Sustainability & Green Build
Rainwater Harvesting Calculator
Estimates tank size and annual saving by roof area and rainfall
Updated 13 May 2026 · Live
What this tool does
Estimates the volume of rainwater you could harvest per year from a roof catchment area, rainfall, and collection efficiency.
Rainwater Harvesting Calculator — estimate harvestable rainwater (litres/yr) from {roof_area_m2} m² of catchment area, {annual_rainfall_mm} mm annual rainfall, and {efficiency_pct}% collection efficiency, giving {result}.
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Formula Used
How the rainwater harvesting calculator works
Estimates tank size and annual saving by roof area and rainfall. The calculator takes your dimensions and supplier rates, applies a standard UK trade formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical UK sustainability and green build wastage
Carbon and energy calculations are sensitivity-driven, not wastage-driven. The biggest errors come from optimistic self-use figures on PV or unrealistic air-tightness on fabric. Our defaults reflect common UK trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm Building Regulations compliance. Those remain the responsibility of a suitably qualified designer, surveyor, or your building control officer.
On-site considerations for rainwater harvesting
MCS certification is required on any installation claiming SEG export tariff, RHI grants, or the Boiler Upgrade Scheme. Uncertified installers can void both grant and warranty.
Building Regulations and compliance
Approved Document L (energy) and the Future Homes Standard trajectory are raising fabric requirements every cycle. Build ahead of the current minimum to futureproof. When in doubt, a pre-application enquiry to the local authority can give early clarity, which tends to be less costly than retrospective correction.
Before you order
Order PV with a minimum 25-year product warranty and a DNO-approved inverter. Cheap panels are a false economy if the warranty fails. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this rainwater harvesting calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labour-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the maths.
Sources & methodology
Estimates tank size and annual saving by roof area and rainfall. Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are rainwater harvesting calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What collection efficiency should I use?
The calculator defaults to 85%, a typical UK allowance that accounts for losses from evaporation, first-flush diversion, overflow, and filtration. Well-maintained systems with good filtration can achieve slightly higher efficiency; older or poorly maintained guttering may be lower. Adjust the figure to match your system.
Does this replace professional advice?
No — this tool is a planning estimator only. For works that affect structure, Building Regulations, Party Wall matters, gas, electrics, or drainage to a public sewer, consult a suitably qualified professional.
Can I use this figure to estimate money saved on water bills?
The calculator reports harvestable rainwater in litres per year, not a monetary saving. To estimate a financial saving, multiply the litres/yr figure by your local water supplier's rate per litre (or per m³) and adjust for the portion of harvested water you actually use, since tank size and demand also limit real-world savings.
Spotted something off?
Calculations or display — let us know.
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