Everything you need to size a rainwater system on one page: the core formula, collection tables by roof size, garden demand estimates, and storage options compared.
1. The Core Formula
| What You Want | Formula | Example |
|---|---|---|
| Gallons per rain event | Roof sq ft × rainfall inches × 0.623 × 0.9* | 1,000 sq ft × 1" × 0.623 × 0.9 ≈ 561 gal |
| Annual collection potential | Roof sq ft × annual rainfall × 0.623 × 0.9 | 1,000 sq ft × 30" × 0.623 × 0.9 ≈ 16,800 gal/yr |
| Roof "footprint" | Length × width of the building (not roof surface) | Slope doesn't change catchment area |
* 0.623 converts inches over 1 sq ft to gallons; 0.9 accounts for ~10% loss to evaporation, splash, and first-flush diversion (metal roofs ~0.95, asphalt shingle ~0.85–0.9).
2. Gallons Collected per 1 Inch of Rain
| Roof Footprint | Per 1" Rain | Per 5" Month | Per 30" Year | 55-gal Barrels Filled (1" rain) |
|---|---|---|---|---|
| 500 sq ft | 280 gal | 1,400 gal | 8,400 gal | 5 |
| 800 sq ft | 449 gal | 2,240 gal | 13,450 gal | 8 |
| 1,000 sq ft | 561 gal | 2,800 gal | 16,820 gal | 10 |
| 1,500 sq ft | 841 gal | 4,210 gal | 25,230 gal | 15 |
| 2,000 sq ft | 1,121 gal | 5,610 gal | 33,640 gal | 20 |
| 3,000 sq ft | 1,682 gal | 8,410 gal | 50,460 gal | 30 |
Values include the 0.9 efficiency factor, rounded. A single downspout typically drains 25–50% of the roof — size each barrel to its downspout share.
3. How Much Water Does the Garden Need?
| Garden Size | Weekly Need (1" equivalent) | Monthly Need | Storage to Cover 3 Dry Weeks |
|---|---|---|---|
| 100 sq ft (small bed) | 62 gal | 250 gal | ~190 gal (4 barrels) |
| 250 sq ft | 156 gal | 625 gal | ~470 gal (9 barrels) |
| 500 sq ft | 312 gal | 1,250 gal | ~940 gal (IBC tote × 3–4) |
| 1,000 sq ft | 623 gal | 2,500 gal | ~1,870 gal (cistern territory) |
Rule of thumb: vegetables need ~1 inch of water per week (0.623 gal per sq ft). Drip irrigation can cut usage 30–50%.
4. Storage Options at a Glance
| Container | Capacity | Typical Cost | Best For |
|---|---|---|---|
| Rain barrel | 50–80 gal | $80–$180 | Single downspout, patio containers, small beds |
| IBC tote (food-grade) | 275–330 gal | $100–$250 used | Serious garden irrigation on a budget |
| Poly tank / cistern | 500–5,000 gal | $400–$3,000+ | Homestead-scale storage, livestock backup |
| Underground cistern | 1,000–10,000+ gal | $2,000–$15,000+ | Whole-property supply, freeze protection |
🚱 First Flush
Divert the first 1–2 gallons per 100 sq ft of roof each rain — it carries most of the dust, pollen, and bird droppings.
🦟 Mosquito Control
Screen every inlet and overflow. Use mosquito dunks (BTI) in open barrels — safe for plants and pets.
❄️ Winterizing
Drain barrels before hard freezes, leave valves open, and store upside down or connect a winter bypass on the downspout.
⚖️ Legality
Most US states allow residential collection; a few (e.g., Colorado) cap it (2 barrels / 110 gal). Check state and HOA rules first.