Input guide
How much rainwater can you collect and use?
Estimate potential collection from roof area, rainfall and losses, then calculate demand from the uses you want to supply. Keep these two quantities separate: water reaching the tank is not necessarily water available when you need it.
Quick answer: roof area in m² × rainfall in mm × collection efficiency gives potential litres before overflow. For 100 m², 600 mm and 80% efficiency, that is 48,000 L. Storage and daily water use decide how much you can actually use.
Estimate your roof and water use → or start with a garden example.
Rainwater collection formula and worked example
One millimetre of rain falling on one square metre of roof footprint equals one litre before losses. Use the horizontal area connected to the tank, not the sloped roof surface.
Potential collection (L) = roof footprint (m²) × rainfall (mm) × collection efficiency
For an illustrative 100 m² roof, 600 mm of annual rainfall and 80% collection efficiency: 100 × 600 × 0.8 = 48,000 L per year, approximately 12,680 US gallons. This is potential inflow before tank overflow, not guaranteed water supply or a tank-size recommendation.
For US inputs, use roof area in ft² × rainfall in inches × 0.6234 × efficiency to estimate US gallons. For example, 1,000 ft² × 1 inch × 0.6234 × 0.8 gives approximately 499 US gallons before tank overflow. Use 0.8 for 80%, not 80.
Annual yield alone cannot tell you when water is available. The same-rainfall storage example keeps the roof and annual rainfall fixed but changes the wet/dry sequence. Use daily rainfall data and a tank simulation to compare useful supply and overflow.
Start with the connected catchment
If measuring is stopping you from trying the calculator, choose a quick roof estimate: 30 m² for a small outbuilding, or 80, 120 or 200 m² as larger starting footprints. These are illustrative rectangles of 6 × 5, 8 × 10, 10 × 12 and 10 × 20 metres, not surveyed averages for a house type or country. The labels and areas convert to US units when selected. You can edit the area at any time.
Use the footprint of the roof that will drain to the tank, rather than total floor area across several storeys or the sloping roof surface. Reduce the estimate when only part of the roof is connected. Try more than one area to see whether the decision changes, then refine the catchment from plans or measurements before buying. Quick estimates stay labelled in the result and printable summary.
Measure the horizontal footprint of each roof section that will actually drain to the tank. A simple rectangle is length × width; divide an irregular roof plan into smaller shapes and add only connected sections. Do not multiply by roof slope. If only half the downpipes will be connected, only that drainage area belongs in the input.
Check the route physically. Valleys, parapets, separate gutter runs, overflow points, leaf guards, and future extensions can make a drawing differ from the useful catchment.
Build demand from uses
For each intended non-potable use, estimate litres per event and events over a representative period:
Average daily demand = sum of (volume per event × events in period) ÷ days in period
The calculator's optional helpers make two estimates explicit. Garden use is garden area in m² × planned weekly watering in mm ÷ 7, giving average L/day. Toilet use is people × flushes per person per day × litres per flush. The prefilled helper values are editable illustrations, not a recommendation for your plants, climate, household or fixtures. Check your own watering plan and flush volume.
Where possible, use a meter, a timed container test, appliance documentation, or actual bills rather than memory. Keep each component visible so it can be revised. Avoid including potable use unless a qualified design and local rules explicitly support it; this planner does not model treatment or water quality.
Already have a tank or rain barrel? Check your stored-water budget and gravity supply to the garden, including a refill reserve and the kit's pressure requirement when known.
Handle seasonal use deliberately
For seasonal irrigation, enable Use water only in selected months and choose your watering months. The daily amount is used in those months; demand is zero in the others. Rainfall still fills the tank outside the season. Choose months for your own climate and usage, rather than copying the examples' seasons.
The helper spreads weekly watering evenly across seven days. Real watering on one or two days can produce different shortfalls. The model does not represent weekday-only operations or varying use within a month. Use separate runs to bracket the decision:
- Low case: efficient fixtures, restrained irrigation, or lower occupancy.
- Central case: the best supported normal-use estimate.
- High case: hotter or busier periods, leakage allowance, or less conservation.
Enter demand for the active season rather than spreading it across the whole year. For example, 70 m² watered at 10 mm per week means 100 L/day during the chosen months. Selecting April–October makes the other months zero; it does not reduce the active-season amount. This is an arithmetic example, not a watering recommendation.
Automatic water-use sensitivity
The automatic water-use sensitivity table checks low at 75%, normal at 100% and high at 125% of the entered use. Selected months stay the same, including zero use outside the season. It compares coverage and longest shortfall for the tank highlighted in the normal case, and shows which size each case highlights. These percentages are illustrative variations, not forecasts, confidence bounds or recommended household use. Roof area, rainfall and efficiency remain fixed; check those separately.
What collection efficiency represents
Collection efficiency converts roof rainfall into modeled tank inflow. It combines several losses that the current calculator does not model separately:
- first-flush diversion and deliberate bypass;
- filter and screen losses;
- gutter, downpipe, and inlet overflow;
- wind-driven rain that misses the effective catchment;
- splash, leakage, debris, and maintenance condition;
- water below a usable outlet or other effective dead storage.
The U.S. Department of Energy technology review cites 75%–90% as a common range. Treat that as a starting range from a general review, not a certification of your system. Roof material, rainfall intensity, first-flush settings, gutter capacity, and maintenance can move the real result.
Choose efficiency cases
A practical first screen is to run a conservative, central, and optimistic efficiency while holding all other inputs fixed. Record why each number was chosen. If the capacity ranking stays similar, the decision is less sensitive to this assumption. If it flips, inspect gutters, filters, first flush, and usable tank volume before spending money.
Starting storage is not free water
The starting percentage gives each simulated tank an initial volume. Its effect usually fades over a long record, but it can influence early days and unusually dry series. Compare empty and half-full starts when you want to understand that sensitivity. Do not use a full start merely to improve the result unless it represents the question you are asking.
A compact input audit
- Mark connected roof sections and calculate their horizontal area.
- List every intended use with a measurable volume and frequency.
- Choose low, central, and high demand cases.
- Choose at least two defensible collection efficiencies.
- Enter only tank sizes that could physically and commercially be considered.
- Keep rainfall source and period identical while comparing the input cases.
- Write down the assumptions alongside exported results.
Once the inputs are ready, follow the tank sizing workflow and check the buying and installation questions.