Rainwater Yield and Storage Scenario Calculator

Inputs

• Catchment area in m² or ft², measured on plan (the horizontal projection), counting only the surfaces that drain to this tank.
• Twelve monthly rainfall totals in mm from a local record — IMD's district rainfall statistics, a nearby station or your own gauge. Zero is a valid month; a blank month is missing. An annual total alone is not accepted, because it cannot size a tank.
• Runoff coefficient (more than 0, at most 1) for the surface, which you justify. CGWB's Manual on Artificial Recharge of Ground Water (2007) tabulates GI sheet 0.9, asbestos 0.8, tiled 0.75 and concrete 0.7; its January 2025 guide uses 0.8 for roofs and says asbestos roofs are not suitable for harvesting.
• Capture factor (more than 0, at most 1) for first-flush diversion, gutter overshoot in heavy rain and filter losses.
• Monthly demand on the harvested water — m³ in the month, or KLD multiplied by the days in that month (non-leap year).
• Tank capacity (usable volume) in m³ or litres, the water in it when the run starts, and the month the run starts in.
• Optional: what a kL of the water it replaces costs you, to put a gross value on the water used.

Method

1. Capture in each month (m³) = area (m²) × rainfall (mm) ÷ 1000 × runoff coefficient × capture factor. 1 mm of rain on 1 m² of plan area is exactly 1 litre.
2. Gross annual capture = the sum of the twelve months. It is reported on its own: it is the catchment's yield, not a tank size.
3. The twelve months then run through the tank in order from the start month. Each month: available = storage at the start + capture; used = the smaller of the demand and the available water; overflow = whatever is left above the tank's capacity after use; the rest is carried into the next month.
4. Over the year, gross capture + starting storage = used + overflow + water left at the end. The tool shows this check with every result.
5. A cautious second run uses only the water already in the tank at the start of each month and lets that month's capture spill first, to bracket what short, heavy storms do to a monthly balance.
6. Sensitivity: the same year is rerun for tanks of ½, 1½ and 2 times the size entered (each starting as full, proportionally), for a year with 25% less rain in every month, and with the cautious rule.

Assumptions

• Monthly steps: rain and use within a month are netted before the tank's capacity applies. That flatters small tanks, which in reality overflow in a storm and run dry in the gap before the next; the cautious run shows how far the answer can fall.
• The runoff coefficient and capture factor apply to every month alike; the first storms after a dry spell lose more to wetting the roof and to first flush.
• Overflow is reported as water leaving the tank — to a recharge structure or a drain. Whether it can be recharged is not assessed.
• The rainfall entered is the year being planned for. A drier year delivers less; the sensitivity table shows one with 25% less rain.

Limitations

• Does not assess recharge. Water-table depth, soil and aquifer, contamination risk and permission are site- and authority-specific: CGWB's 2025 guide does not recommend recharge structures where the post-monsoon water level is shallower than 3 m below ground, and on sites with industrial, chemical or hazardous waste it limits recharge to surface spreading.
• Does not size gutters, downpipes, first-flush devices, filters or overflow pipes — those follow storm intensity, not monthly totals.
• Says nothing about water quality or fitness for any use; treatment follows the use and test results.
• Does not check what your building bye-laws or groundwater conditions require — they set their own volumes and wording.