Rainwater Harvesting in Hawaii
Hawaii (HI) receives an average of 63.7 inches of rainfall per year, placing it in the high-rainfall category among US states. The wettest month is typically Nov, and the driest is Jun. Rainwater harvesting is fully legal in Hawaii. A standard 1,500-square-foot asphalt shingle roof can collect approximately 45,229 gal per year, saving an estimated $160.50 annually at the state's average municipal water rate of 1.5 cents per gallon.
Monthly Rainfall in Hawaii
Understanding Hawaii's monthly rainfall distribution is essential for sizing your storage tank correctly. The table below shows average monthly rainfall and the resulting harvest from a 1,500-square-foot roof. With 63.7 inches annually, Hawaii provides abundant raw material for rainwater collection. The key is matching your storage capacity to the seasonal pattern — storing surplus from wet months to use during dry periods.
| Month | Rainfall (in) | Harvest (1,500 sq ft) |
|---|---|---|
| Jan | 5.0" | 3,738 gal |
| Feb | 4.2" | 3,140 gal |
| Mar | 6.0" | 4,486 gal |
| Apr | 5.5" | 4,112 gal |
| May | 3.8" | 2,841 gal |
| Jun | 3.2" | 2,392 gal |
| Jul | 4.5" | 3,364 gal |
| Aug | 4.8" | 3,588 gal |
| Sep | 5.0" | 3,738 gal |
| Oct | 5.8" | 4,336 gal |
| Nov | 6.5" | 4,859 gal |
| Dec | 6.2" | 4,635 gal |
| Total | 63.7" | 45,229 gal |
Harvest by Roof Size in Hawaii
Your actual harvest depends on how much roof area you connect to your collection system. Not every square foot of your roof needs to drain into your tank — many homeowners start by connecting just one or two downspouts covering a portion of their roof. The table below shows what different collection areas yield annually in Hawaii using an asphalt shingle roof at 80% efficiency.
| Roof Area | Annual Harvest | Rec. Tank Size | Annual Savings |
|---|---|---|---|
| 1,000 sq ft | 30,153 gal | 5,000 gal | $160.50 |
| 1,500 sq ft | 45,229 gal | 5,000 gal | $160.50 |
| 2,000 sq ft | 60,307 gal | 10,000 gal | $160.50 |
| 2,500 sq ft | 75,383 gal | 10,000 gal | $160.50 |
Upgrading to a metal roof (95% efficiency) boosts collection by about 19% compared to asphalt shingle. For example, a 1,500-square-foot metal roof in Hawaii captures approximately 53,712 gal per year when used for garden irrigation and toilet flushing for a household of two.
Rainwater Harvesting Laws in Hawaii
Hawaii encourages rainwater harvesting, and many rural homes rely on catchment systems as their primary water source. Over 60,000 residents use rainwater catchment. The state Department of Health provides guidelines for potable rainwater systems.
Regulation Summary for Hawaii
- Legal Status: Legal — no restrictions
- Max Collection: No limit
- Permit Required: No — for standard residential systems
- Municipal Water Rate: 1.5 cents per gallon (state average)
Popular Uses for Harvested Rainwater in Hawaii
Hawaii homeowners use harvested rainwater for a variety of purposes. The most common applications are garden irrigation, lawn watering, and toilet flushing. Each use has different water requirements and may require different levels of filtration. Explore detailed guides for the most popular uses in Hawaii:
How Hawaii Compares to Similar States
The table below compares Hawaii with five states that have similar annual rainfall patterns. All calculations use a standard 1,500-square-foot asphalt shingle roof collecting water for garden irrigation.
| State | Annual Rainfall | Legal Status | Annual Harvest | Water Cost | Annual Savings |
|---|---|---|---|---|---|
| Hawaii | 63.7" | Legal | 45,229 gal | 1.5 c/gal | $160.50 |
| Louisiana | 60.1" | Legal | 46,201 gal | 0.4 c/gal | $42.80 |
| Mississippi | 56.9" | Legal | 41,865 gal | 0.4 c/gal | $42.80 |
| Alabama | 56" | Legal | 40,371 gal | 0.5 c/gal | $53.50 |
| Florida | 54.5" | Legal | 39,622 gal | 0.5 c/gal | $53.50 |
| Tennessee | 53.7" | Legal | 38,274 gal | 0.4 c/gal | $42.80 |
Tips for Rainwater Harvesting in Hawaii
With 63.7 inches of annual rainfall, Hawaii is well-suited for rainwater harvesting. You can start with a simple 55-gallon rain barrel and upgrade as you see the benefits. Since rainfall is abundant, even a small roof area connected to a modest barrel provides useful water for potted plants, container gardens, and spot watering. For more serious collection, a 500-1,000 gallon tank connected to multiple downspouts can supply most of your garden irrigation needs. Consider a first-flush diverter to improve water quality, especially after dry spells when dust and debris accumulate on your roof. In Hawaii's climate, be sure to protect above-ground tanks from direct sunlight to prevent algae growth and maintain water quality.
Calculate Your Specific Harvest in Hawaii
The estimates above use standardized assumptions. Use our Rainwater Harvesting Calculator to enter your exact roof area, material, household size, and intended uses for a customized analysis tailored to your property in Hawaii. You can also browse all states to compare potential across the country, or explore tank sizes and water uses for detailed information on system components and water budgeting.