YielixRainwater Harvesting Calculator
Estimate potential runoff from a roof or covered farm structure from its effective horizontal catchment area, supplied rainfall and your collection efficiency. Add matching monthly non-potable demand and optional tank information to explore coverage and a coarse storage balance; no rainfall month, annual total or water quality is assumed.
Roof Capture Is Not Field Rainfall
One millimetre of rain falling on one square metre of effective horizontal catchment produces one litre before collection losses. This calculator applies your efficiency to that physical identity and sums only records you provide.
Roof runoff is a potential supply. It is different from rain falling onto a field: use Effective Rainfall Calculator and Irrigation Water Requirement Calculator for field-water questions, not as roof-capture replacements.
Plan Collected Water as a Separate Supply
Potential roof capture is not proof that water reaches a field or replaces a scheduled irrigation. Compare matching supplied periods with your own non-potable demand, then account for delivery, storage and local operating constraints before relying on it. The Irrigation Runtime Calculator can help express a known irrigation volume as a system run time; it does not turn a roof-capture estimate into a crop-water recommendation.
If roof water reduces purchased, pumped or hauled water, use the Water Cost Calculator to examine the entered cost of that water separately. It does not price tank installation, treatment, maintenance, permits or the water itself, and it cannot determine whether collection is allowed locally.
Inputs Explained
- Horizontal catchment area
- Effective plan-view roof or covered-surface area that drains to the collection point, in m² or ft².
- Supplied rainfall
- One event or only the months you know, in mm or inches. Missing months are never filled with averages.
- Collection efficiency
- Your editable fraction of roof rainfall expected to reach storage, from 0% through 100%.
- Matching monthly demand
- Optional non-potable demand for every supplied month. Omitted demand is not zero; explicit zero is allowed.
- Tank capacity and starting volume
- Optional monthly planning inputs. They produce a coarse period-to-period balance, not a tank-sizing prediction.
How This Calculator Works
Formula
Captured litres = horizontal roof area (m²) × rainfall (mm) × efficiency / 100- One millimetre over one square metre is one litre.
Available = starting stored water + captured; used = min(available, demand)- Only matching monthly demand may be compared.
Ending tank = min(capacity, max(0, available − used))- Overflow and unmet demand are reported from the same coarse monthly balance.
Source
- Harvesting Rain in Cochise County (opens in a new tab), University of Arizona Cooperative Extension — Background on roof catchment and rainwater-harvesting planning.
- Rain Barrels in Utah (opens in a new tab), Utah State University Extension — Supports roof-runoff collection arithmetic and practical storage context.
Assumptions
- Each supplied rainfall record is independent input; no annual rainfall, unentered month or water use is invented.
- Tank outputs are approximate month-level planning values. Daily or event chronology is needed to size storage or predict intramonth overflow.
- First-flush treatment is not modelled. Any future first-flush deduction belongs to each event, not once to an annual total.
Worked Example
W01 fixture: select Metric, enter 100 m² horizontal catchment, 50 mm for April and 80% collection efficiency.
- 1. Convert rain over area
100 m² × 50 mm = 5,000 L before collection losses.
- 2. Apply collection efficiency
5,000 L × 80% = 4,000 L potential captured water.
Potential April capture is 4,000 L (4 m³, about 1,057 US gal). This does not predict water quality or an unentered month.
How to Interpret Your Results
| Condition | What It Means |
|---|---|
| Demand is omitted | Potential capture is still shown, but coverage is unavailable rather than assumed to be zero. |
| Demand is explicitly zero | Coverage has no meaningful denominator, so it is shown as not applicable. |
| Tank mode is enabled | Ending tank, overflow and shortfall are an approximate supplied-month sequence, not intramonth storage sizing. |
Limitations and Cautions
- Roof runoff is not automatically potable or suitable for crop-contact irrigation. Check treatment, water quality and local requirements.
- The calculator does not model gutter capacity, first flush, debris, evaporation, leakage, pump capacity, rainfall intensity or irrigation scheduling.
- Monthly storage balances cannot resolve rain and demand timing within a month; use daily or event data for detailed tank sizing.
Common Mistakes to Avoid
Using sloped roof surface area
Use effective horizontal catchment area unless a verified drainage design gives another effective value.
Treating roof capture as effective field rainfall
Roof collection and rainfall retained in a crop root zone are different physical questions.
Filling omitted months with a regional average
Add only actual supplied records; this calculator intentionally leaves missing calendar periods unmodelled.
References
- Harvesting Rain in Cochise County (opens in a new tab), University of Arizona Cooperative Extension — Background on roof catchment and rainwater-harvesting planning.
- Rain Barrels in Utah (opens in a new tab), Utah State University Extension — Supports roof-runoff collection arithmetic and practical storage context.
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