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Rainwater collection estimate
Estimate rainwater reaching a tank inlet from horizontal catchment area, cumulative rainfall and assumed efficiency. Separate inflow from stored water.
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Estimated flow reaching the tank inlet
800L
The answer is estimated total inflow before the tank under your assumptions. Tank overflow and water used during the period have not been deducted, so this is not the water left in storage at the end. Water quality and drinking suitability are not assessed.
Theoretical rainwater on the catchment1,000 L
Assumed overall collection efficiency80 %
Estimated loss before the tank inlet200 L
Inlet flow in cubic meters0.8 m³
Worked steps
- 50 × 20 = 1000
- 1000 × (80 ÷ 100) ≈ 800
One square meter covered by one millimeter of rain is one liter. Theoretical liters equal area in m² times rainfall in mm. Multiply by efficiency/100 for inlet flow; subtract that from the theoretical amount for estimated collection loss. Divide liters by 1000 for m³. The answer is estimated total inflow before the tank under your assumptions. Tank overflow and water used during the period have not been deducted, so this is not the water left in storage at the end. Water quality and drinking suitability are not assessed. The main answer is total potential inlet flow before tank capacity or overflow constraints. It does not mean that amount remains at the end of the period. Water drawn out, later leaks and overflow require a separate time-based storage balance. Positive amounts below 0.001 L are shown as less than 0.001 L rather than zero. Rainfall, roof condition and system performance vary. No live forecast, universal 80% efficiency, drinking-water assessment, supply-days prediction or guaranteed savings is provided. Use records for your catchment, save the assumptions and compare cases. Taiwan WRA guidance gives local context; this calculator does not decide legal compliance or replace professional storage, overflow and treatment design.
Method and scope
The answer is estimated total inflow before the tank under your assumptions. Tank overflow and water used during the period have not been deducted, so this is not the water left in storage at the end. Water quality and drinking suitability are not assessed.
Rainfall, roof condition and system performance vary. No live forecast, universal 80% efficiency, drinking-water assessment, supply-days prediction or guaranteed savings is provided. Use records for your catchment, save the assumptions and compare cases. Taiwan WRA guidance gives local context; this calculator does not decide legal compliance or replace professional storage, overflow and treatment design.
References:Taiwan MOA: projected area, rainfall and a measured site-specific yield、US DOE: rainfall, area, efficiency and model limitations、Taiwan WRA: rainwater use and storage concepts
What this tool helps you do
Estimate water from one rain event or a defined period using catchment area and collection efficiency. This is a simple inflow calculation, separate from tank capacity, remaining storage or construction design.
What to enter
Use the horizontal area draining to the pipe and cumulative rainfall in millimeters for the same place and period. Enter efficiency as a percentage: 80, not 0.8, for 80%. Account for roof, first-flush and pipe losses together once.
How it is calculated
One square meter covered by one millimeter of rain is one liter. Theoretical liters equal area in m² times rainfall in mm. Multiply by efficiency/100 for inlet flow; subtract that from the theoretical amount for estimated collection loss. Divide liters by 1000 for m³.
Worked example
Synthetic example: 50 m² receives 20 mm of rain, giving 1000 liters on the catchment. At an assumed 80% efficiency, 800 liters reaches the tank inlet and 200 liters is lost before it. The 800 liters is 0.8 m³, not a promise of stored water.
Understanding the answer
The main answer is total potential inlet flow before tank capacity or overflow constraints. It does not mean that amount remains at the end of the period. Water drawn out, later leaks and overflow require a separate time-based storage balance. Positive amounts below 0.001 L are shown as less than 0.001 L rather than zero.
Scope and limitations
Rainfall, roof condition and system performance vary. No live forecast, universal 80% efficiency, drinking-water assessment, supply-days prediction or guaranteed savings is provided. Use records for your catchment, save the assumptions and compare cases. Taiwan WRA guidance gives local context; this calculator does not decide legal compliance or replace professional storage, overflow and treatment design.