Rainwater Harvesting Gallon Collection Estimator
1. Roof Area & Rainfall
Illustrative rainfall example: 36 inches per year. These values are invented for demonstration, not local historical averages. Replace them with your own monthly totals.
Rain falls on the horizontal roof footprint. Sloped surface area would overstate catchment. Monthly rainfall range: 0–1,000 inches or 0–25,400 mm.
Estimated annual collection
38,150.65 US gal annual potential collection
Annual potential collection before tank overflow.
- Wettest collection months
- May
- Driest collection months
- January, February, August, September
Ties are included. Tank demand and storage settings follow below.
2. Monthly Collection Results
Before tank overflow. Wettest collection months: May. Driest: January, February, August, September. Ties are included.
- January2,119.48 US gal · driest
- February2,119.48 US gal · driest
- March3,179.22 US gal
- April4,238.96 US gal
- May5,298.7 US gal · wettest
- June4,238.96 US gal
- July3,179.22 US gal
- August2,119.48 US gal · driest
- September2,119.48 US gal · driest
- October3,179.22 US gal
- November3,179.22 US gal
- December3,179.22 US gal
3. Tank Size & Water Demand
Non-leap year (365 days). Start empty; repeat the same 12 months until January storage changes by at most 0.0000001 US gal, up to 1,000 years. Monthly inflow arrives before monthly demand; storm timing is unresolved and coverage may be optimistic. Efficiency excludes the separately entered monthly first-flush allowance, capped at available runoff; no event counts are inferred. Display rounds to at most 2 decimals; intermediates are unrounded. Non-potable planning only.
Tested capacities (US gal): 50, 100, 250, 500, 1000, 1500, 2500, 5000, 10000, 20000. Recommendation is the smallest tested tank meeting your annual coverage target in a converged seasonal cycle.
Recommended tested tank: 1,500 US gal
Selected tank supplies 65.75% of demand. Captured after overflow: 12,000 US gal; supplied: 12,000 US gal; unmet: 6,250 US gal; overflow: 26,150.65 US gal.
Seasonal simulation: 1 repeated years; converged. Reported January starting storage: 0 US gal.
| Month | Days | Starting | Potential inflow | Captured | Overflow | Demand | Supplied | Unmet | Ending |
|---|---|---|---|---|---|---|---|---|---|
| January | 31 | 0 | 2,119.48 | 1,000 | 1,119.48 | 1,550 | 1,000 | 550 | 0 |
| February | 28 | 0 | 2,119.48 | 1,000 | 1,119.48 | 1,400 | 1,000 | 400 | 0 |
| March | 31 | 0 | 3,179.22 | 1,000 | 2,179.22 | 1,550 | 1,000 | 550 | 0 |
| April | 30 | 0 | 4,238.96 | 1,000 | 3,238.96 | 1,500 | 1,000 | 500 | 0 |
| May | 31 | 0 | 5,298.7 | 1,000 | 4,298.7 | 1,550 | 1,000 | 550 | 0 |
| June | 30 | 0 | 4,238.96 | 1,000 | 3,238.96 | 1,500 | 1,000 | 500 | 0 |
| July | 31 | 0 | 3,179.22 | 1,000 | 2,179.22 | 1,550 | 1,000 | 550 | 0 |
| August | 31 | 0 | 2,119.48 | 1,000 | 1,119.48 | 1,550 | 1,000 | 550 | 0 |
| September | 30 | 0 | 2,119.48 | 1,000 | 1,119.48 | 1,500 | 1,000 | 500 | 0 |
| October | 31 | 0 | 3,179.22 | 1,000 | 2,179.22 | 1,550 | 1,000 | 550 | 0 |
| November | 30 | 0 | 3,179.22 | 1,000 | 2,179.22 | 1,500 | 1,000 | 500 | 0 |
| December | 31 | 0 | 3,179.22 | 1,000 | 2,179.22 | 1,550 | 1,000 | 550 | 0 |
How to use this calculator
Estimate monthly and annual rainwater collection from your roof, compare seasonal rainfall, and find a practical rainwater tank size based on water demand.
Enter catchment area, monthly rainfall, collection efficiency and optional monthly first-flush diversion. Compare potential collection, then edit outdoor demand and a tank capacity. All calculation inputs stay in your browser; no account or location lookup is used.
Collection and storage methodology
US gallons = roof area in square feet × rainfall in inches × (144 / 231) × efficiency fraction. One square meter is 1 / 0.09290304 square feet; one inch is 25.4 millimeters; one US gallon is 3.785411784 liters. Subtract the separately entered monthly first-flush allowance, capped at runoff. Never round intermediate results.
For each month: overflow = max(0, starting storage + inflow − capacity); captured = inflow − overflow; available = min(capacity, starting storage + inflow); supplied = min(available, daily demand × days); unmet = demand − supplied; ending storage = available − supplied. Ending storage becomes next month’s starting storage. Days follow a non-leap calendar with 28 days in February. Monthly inflow arrives before demand.
Repeat from an empty tank until annual boundary storage converges to within 10⁻⁷ US gallon, or stop after 1,000 years. A candidate that does not converge cannot be recommended. The selected tank’s results are separate from the recommendation. No demand means no tank recommendation. Display uses at most two decimals; CSV numeric results use six decimals.
Exact conversion worked example
For 2,000 sq ft, 3 inches and 85% efficiency, the rounded 0.623 factor gives 3,177.3 US gallons. The exact 144 / 231 factor gives 3,179.220779… US gallons, displayed as 3,179.22. For 36 inches annually, the rounded factor gives 38,127.6 gallons; the exact factor gives 38,150.649350… gallons, displayed as 38,150.65. The difference is 23.049350… gallons per year. Neither method gives 38,144 gallons. These figures exclude first-flush diversion and tank overflow.
Limitations and safe use
Actual harvest depends on rainfall timing, roof contamination, gutter losses, first-flush diversion, overflow and maintenance. Monthly averages cannot guarantee drought resilience. Efficiency presets, outdoor demand defaults and candidate capacities are planning assumptions, not product specifications. Demand is constant every day; soil, planted area, evaporation and seasonal watering restrictions are not modeled.
This estimate does not establish potable-water suitability or claim water is safe to drink. Local rules, permits, appropriate treatment, testing and backflow protection may apply. Keep harvested water separate from drinking-water plumbing and obtain local guidance. No historical weather integration is included: a documented source, coverage, period, units and sustainable access need verification before adding location lookup.
For a separately measured pipe segment, check pipe volume and average water velocity using its actual inside diameter and the applicable length or flow inputs. That model does not design gutters, tank plumbing or pumps, or establish potable-water safety.
FAQ
Why use the roof footprint?
Vertical rainfall reaches the horizontal projected area. Use only the area connected to your collection system, rather than the larger sloped surface.
Is the example real local rainfall?
No. The 12 example values are illustrative and sum to 36 inches. Enter measured totals or documented local monthly averages. Location lookup is deferred; no city or postal averages are invented and no location permission is requested.
What are the material efficiencies?
Metal 90%, asphalt shingles 85%, tile 80%, flat roof 75% are illustrative editable defaults. Roof condition, gutter performance and maintenance affect actual collection.
How is first flush calculated?
Enter your total diversion allowance per month in US gallons. It is subtracted after efficiency and capped at available runoff. Do not also include that diversion in efficiency. Per-event diversion is not modeled; sum your expected diversions independently.
Does annual collection mean usable supply?
Potential collection is runoff after efficiency and first flush, before overflow. Captured water subtracts tank overflow; supplied water is the portion meeting demand. Storage carries between months.
How are tank sizes recommended?
Tested tanks range from 50 to 20,000 US gallons. The smallest converged candidate meeting the editable annual demand-coverage target is recommended. If none meets it, no recommendation is made.
Does the model start with a full tank?
It starts empty, then repeats the same non-leap rainfall year until January storage changes by at most 0.0000001 gallon, with a 1,000-year limit. Results describe the stable seasonal cycle rather than first-year performance.
Does it model storms and dry spells?
No. Each month adds all inflow before removing demand. This coarse assumption can overstate supply and does not resolve storms or dry spells. Use daily records for detailed sizing.
Can I switch to metric units?
Yes. Area and rainfall values convert on unit changes. Results can show liters; demand, tank and first-flush input labels remain explicitly in US gallons. Exports use US gallons with unit labels.
Is collected rainwater safe to drink?
This calculator estimates quantities for non-potable planning. It does not establish water quality. Contaminants, treatment, testing, local permits and backflow protection need separate assessment.
Sources and method notes
TWDB describes roof catchment and collection losses; CDC describes rainwater quality and separation from drinking-water plumbing. These references do not validate this tool’s illustrative efficiency or demand presets. The explicit formulas above define this estimator.