter. This direct relationship makes a 40 m² area receiving 25 mm of rain equal 1,000 liters theoretically.
Estimating rain barrel or tank capacity
Divide adjusted collection volume by usable container capacity. Storage estimates round up to whole containers for planning, but they do not prescribe a suitable tank or account for existing water, overflow, demand, plumbing design or local requirements.
Common rainwater calculation mistakes
- Using building footprint as exact roof area without checking the catchment.
- Mixing inches with square meters or millimeters with square feet.
- Applying an efficiency percentage twice.
- Assuming storage capacity equals usable capacity in every installation.
- Treating the estimate as guaranteed captured or potable water.
Frequently asked questions
How much water can I collect from 1 inch of rain?
One inch over one square foot is approximately 0.6233766 US gallons. Multiply that relationship by the catchment area and your selected efficiency.
How much water does 1 mm of rain produce?
One millimeter of rain over one square meter equals one liter of water before collection losses.
Why is collection efficiency included?
Some water may not reach storage because of splash, first flush, overflow, gutter losses or other system conditions. Efficiency lets you apply your own assumption rather than relying on a universal percentage.
Can I use roof length and width?
Yes. Length × width provides a simplified rectangular catchment area. It does not model roof pitch, overhangs or complex roof sections.
Can I enter my known roof area instead?
Yes. Select Known area and enter square feet or square meters.
How many rain barrels would I need?
Enable the storage estimate, choose a capacity and the calculator divides adjusted collection by that capacity, rounding up to a whole container.
Is collected rainwater automatically safe to drink?
No. This calculator estimates volume only. Potable use may require suitable treatment and compliance with local rules.