📖 Overview
This calculator converts roof area and rainfall into realistic annual harvest potential.
It also sizes a storage tank based on planned demand and autonomy target.
🧪 Example Scenarios
Use these default and higher-pressure example inputs to explore how sensitive this calculator is before using your real numbers.
| Input | Base Case | Higher Pressure Case |
|---|---|---|
| Roof Collection Area (sq ft) | 2,200 | 2,530 |
| Annual Rainfall (in) | 34 | 39.1 |
| Capture Efficiency (%) | 82 | 98.4 |
| Daily Non Potable Demand (gal) | 65 | 74.75 |
| Desired Autonomy Days | 21 | 25.2 |
⚙️ How It Works
Projects annual rainwater capture from roof area rainfall and collection efficiency, then sizes storage around usage autonomy needs.
The Formula
Annual Gallons = RoofArea × RainInches × 0.623 × Efficiency
| Roof Area | Effective catchment area in square feet |
| Rainfall | Average annual rainfall in inches |
| Efficiency | Collection efficiency after system losses |
| Daily Demand | Planned non-potable daily usage |
| Autonomy Days | Storage continuity target without rainfall |
💡This calculator is scenario-based. Better input quality leads to better decision quality.
Quick Reference
| Roof × Rain | Efficiency | Annual Capture | Suggested Cistern |
|---|---|---|---|
| 1,500 sq ft × 25 in | 80% | ~18,700 gal | Demand × autonomy |
| 2,200 sq ft × 34 in | 82% | ~38,300 gal | Demand × autonomy |
| 3,000 sq ft × 40 in | 85% | ~63,500 gal | Demand × autonomy |
When To Use This
- Use this tool when you need a fast decision during active planning or execution.
- Use this before committing money, time, or tradeoffs that are hard to reverse.
- Use this to compare options using the same assumptions across scenarios.
Edge Cases To Watch
- Results can be misleading if key inputs are missing, stale, or unrealistic.
- Very small or very large values may amplify rounding effects and interpretation risk.
- If assumptions change mid-decision, recalculate before acting.
Practical Tips
💡 Use realistic roof runoff coefficients for your roof material.
💡 Do not size storage only from annual totals; include dry-season patterns.
💡 Separate potable and non-potable system design requirements.
Frequently Asked Questions
❓ What does 0.623 represent?
It converts one inch of rain on one square foot into gallons of water.
❓ How should I choose autonomy days?
Use your longest expected dry spell plus a safety margin for reliability.