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A practical rain barrel savings scenario
For this example, use connected roof area of 80 m², monthly rainfall of 65 mm, collection efficiency of 80 %, mains water price per m³ of 3.5, and storage capacity of 500 L. These are starting values, so replace them with numbers that match your situation.
Connected roof area: 25% lower
Change connected roof area to 60 m² while keeping the other example values the same.
Connected roof area: 25% higher
Change connected roof area to 100 m² while keeping the other example values the same.
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Quick answer
What does the Rain barrel savings calculate?
How much mains water can collected rain replace? This calculator uses connected roof area, monthly rainfall, collection efficiency, mains water price per m³, and storage capacity to estimate rainwater collection value immediately in your browser.
With the values currently entered, the result is 132 gal — usable water per fill cycle. It also shows collection potential, and mains water value.
How to use the Rain barrel savings
- Replace the example values with your own numbers.
- Review the result and supporting figures as they update automatically.
- Check the formula and assumptions before using the estimate for a decision.
Inputs used
- Connected roof area — entered in m²
- Monthly rainfall — entered in mm
- Collection efficiency — entered in %
- Mains water price per m³
- Storage capacity — entered in L
Rain barrel savings formula
Roof area × rainfall × collection efficiency, capped by usable storage
Assumptions
- One millimetre of rain on one square metre equals one litre.
- Overflow and actual garden demand can reduce useful collection.
Practical guide
Rain barrel savings example and edge cases
How much mains water can collected rain replace? Let's use a concrete example, then look at the assumptions that can move the answer.
Example: A practical rain barrel savings scenario
For this example, use connected roof area of 80 m², monthly rainfall of 65 mm, collection efficiency of 80 %, mains water price per m³ of 3.5, and storage capacity of 500 L. These are starting values, so replace them with numbers that match your situation.
- Connected roof area
- 80 m²
- Monthly rainfall
- 65 mm
- Collection efficiency
- 80 %
- Mains water price per m³
- 3.5
- Storage capacity
- 500 L
Calculated result132 galusable water per fill cycle
Start with usable water per fill cycle. Then check collection potential, and mains water value to understand what sits behind the main result.
Example results use the default display profile. The calculator above follows your selected country and units.
How to read the result
- Read the main result first. The supporting figures for collection potential, and mains water value explain how the estimate is built.
- The method is Roof area × rainfall × collection efficiency, capped by usable storage. Keep the units consistent and use values from the same time period.
Edge cases worth checking
When connected roof area is unusual
One millimetre of rain on one square metre equals one litre. Double-check this input before relying on the result.
When storage capacity is uncertain
Overflow and actual garden demand can reduce useful collection. Run a lower and higher value to see a useful range.
What changes the result most
Connected roof area
Measure connected roof area with the same unit shown beside the input. Convert first if your source uses another unit.
Monthly rainfall
Measure monthly rainfall with the same unit shown beside the input. Convert first if your source uses another unit.
Collection efficiency
Test a lower and higher collection efficiency. A small percentage change can move the final result more than expected.
Try a different scenario
Small changes show whether the answer is stable or sensitive.
Connected roof area: 25% lower
60 m²132 galusable water per fill cycle
Connected roof area: 25% higher
100 m²132 galusable water per fill cycle
Monthly rainfall: 25% higher
81 mm132 galusable water per fill cycle
Common mistakes
Check connected roof area
One millimetre of rain on one square metre equals one litre. Make sure this matches the number you enter.
Keep storage capacity consistent
Overflow and actual garden demand can reduce useful collection. Use the same units and time period throughout the calculation.
Do not rely on one rain barrel savings scenario
Run a cautious case and an optimistic case. The range is often more useful than one exact-looking number.
Use this result well
How much mains water can collected rain replace?
Weather, soil, product coverage, and site conditions can change the quantity or cost.