Rainwater Harvesting Calculator — Tank Size & Daily Balance
Compare rainwater tanks using a storm and dry spell or your daily rainfall data. See water supplied, shortages, overflow and the reserve left each day.
Water used in this scenario
250 US gal100% of the 250 US gal demand supplied during 10 dry days.
Adjust roof, rainfall and tankPlan your rainwater storage
Paste 1–366 consecutive days: date,rainfall,demand. Use YYYY-MM-DD dates and decimal points. Rainfall is in in; optional demand is in US gal. Blank demand uses the daily demand above.
The starting data is an illustrative seven-day example, not local weather. Replace it with your measured daily totals. Rain enters before each day's withdrawal; within-day timing is not modelled.
Where the water goes
- Collected after losses
- 561.04 US gal
- Overflow lost
- 311.04 US gal
- Demand not supplied
- 0 US gal
- Ending reserve
- 0 US gal
Stored water over the scenario
End-of-day stored water, from the collection day through 10 dry days. No demand on collection day.
Closing reserve. Lines connect daily observations; they do not show the timing of rainfall or use within a day. Exact values are in the daily table below.
0 initial + 561.04 collected = 250 used + 311.04 overflow + 0 ending (US gal).
No unmet demand in this scenario.
Compare tank capacities
Same roof, rainfall, demand and initial water. These are comparison sizes, not installation recommendations. Scroll the table horizontally on small screens.
| Tank | Used | Unmet | Overflow | Ending |
|---|---|---|---|---|
| 55 | 55 | 195 | 506.04 | 0 |
| 250 (selected) | 250 | 0 | 311.04 | 0 |
| 275 | 250 | 0 | 286.04 | 25 |
| 500 | 250 | 0 | 61.04 | 250 |
| 1,000 | 250 | 0 | 0 | 311.04 |
Daily water balance
| Day | Rain | Opening | Collected | Demand | Used | Unmet | Overflow | Closing |
|---|
How to calculate rainwater harvesting
Collected litres = roof plan area in m² × rainfall in mm × collection efficiency. In US units, one inch on one square foot is about 0.6234 US gallons before collection losses. The calculator uses exact unit conversions internally.
For each day, add the collected water to the opening reserve, discard any water above usable tank capacity, and then supply as much of that day's demand as the reserve allows. Unmet demand is the amount you would need from another source. The remaining water carries into the next day.
Why annual rainfall is not enough to choose a tank
The same rainfall total can arrive in many small showers or in one downpour. A small tank can spill most of a downpour and then run dry. Use a daily series representative of your site, including dry periods, and compare multiple years separately. This tool reports only the entered period and starts with exactly the initial reserve you specify.
Does a bigger rainwater tank always help?
It can reduce overflow and preserve more water for later use, but cannot create rainfall. Compare both unmet demand and ending reserve: a tank may finish with spare water after an earlier shortage. Increasing capacity may have little benefit if collection or demand timing is the limiting factor.
How are first-flush and other losses handled?
Efficiency is one combined allowance for water that does not enter the tank. If your efficiency already accounts for diversion and filtration, do not deduct those losses again. This simplified model has no separate first-flush event volume, evaporation from storage, leakage, pumping limit or automatic top-up.
Collection formula reference: University of Arizona Cooperative Extension. This is a non-potable water budget, not a water-quality assessment, local design standard or a specification for plumbing and overflow outlets.
Planning where excess water goes? Explore the dry well storage calculator and French drain materials calculator.
