Rain Barrel Size Calculator
Calculate the right rain barrel or cistern size based on your roof area, local rainfall, and intended water use.
Rainwater harvesting captures water from your roof and stores it for garden irrigation, reducing your water bill and conserving municipal water. Sizing your rain barrel or cistern correctly ensures you capture enough water without overflow during storms and have enough stored for dry periods.
Rainwater Collection Formula:
Gallons collected = Roof area (sq ft) × Rainfall (inches) × 0.623
The 0.623 factor converts square feet of roof area receiving 1 inch of rain into gallons (1 square foot receiving 1 inch of rain = 0.623 gallons). A more conservative estimate uses 0.5 to account for evaporation, splash, and first-flush diverter losses.
In metric:
Liters collected = Roof area (m²) × Rainfall (mm)
(1 mm of rain on 1 m² = 1 liter, minus ~20% for losses)
How Much Rain Does Your Roof Collect?
| Roof Area | Per 1 inch rain | Per 1 cm rain |
|---|---|---|
| 500 sq ft (46 m²) | ~310 gal (1,175 L) | ~46 L |
| 1,000 sq ft (93 m²) | ~623 gal (2,360 L) | ~93 L |
| 1,500 sq ft (139 m²) | ~935 gal (3,540 L) | ~139 L |
| 2,000 sq ft (186 m²) | ~1,246 gal (4,720 L) | ~186 L |
Even a moderate 1,000 sq ft roof section produces over 600 gallons from just 1 inch of rain — that is a LOT of water.
Garden Water Usage Estimates:
| Garden Size | Water Need/Week |
|---|---|
| Small (100 sq ft / 9 m²) | 60 gal (227 L) |
| Medium (300 sq ft / 28 m²) | 180 gal (680 L) |
| Large (500 sq ft / 46 m²) | 300 gal (1,135 L) |
| Lawn (1,000 sq ft / 93 m²) | 600 gal (2,270 L) |
Gardens typically need about 1 inch of water per week during the growing season, which translates to approximately 0.6 gallons per square foot per week.
Rain Barrel Sizes:
| Type | Capacity | Best For |
|---|---|---|
| Small barrel | 50 gal (190 L) | Container gardens, small beds |
| Standard barrel | 55–65 gal (210–250 L) | Small to medium gardens |
| Large barrel | 80–100 gal (300–380 L) | Medium gardens |
| Slim tank | 100–200 gal (380–760 L) | Tight spaces along walls |
| Cistern | 250–1,000+ gal | Large gardens, serious collection |
| Underground tank | 500–5,000 gal | Whole-property irrigation |
Sizing Strategy:
- Calculate how much rain your roof section produces per average storm
- Calculate how much water your garden uses between storms
- Size your storage to bridge the gap between rain events
For most residential gardens, 1–2 standard 55-gallon barrels connected in series is a practical starting point. Each downspout can feed a separate barrel or chain of barrels.
Important Components:
- First-flush diverter: Diverts the first gallon of dirty roof water (bird droppings, dust) away from your barrel
- Overflow outlet: Directs excess water away from your foundation when the barrel is full
- Screen/filter: Keeps mosquitoes, leaves, and debris out
- Spigot: Near the bottom for gravity-fed garden hose connection
- Elevated stand: Raise the barrel 12–24 inches for better water pressure
Maintenance: Clean the barrel once per year. Check screens and filters after storms. In freezing climates, drain and disconnect barrels before winter to prevent ice damage. Store upside-down or indoors during winter.
Legal Note: Most states in the United States allow residential rainwater collection, but a few have restrictions. Colorado limited collection until 2016, and some states require permits for large cisterns. Check your local regulations before installing a system.