Rainwater Harvest and Tank Overflow Calculator
Estimate one rainfall event’s collection, diversion, overflow and stored-water supply from entered catchment and tank data.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Multiply horizontal projected area by rainfall depth. Divert the entered initial depth first, then apply collection efficiency to the remaining runoff. Add the initial stored volume, cap it at tank capacity and report overflow.
Collected=area×max(0,rain−diverted depth)×efficiency; overflow=max(0,initial+collected−capacity)
Worked example
100 m², 20 mm rain, 1 mm initial diversion and 80% efficiency collect 1520 L. A 1000 L tank initially holding 200 L overflows by 720 L and ends full, enough for 10 days at 100 L/day.
Assumptions and input details
One event with no water use during rainfall. Use horizontal projected catchment area, not sloped roof surface area. Initial diversion is deducted once per event before the efficiency factor; efficiency describes additional losses only. Initial storage cannot exceed usable capacity. Supply days assume no further rain or losses and constant entered demand. Zero demand has no depletion date. This volume model does not determine water quality, potability, treatment requirements, structural capacity or local permissions.
References: Texas Water Development Board: catchment area and rainfall
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
The 36 volume definitions distinguish US liquid, US dry and Imperial measures. Cups carry an explicit definition. Japanese shō is 2401/1331 L and a gō is one tenth of it; the explicitly labelled 180 mL rice cup is separate.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · UK Imperial definitions · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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