Score from the build-order list
This calculator is item #40 — Rainwater Harvesting Tank Sizer from the build-order list, with score:
Vol / Comp / Ratio = 3 / 1 / 3.0It is the fourth calculator from the research-prioritised group after cabinet paint, pond pump/filter, and compost C:N.
What this estimate does and does not do
This calculator estimates rainwater collection and storage from a roof catchment. It calculates annual harvest, average monthly harvest, dry-spell storage need, selected storm capture, overflow from a current tank, and a recommended practical tank size.
It does not design a potable water system. Drinking-water use requires appropriate roof materials, first-flush diversion, sediment filtration, disinfection, testing, backflow prevention, and compliance with local regulations. For household supply, fire storage, code-required cisterns, or off-grid potable systems, use local professional guidance.
Formula / method used
Imperial catchment formula
One inch of rain on one square foot yields about 0.623 US gallons before losses:
annual harvest gallons = roof area ft² × annual rainfall in × 0.623 × net efficiencyMetric catchment formula
One millimetre of rain on one square metre yields about one litre before losses:
annual harvest litres = roof area m² × annual rainfall mm × net efficiencyNet efficiency
net efficiency = roof runoff efficiency × (1 - first-flush/loss %)Default roof efficiencies:
| Roof material | Efficiency used |
|---|---|
| Metal roof | 85% |
| Tile / slate roof | 80% |
| Composition shingle roof | 75% |
| Flat / low-slope roof | 70% |
| Custom | user input |
Demand and dry-spell storage
annual demand = daily demand × use days per year
dry-spell need = daily demand × dry-spell days × (1 + reserve %)Storm capture and tank target
The selected storm depth estimates how much water a single storm could send to the tank if it starts empty:
storm capture = roof area × storm rainfall × conversion factor × net efficiencyThe storage target is whichever is larger:
storage target = max(dry-spell need, storm capture)The calculator rounds that target up to a common tank tier.
Worked example
For a 1,500 sq ft metal roof, 30 inches of annual rain, 5% first-flush/loss allowance, 20 gallons/day garden demand for 120 days/year, a 21-day dry-spell target, 15% reserve, and a 1-inch storm capture target:
net efficiency = 85% × (1 - 5%) = 80.75%
gallons per inch = 1,500 × 0.623 × 0.8075 ≈ 754.6 gal/in
annual harvest = 754.6 × 30 ≈ 22,638 gal/yearDry-spell need:
dry-spell storage = 20 × 21 × 1.15 = 483 galStorm capture:
1-inch storm capture ≈ 754.6 galThe larger target is about 755 gallons, so the calculator rounds up to a common 1,000 gallon tank tier.
How to use the result
- Use actual roof catchment area, not whole-lot area.
- Use local rainfall data and realistic dry-spell assumptions.
- Use daily demand for the actual use: irrigation, animals, washing, toilet flushing, or household supply.
- Size for dry-spell storage if your goal is water availability.
- Size for storm capture if your goal is runoff reduction or making the most of large storms.
- Route overflow away from foundations and erosion-prone areas.
- Add leaf screens, first-flush diversion, mosquito screening, and clean-out access.
Assumptions and limitations
- Annual rainfall can hide long dry seasons. Monthly rainfall distribution matters.
- Roof material, slope, gutters, leaf screens, first flush, evaporation, and leaks all affect collection.
- Tank size does not guarantee water availability if annual demand exceeds annual harvest.
- First-flush requirements vary by roof, debris, air quality, and intended use.
- Potable systems require much more than sizing math.
- Local rules may restrict rainwater harvesting, require permits, or require backflow protection.
For related garden/outdoor calculators, see the Compost C:N Ratio Blender and Pond Pump and Filter Turnover Calculator.
Frequently asked questions
How much rainwater can I collect from my roof?
In imperial units, estimated annual gallons are roof area in square feet multiplied by rainfall in inches, 0.623, and a collection efficiency. In metric, 1 mm of rain on 1 m² is about 1 litre before losses.
What size rainwater tank do I need?
Tank size depends on your dry-spell water demand, roof catchment, rainfall pattern, and whether you want to capture a chosen storm depth. This calculator recommends the larger of dry-spell storage need and selected storm capture, rounded up to a common tank size.
What roof efficiency should I use?
Metal roofs often collect efficiently, around 80–90%. Tile and shingle roofs are usually lower due to roughness, absorption, and debris. Use a lower efficiency if you have long gutters, leaf screens, or heavy first-flush diversion.
Can a larger tank fix low rainfall?
No. A larger tank stores more when rain arrives, but it cannot create water. If annual harvest is below annual demand, reduce demand, add roof area, or use another water source.
Can I drink harvested rainwater?
Only with proper design, filtration, disinfection, testing, backflow prevention, and compliance with local rules. Roof runoff can contain biological and chemical contamination.