Rainwater Harvesting Calculator for UK Farms
Many rainwater calculators use one figure for the whole country. That is fine for a rough idea and less useful for sizing a tank, because rainfall in the UK ranges from around 530 mm per year in south Essex to over 1,800 mm per year in the central Lake District. A farm in the wrong half of that range gets an answer that is out by more than double.
This calculator uses the Met Office 1 km rainfall record for your own postcode district, including the autumn and winter share, which is what matters if your stock are housed.
Figures need to be rounded up or down to what tanks can most closely manage: different volume tanks cannot be used in the same system. So for instance, a 22,000 litre requirement could be rounded up to a single 25,000 litre tank, or rounded down to 2x 10,000 litre tanks (or a single 20,000 litre volume tank).
Your figures
Enter postcode district for your farm to use the calculator. We will use the long-term rainfall for your postcode rather than a national average.
The first part is enough - SY1, LN4, CW11.
Mid-range planning figures from AHDB. Beef and sheep from the rainwater leaflet (dry cow 15–40 L, lactating 40–70 L, sheep 3.3–7.5 L). Dairy from the hot-weather note (120–140 L). Finishing pigs from the pig water-quantity table (5–6 L). Grass figures allow for the AHDB 10–20% heat uplift. Intake still moves with diet, milk yield and weather.
Used to show what you would draw over that period. It does not set the tank size - see the note with the result.
Enter your postcode to see the numbers for your farm.
| Rainfall where you are | mm a year |
|---|---|
| Your roof | m² |
| Collected in a year | litres |
| Collected autumn and winter | litres |
| What you'd draw | litres |
| What the roof covers |
Indicated water storage
-
litres
A store in this range is what these inputs point to. Not a recommendation or a quote.
Start an enquiry
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An estimate for planning, not a specification. Gutter condition, pitch and how much you lose to first flush all move the real figure. A quote is where tangible figures emerge.
Rainfall from Met Office HadUK-Grid 1km gridded observations, 1991–2020, used under the Open Government Licence v3. Postcode locations from the ONS Postcode Directory; contains OS data © Crown copyright and database right, and Royal Mail data © Royal Mail copyright and database right. Storage is a planning figure from roof yield and how the water is used, not a performance guarantee. Design reference: BS EN 16941-1.
How much rainwater can I collect from my roof?
The calculator uses the same arithmetic as the guide:
roof area (m²) × rainfall (mm) × roof coefficient × filter efficiency = litres per year
Area is the plan footprint, not the slope. Rainfall is the Met Office record for your postcode district, not a UK average. Filter efficiency is taken as 0.9. Roof coefficients and which surfaces are fit for troughs sit in the farm rainwater harvesting guide.
Why the answer is the smaller of two numbers
Storage is the lesser of what the roof can deliver in the relevant season and what you actually need to hold. Size to demand alone and you can buy a tank the roof will never fill. Size to the roof alone and the store can run dry in the weeks you need it.
The calculator tool names which of the two is binding. If the roof is the limit, more tank is not the answer. If draw is the limit, the question is how much you want to carry.
Housed stock, grazing, spraying and washdown are different jobs. Do not enter dairy drinking to check a yard-wash figure, or the other way round.
What it does not account for
- Gutters that cannot take what the roof sheds.
- A year that is drier than the thirty-year average.
- A roof paced short.
- Access, base, offload or planning.
The result is a planning figure, not a specification.
What to do with the number
Send the figures through and we will come back with a planning specification. A quote is where the numbers become firmer.
Background, rules and worked examples: farm rainwater harvesting systems. Tanks: water storage tanks for farms.
Where the figures come from
Met Office HadUK-Grid 1 km gridded observations, 1991–2020. For each postcode district we average the 1 km cells that contain a postcode, rather than the geographic centre of the district.
Livestock rates in the calculator tool are mid-range AHDB planning figures, not a ration for your herd.
Rainfall derived from Met Office HadUK-Grid 1 km gridded observations, used under the Open Government Licence v3. Citation: Met Office; Hollis, D.; Carlisle, E.; Kendon, M.; Packman, S.; Doherty, A. HadUK-Grid Gridded Climate Observations on a 1 km grid over the UK. Postcode locations from the ONS Postcode Directory; contains OS data © Crown copyright and database right, and Royal Mail data © Royal Mail copyright and database right.
This calculator is provided as a planning aid. Figures are estimates based on long-term averages and the information entered, and are not a guarantee of performance or a specification for any particular installation.