Rainwater Harvesting Calculator
Contact Us
Rainwater Harvesting Calculator
Work out how much rainwater a roof can collect, the non-potable demand it could meet, and the storage tank size — the yield, demand and tank trio. Based on The SuDS Manual (CIRIA C753, Chapter 11) and BS EN 16941-1:2024.
Collectable yield
Annual yield = collection area × runoff coefficient × annual rainfall × filter efficiency (C753 Box 11.2).
Non-potable demand
The demand rainwater can supply — WC flushing, laundry and garden/irrigation (C753 Box 11.3).
Storage tank size
The intermediate method sizes the tank as the smaller of 5% of annual yield and 5% of annual demand (about 18 days), per BS EN 16941-1 / BS 8515.
| Surface | Coefficient |
|---|---|
| Pitched roof, profiled metal | 0.95 |
| Pitched roof, tiles | 0.90 |
| Flat roof, no gravel | 0.80 |
| Flat roof with gravel | 0.60 |
| Green roof, extensive* | 0.50 |
| Green roof, intensive* | 0.30 |
| Permeable pavement | 0.80 |
*Green roof yield is uncertain and varies with season – combining a green roof with rainwater harvesting is rarely efficient on small roofs. The 5% rule assumes regular demand and fairly even monthly rainfall. For surface-water (attenuation) duty as well as supply, the tank needs active control / additional storage – see BS EN 16941-1 and C753 §11.3.3.
This rainwater harvesting calculator works out how much rainwater a roof can collect, the non-potable demand it could meet, and the storage tank size — the yield, demand and tank trio. It is based on The SuDS Manual (CIRIA C753, Chapter 11), BS EN 16941-1:2024 and BS 8515. Collecting and reusing rainwater supports the water-efficiency aims of Approved Document G.
Collectable yield from the roof
The annual yield is the collection area times the runoff coefficient, the annual rainfall and the filter efficiency. Enter the plan (footprint) roof area, the surface type, the local average annual rainfall and the filter efficiency, and the calculator returns the average annual yield you could collect.
Working out non-potable demand
Rainwater can supply WC flushing, laundry and the garden. Enter the occupants and uses and the calculator returns the annual non-potable demand — the figure that, alongside yield, sizes the system.
Non-Potable Water Demand Calculator
Work out the annual non-potable water demand — WC and urinal flushing, laundry and irrigation — for a home or a non-domestic building. The figure to size a rainwater harvesting or greywater system. Based on The SuDS Manual (CIRIA C753 Box 11.3) and BS EN 16941 / BS 8525.
Domestic non-potable demand
Demand = daily demand per person × occupants × 365 (C753 Box 11.3).
Non-domestic demand
For offices, schools and similar — demand applies only on occupied/working days, so the annual figure uses working days, not 365.
Typical figures
Indicative non-potable end-use figures and domestic occupancy. Confirm against the actual fittings and BS EN 16941 / BS 8525.
| Use | Typical |
|---|---|
| WC flush (dual-flush) | ~5–6 L/flush |
| WC use, home | ~5 flushes/person/day |
| WC use, office | ~3 uses/person/working day |
| Washing machine | ~15 L/person/day |
| Urinal (sensor/low-flush) | ~6 L/person/working day |
| Garden / irrigation | seasonal (annual average input) |
| Bedrooms | Typical occupancy |
|---|---|
| 1 | ~1.5 |
| 2 | ~2.5 |
| 3 | ~2.8 |
| 4 | ~3.5 |
| 5+ | ~3.9 |
Method. Domestic demand = per-person daily demand × occupants × 365 (C753 Box 11.3). Non-domestic demand = per-person daily demand × people × working days (not 365), because the building is only occupied part of the year.
Use. This demand feeds the tank sizing for a rainwater harvesting (BS EN 16941-1:2024) or greywater (BS EN 16941-2:2021) system. Only connect genuinely non-potable uses, with separate labelled pipework and backflow protection to the Water Fittings Regulations.
Sizing the storage tank
The tank is sized from yield and demand using the BS 8515 / BS EN 16941-1 intermediate (5%) method. You can size it in one step from the roof and household, or directly from a known yield and demand.
Rainwater Harvesting Tank Sizing Calculator
Size a rainwater harvesting storage tank — straight from the roof and household, or from a known yield and demand — using the BS 8515 / BS EN 16941-1 intermediate (5%) method. Based on The SuDS Manual (CIRIA C753, §11.3.2).
One-step tank size
Enter the roof and the household — the tool works out yield and demand and applies the 5% rule (the BS 8515 simple-lookup approach).
From a known yield & demand
If you already have the annual yield and demand (e.g. from the yield and demand calculators), size the tank directly.
Method & notes
How the tank is sized and what to watch for.
Intermediate (5%) method. Tank size = the smaller of 5% of the annual yield and 5% of the annual non-potable demand. The 5% represents about 18 days — long enough for the yield to average out, while not over-sizing. It assumes a reliable regular demand and fairly even monthly rainfall.
Yield / demand ratio. If demand is far larger than yield, the roof governs (mains top-up covers the rest); if yield is far larger than demand, demand governs (surplus overflows, ideally to a SuDS feature). Where demand is more than ~3× the yield, the yield (roof) sets the tank.
| Item | Value |
|---|---|
| Annual yield | 64,800 L |
| Annual demand | 43,800 L |
| 5% of yield | 3,240 L |
| 5% of demand | 2,190 L |
| Tank (lesser) | 2,190 L |
Standards & dual purpose. Design to BS EN 16941-1:2024 (which supersedes BS 8515; the 5% method is carried forward). This sizes a water-conservation tank only. If the tank must also provide surface-water attenuation, add storage and active control so the required volume is empty before a storm (C753 §11.3.3) – a larger combined tank than supply alone. Connect non-potable uses only, with labelled separate pipework and backflow protection (Water Fittings Regulations). For high-value or irregular-demand schemes, use continuous (detailed) simulation rather than the 5% method.
Further reading: integrating rainwater harvesting into SuDS design · the 2025 national standards for SuDS · Approved Document G.
Frequently asked questions
Multiply the plan roof area by the runoff coefficient, the annual rainfall and the filter efficiency. The calculator does this and returns the average annual yield.
Enough to bridge typical dry spells without being oversized. The calculator applies the BS 8515 5% method to size it from yield and demand.
Non-potable uses — WC flushing, laundry and the garden. Potable use needs full treatment and is rarely worthwhile for a single dwelling.
One of our experienced Flood Risk Consultants will get back to you within 60 minutes