Decision checklist
Rainwater tank buying checklist
A modeled capacity is only one part of a workable system. Use this checklist to turn calculator results into better questions for suppliers, installers, local authorities, and qualified designers.
Before asking for a quote: save the modeled comparison, record your rainfall and water-use assumptions, and check the space, delivery route and installation requirements. Nominal capacity alone is not enough to choose a tank.
Build a tank comparison → then use Print / Save PDF in the results to keep the assumptions and candidate sizes together.
1. Define exactly what the water will supply
List the intended uses before comparing tanks. Garden irrigation, toilet flushing, laundry, equipment wash-down, stormwater control, emergency reserve, and whole-house supply have different timing, plumbing, treatment, and legal requirements. Keep potable use outside a preliminary calculator unless local qualified guidance explicitly addresses collection, treatment, testing, and maintenance.
2. Measure the roof that will actually be connected
Use the horizontal footprint of the roof sections draining through connected gutters and downpipes. Exclude roof areas that discharge elsewhere. Check drawings against the physical gutter layout, valleys, parapets, leaf guards, and available downpipe routes.
3. Test demand instead of guessing one perfect number
Build demand from measurable uses: volume per event × events over a stated period. Run low, central, and high cases. Outdoor demand is often seasonal. Choose the active months in the calculator and estimate average daily use during that season; rainfall still fills the tank in other months. Treat a sensitive result as a reason to improve the demand estimate.
4. Use rainfall timing, not only an annual total
Annual rainfall shows potential yield but not whether water arrives before the days it is needed. Compare candidate capacities against a multi-year daily series, then check a nearby quality-controlled gauge when the investment depends on site-specific accuracy. The planner’s NASA POWER series is a consistent global screen, not a property rain gauge.
5. Compare a capacity ladder and read the whole result
Test sizes that are commercially and physically realistic. Compare demand coverage, full-demand days, annual supply, overflow, and longest shortfall together. The highlighted near-plateau tank only identifies diminishing modeled returns among the capacities entered; it does not know price, installation constraints, resilience goals, or local requirements.
6. Confirm space, access, support, and full-tank load
- Measure the installed footprint, height, clearances, and maintenance access—not only nominal volume.
- Confirm delivery access through gates, paths, cranes, excavation routes, or the building.
- Ask a qualified person what base, foundation, restraint, burial, groundwater, wind, seismic, or structural checks apply.
- Remember that one litre of water has a mass of about one kilogram before adding the tank and equipment.
7. Design collection, first flush, inlet, and safe overflow
A tank needs more than a downpipe. Check gutter and inlet capacity, debris screening, first-flush arrangements, calming inlet, ventilation, access covers, mosquito and pest exclusion, usable dead volume, and an overflow route that will not damage foundations or neighboring property. Overflow may need to connect to an approved drainage or infiltration system.
8. Resolve pumps, pressure, backflow, and water quality
For garden use, check how long stored water lasts and compare gravity pressure at current and refill levels. A sufficient static pressure ceiling still needs a delivery test with the actual kit.
Gravity supply and pumped indoor uses have different equipment and energy needs. Where rainwater and mains plumbing interact, appropriate separation and backflow protection are essential. Treatment and monitoring depend on the source, roof and tank materials, intended use, and local health guidance; the sizing result does not establish water safety.
9. Check the rules for the actual property
Rules can differ by country, state, municipality, water supplier, building type, end use, and whether the system also manages stormwater. Use current primary sources and ask the relevant authority or qualified professional. Do not treat a general international page as permission to install.
- United States: the U.S. Department of Energy technology review explains sizing, system components, maintenance, and the need to check local permits.
- United Kingdom: the current national SuDS standards reference BS EN 16941 and rainfall time-series evidence for relevant designs.
- Australia: the government-backed YourHome rainwater guide covers tank sizing, uses, plumbing separation, maintenance, and jurisdiction-specific requirements.
10. Compare lifetime cost and maintenance, not only tank price
Record the tank, delivery, base or excavation, plumbing, pump, controls, filters, first flush, overflow work, treatment, inspections, electricity, consumables, repairs, and eventual replacement. Ask what must be cleaned or tested and how safely it can be accessed. Use measured local water prices and a conservative modeled annual supply if you calculate savings.
Finding a supplier for a modeled size
After calculating, choose your delivery country in the supplier section. The planner shows up to three nearby nominal sizes from a small, manually reviewed selection. It ranks by capacity difference, not commission, price or supplier quality. This is a starting point, not a complete market comparison.
Listings within 30% of the highlighted capacity can appear. A smaller or larger product is labelled with that difference. Use Compare this size to add its stated capacity, then press Compare tank sizes again. A supplier's nominal capacity may also differ from its usable installed volume.
For countries or sizes outside this selection, the local-search link uses the tank size and selected country. No coordinates, household details or uploaded rainfall are included. Prices, stock, delivery eligibility and installation type must be checked directly with the seller.
- Norway: examples from Importpris and vidaXL are collapsible garden containers, labelled as such. Larger sizes use the local-search fallback.
- UK: public listings from Enduramaxx, The Tank Shop, Tanks for Everything, and Aquatic Living.
- US: Bushman USA lists rain-harvesting tanks in US gallons and sells through distributors. Some links go to dealer-contact/specification pages; retail examples include Rainwater Collection Concepts.
- Australia: QTank lists several standard litre capacities. Its main delivery area is south-east Queensland and northern NSW; ask about delivery elsewhere.
Sources reviewed 7 October 2026. These are ordinary unpaid links. No prices or stock claims are copied into the planner. Always compare delivery, access, dimensions, equipment and installation requirements before choosing a product.
Before shortlisting products, check the tank sizing workflow and roof-yield and water-demand assumptions. A nominal capacity alone does not establish suitability.
Take this brief to a supplier
- Export the candidate-size comparison from the planner.
- Write down the rainfall source, period, roof area, efficiency, demand cases, and intended uses.
- Mark the available space, access route, proposed inlet, overflow destination, and uses on a simple site sketch.
- Ask each supplier to identify assumptions, exclusions, standards, maintenance, warranty, and responsibilities in writing.
- Re-run the model if a quote changes usable volume, connected roof area, losses, or demand.