Using Harvested Rainwater for Toilet Flushing
Toilet flushing accounts for approximately 30% of indoor household water use, making it the largest single indoor water consumer. Each person flushes an average of 5-6 times per day, using 1.6-3.5 gallons per flush depending on toilet age. Modern low-flow toilets use 1.28 gallons per flush. Our estimate of 20 gallons per person per day assumes standard fixtures. Connecting a rainwater system to toilet supply lines requires a dedicated non-potable plumbing line (typically marked with purple pipe) and may require a building permit. The system needs a pump, pressure tank, and a backup connection to municipal water for dry periods. This indoor use provides year-round savings regardless of season, making it one of the most cost-effective applications of harvested rainwater.
Toilet Flushing Potential by State
How much of your toilet flushing demand can rainwater meet? It depends on where you live. The table below shows the annual harvest, demand coverage, and recommended tank size for toilet flushing in eight representative states, using a 1,500-square-foot asphalt shingle roof and a household of 2.
| State | Annual Rainfall | Annual Harvest | Demand Met | Rec. Tank | Savings/Year |
|---|---|---|---|---|---|
| California | 22.2" | 13,756 gal | 94.2% | 2,500 gal | $123.80 |
| Texas | 28.9" | 22,277 gal | 100.0% | 1,500 gal | $73.00 |
| Florida | 54.5" | 39,622 gal | 100.0% | 5,000 gal | $73.00 |
| New York | 46.2" | 33,641 gal | 100.0% | 2,500 gal | $131.40 |
| Colorado | 15.9" | 11,589 gal | 79.4% | 500 gal | $57.95 |
| Hawaii | 63.7" | 45,229 gal | 100.0% | 5,000 gal | $219.00 |
| Arizona | 13.6" | 8,373 gal | 57.3% | 500 gal | $50.24 |
| Washington | 38.4" | 27,511 gal | 100.0% | 5,000 gal | $87.60 |
Setup Requirements for Toilet Flushing
Connecting rainwater to your toilets is one of the most impactful indoor applications because toilets represent roughly 30% of all indoor water use. The installation requires a dedicated non-potable plumbing line (purple pipe per most building codes) separate from your potable water supply. A pump draws water from the storage tank through a sediment filter and delivers it to a small pressure tank that supplies the toilet fill valves.
A critical component is the municipal water backup: when your rainwater tank runs low, a float-activated valve allows municipal water to refill a small header tank or directly supply the toilet line. An air gap or reduced-pressure backflow preventer between the municipal and rainwater systems prevents cross-contamination — this is a code requirement in virtually all jurisdictions. Most plumbing codes require a permit for this type of installation, and inspection may be required. Plan for a minimum 500-gallon tank dedicated to indoor use to ensure continuity during dry periods.
Recommended Equipment
- Storage tank (500-1,000 gal, dedicated to indoor use)
- Submersible pump with pressure tank
- 20-micron sediment filter
- Purple non-potable piping
- Municipal water backup with air gap
- Float switch and low-level alarm
- Plumbing permit (check local codes)
Other Rainwater Uses
Explore other ways to use harvested rainwater at home:
- Garden Irrigation — 50 gal/day
- Lawn Irrigation — 75 gal/day
- Laundry — 15 gal/day
- Car Washing — 50 gal/day
- Livestock Watering — 30 gal/day
- Pool Top-Off — 10 gal/day
- Drinking and Cooking — 1 gal/day (potable treatment required)
- General Outdoor Use — 30 gal/day
- Pressure Washing — 100 gal/day
Calculate Your Toilet Flushing Potential
Use our Rainwater Harvesting Calculator to see how much of your toilet flushing demand can be met by rainwater in your specific location. Select toilet flushing from the use checkboxes along with any other uses you are considering, and get a personalized monthly supply vs. demand analysis with tank size recommendation and cost savings estimate.