This rainwater harvesting calculator helps UK households estimate how much water a roof could collect, how much suitable demand it might supply and how much storage may be useful. It works for simple garden water butts and provides an initial assessment for larger non-potable systems.
The calculator provides planning estimates rather than a final system design. Annual rainfall does not arrive evenly, so a property can collect enough water over a year while still running out during a dry spell or overflowing during wet weather.
For broader advice before calculating, read Is Rainwater Harvesting Worth It for a UK Home? and How to Collect and Store Rainwater at Home.
Rainwater harvesting calculator: collect these figures
| Input | Your figure | Unit |
|---|---|---|
| A. Roof area draining to the system | ________ | m² |
| B. Average annual rainfall | ________ | mm per year |
| C. Collection factor | ________ | Decimal |
| D. Annual compatible water demand | ________ | Litres per year |
| E. Average compatible daily demand | ________ | Litres per day |
| F. Desired storage period | ________ | Days |
Step 1: calculate the roof catchment area
Use the horizontal plan area of the roof section draining to the tank, not the larger sloping surface area. Include only roof sections connected through suitable gutters and downpipes.
- Rectangular roof: length in metres × horizontal width in metres
- One side of a simple gable roof: approximately half the building footprint where only one roof slope drains to the system
- Complex roof: divide the plan into rectangles or other simple shapes, calculate each relevant area and add them together
- Several buildings: calculate the connected areas of the house, garage, shed or greenhouse separately, then add them
- Several downpipes: include only the proportion of roof served by downpipes connected to the storage
| Connected roof section | Length | Width | Connected area |
|---|---|---|---|
| Main roof | ________ m | ________ m | ________ m² |
| Garage | ________ m | ________ m | ________ m² |
| Shed or greenhouse | ________ m | ________ m | ________ m² |
| Other | ________ m | ________ m | ________ m² |
| Total connected area A | ________ m² |
Step 2: find the local annual rainfall
Use the Met Office location-specific long-term averages to find rainfall for the nearest suitable location. Use the annual total in millimetres, or add the 12 monthly rainfall figures if an annual total is not displayed.
- Average annual rainfall B: __________ mm
- Climate averaging period: __________
- Met Office location used: ________________________________
- Distance from the property: __________ miles or kilometres
Local rainfall is preferable to a general UK average. Elevation, exposure and regional climate can produce large differences between locations.
Step 3: choose the collection factor
The collection factor allows for water that does not reach storage because of roof wetting, evaporation, small rainfall events, filters and other losses.
Quick planning method
Use a combined collection factor of 0.8. The Business Wales rainwater harvesting method uses this figure as a typical estimate after allowing for filtering losses and rainfall that produces little runoff.
More detailed planning method
Multiply an initial roof runoff coefficient by the documented hydraulic efficiency of the filter. Do not also apply the separate 0.8 quick-planning factor.
| Collection surface | Initial runoff coefficient |
|---|---|
| Pitched roof with profiled metal sheeting | 0.95 |
| Pitched tiled roof | 0.90 |
| Flat roof without gravel | 0.80 |
| Flat roof with gravel | 0.60 |
| Extensive green roof | 0.60 |
| Intensive green roof | 0.30 |
These are initial planning coefficients published in the CIRIA SuDS Manual C753. Green-roof yield is particularly uncertain and varies by season. Use manufacturer or specialist information where a more suitable coefficient is available.
| Detailed factor input | Your figure |
|---|---|
| Roof runoff coefficient | ________ |
| Filter hydraulic efficiency | ________ |
| Collection factor C: coefficient × filter efficiency | ________ |
If the filter efficiency is unknown, use the simpler combined 0.8 method rather than presenting an unsupported detailed result.
Step 4: calculate annual rainwater yield
One millimetre of rain falling on one square metre produces one litre of water before losses.
Annual collectable rainwater in litres = roof area A × annual rainfall B × collection factor C
| Annual yield calculation | Your result |
|---|---|
| Roof area A | ________ m² |
| Annual rainfall B | ________ mm |
| Collection factor C | ________ |
| A × B × C | ________ litres per year |
| Annual yield in cubic metres: litres ÷ 1,000 | ________ m³ per year |
| Annual average: litres ÷ 365 | ________ litres per day |
The daily figure is an annual average only. It does not mean that this quantity will enter the tank every day.
Step 5: calculate collection from one rainfall event
This calculation shows how quickly a water butt or tank could fill during a particular rainfall event.
Event yield in litres = roof area × rainfall depth in mm × collection factor
| Event calculation | Your result |
|---|---|
| Yield from each 1 mm of rain: roof area × factor | ________ litres |
| Yield from 5 mm of rain | ________ litres |
| Yield from 10 mm of rain | ________ litres |
| Yield from 20 mm of rain | ________ litres |
| Yield from 30 mm of rain | ________ litres |
The amount retained cannot exceed the empty space in the tank. If an event could deliver 500 litres but the tank has only 120 litres of available capacity, no more than 120 litres can be stored.
Potential overflow = event yield minus unused storage capacity
Step 6: calculate suitable household demand
Include only uses that the proposed system will actually supply. A simple water butt might serve garden watering and outdoor cleaning. A professionally installed system may also supply toilets or a washing machine where the design, water quality, pipe identification and backflow protection are suitable.
| Proposed use | Calculation | Annual demand |
|---|---|---|
| Garden watering | Litres per session × sessions per year | ________ litres |
| Toilet flushing | People × flushes per day × litres per flush × 365 | ________ litres |
| Washing machine | Cycles per week × litres per cycle × 52 | ________ litres |
| Car or outdoor cleaning | Litres per use × uses per year | ________ litres |
| Other suitable use | Measured quantity × annual frequency | ________ litres |
| Total compatible demand D | Add the annual demand figures | ________ litres |
The recently completed Household Water Use Calculator can help establish normal water consumption before rainwater demand is estimated.
Step 7: compare annual supply and demand
Theoretical maximum matched volume = the lower of annual rainwater yield and annual compatible demand
| Comparison | Your result |
|---|---|
| Annual collectable yield | ________ litres |
| Annual compatible demand | ________ litres |
| Lower of the two figures | ________ litres |
| Theoretical demand coverage: lower figure ÷ demand × 100 | ________% |
| Theoretical yield used: lower figure ÷ yield × 100 | ________% |
This is a theoretical ceiling. Actual mains-water replacement is normally lower because rainfall and demand occur at different times, the tank sometimes overflows and stored water may run out.
| Result | What it suggests |
|---|---|
| Yield is lower than demand | The system is primarily limited by rainfall, connected roof area or collection efficiency |
| Yield is much higher than demand | Additional roof area may add little unless more suitable demand or storage is available |
| Yield and demand are similar | Timing and storage capacity will strongly affect the amount actually used |
| Tank frequently overflows | More storage or more regular use may increase collection, subject to space and cost |
| Tank frequently runs dry | Check whether the cause is insufficient storage, low rainfall, excessive demand or limited roof area |
Step 8: estimate preliminary storage capacity
A simple demand-based calculation can estimate how much usable storage would cover a chosen period without rainfall.
Demand-based storage = average compatible daily demand × desired storage period in days
| Storage calculation | Your result |
|---|---|
| Annual compatible demand D | ________ litres |
| Average daily demand E: annual demand ÷ 365 | ________ litres |
| Desired storage period F | ________ days |
| Demand-based storage: E × F | ________ litres |
| Collection from chosen rainfall event | ________ litres |
| Existing storage capacity | ________ litres |
| Additional capacity under consideration | ________ litres |
For garden watering, use expected demand during the watering season rather than dividing annual garden use evenly across the year. A tank intended to supply toilets or laundry requires a more detailed rainfall and demand assessment.
Do not treat the demand-based figure as a final recommended tank size. A larger tank costs more, occupies more space and may rarely fill. A smaller tank may fill and overflow repeatedly. Systems supplying household plumbing should be sized against a suitable rainfall time series and designed to the current standard by a competent specialist.
Worked rainwater harvesting calculation
A household has 60 m² of connected roof area, average annual rainfall of 900 mm and uses the quick-planning collection factor of 0.8.
- Annual yield: 60 × 900 × 0.8 = 43,200 litres
- Annual yield in cubic metres: 43,200 ÷ 1,000 = 43.2 m³
- Annual daily average: 43,200 ÷ 365 = approximately 118 litres per day
- Yield from each 1 mm of rain: 60 × 0.8 = 48 litres
- Yield from a 10 mm event: 60 × 10 × 0.8 = 480 litres
- Yield from a 20 mm event: 60 × 20 × 0.8 = 960 litres
The household estimates annual compatible demand of 5,000 litres for the garden, 13,140 litres for toilet flushing, 9,360 litres for laundry and 1,000 litres for outdoor cleaning. Total annual demand is therefore 28,500 litres.
- Theoretical maximum matched volume: 28,500 litres
- Theoretical demand coverage: 28,500 ÷ 28,500 × 100 = 100%
- Theoretical proportion of collected yield used: 28,500 ÷ 43,200 × 100 = approximately 66%
- Average compatible demand: 28,500 ÷ 365 = approximately 78 litres per day
- Fourteen-day demand-based storage: 78 × 14 = approximately 1,092 litres
The calculation does not prove that a 1,092-litre tank will meet all demand. Toilet and laundry demand may be relatively regular, while garden demand and rainfall are seasonal. Daily or monthly rainfall modelling could produce a different storage requirement.
Storage and installation checks
- ☐ Confirm that the base can safely support the full tank and remains level.
- ☐ Remember that each litre of stored water weighs approximately one kilogram, excluding the tank itself.
- ☐ Provide a close-fitting, secure cover and protect openings from debris and pests.
- ☐ Fit an appropriate leaf filter or other inlet filtration.
- ☐ Route overflow safely without creating damp, erosion or flooding problems.
- ☐ Make the tank, filters and pumps accessible for inspection and cleaning.
- ☐ Check the manufacturer’s frost, drainage and maintenance instructions.
- ☐ Keep simple water-butt water for appropriate non-potable uses.
- ☐ Do not connect rainwater pipework directly to the mains water supply.
- ☐ Use a competent installer for systems supplying toilets, washing machines or other internal plumbing.
- ☐ Confirm applicable water-fitting, building-control, drainage and planning requirements before installation.
Water quality and connected systems
The current BS EN 16941-1:2024 rainwater standard covers systems using collected rainwater as non-potable water. Drinking, food preparation and personal hygiene are outside its scope.
- Rainwater collected from a roof should not be assumed to meet drinking-water standards.
- Pipework carrying rainwater must remain distinguishable from wholesome-water pipework.
- Systems connected to household plumbing require suitable separation and backflow protection.
- Mains backup must not create an unsafe cross-connection between the two water sources.
- Requirements differ across England, Wales, Scotland and Northern Ireland, so check the rules applying at the property.
- If the system is also intended to control surface-water flooding or provide attenuation, use a separate drainage design. This household calculator does not calculate storm-storage capacity.
The guide to collecting and storing rainwater at home covers practical water-butt use, maintenance and appropriate applications in more detail.
What this calculator does not include
- Variation between individual storms, months and years
- Detailed daily rainfall and demand simulation
- Climate-change allowances for future rainfall
- Stormwater attenuation or flood-storage calculations
- Structural design for above-ground or underground tanks
- Pump sizing, pressure, electrical consumption or backup supply
- Water treatment or confirmation of water quality
- Drainage approval, planning permission or building-control requirements
- Installation costs, maintenance costs or a guaranteed payback period
- Automatic calculation of water and sewerage bill savings
Rainwater harvesting resources and official guidance
- The Met Office location-specific climate averages provide monthly and annual rainfall figures for locations around the UK.
- Business Wales rainwater harvesting guidance explains the simple annual-yield calculation using roof area, rainfall and a combined 0.8 collection factor.
- BSI’s BS EN 16941-1:2024 overview describes the current standard covering the design, sizing, installation, identification, commissioning and maintenance of non-potable rainwater systems.
- The national standards for sustainable drainage systems explain current requirements for drainage and rainwater-management design in England.
- The Environment Agency rainwater harvesting position statement explains when harvesting rainwater in England does not require an abstraction licence.
- Water Regs UK provides separate routes to the water-fitting requirements for England and Wales, Scotland and Northern Ireland.
- The Scottish Government surface-water drainage guidance covers rainwater harvesting, filtration, identification, backflow prevention and safe overflow arrangements in Scotland.
- The Royal Horticultural Society rainwater guide explains practical water-butt installation, garden use, cleaning and stored-water precautions.
