1. Your nation & storage period
Where you farm — and whether you're in a Nitrate Vulnerable Zone — sets the minimum storage period.
2. Livestock on the slurry system
Count animals housed / on the slurry system over the period. Only muck that reaches the store counts.
3. Dirty water & rainfall
Rain and washings draining into the store are slurry too, and must be stored.
Estimated storage capacity required
Then add freeboard. This is slurry volume only. When sizing the store, add freeboard on top — 300 mm (about 12 in) for steel/concrete, 750 mm (about 30 in) for earth-bank. Add silage effluent or other dirty water separately.
Will I have to cover it?
Covering is already required on permitted pig and poultry units and on grant-funded stores. The Clean Air Strategy proposed covering all slurry and digestate stores — originally by 2027, though that date has slipped and a requirement for everyone is expected later this decade as the regulations are reformed. Building any new store covered or cover-ready future-proofs it and keeps more nitrogen in the slurry. A sealed flexible tank is inherently covered; open tanks and lagoons need a cover fitted.
Which store meets it?
Every option below can be sized to your figure. None is best for everyone — it comes down to site, budget, groundwork and how you spread. Honest pros and cons:
Above-ground steel or concrete tank
Earth-bank lagoon — with or without a synthetic liner
Supported flexible tank — also called a bladder or pillow tank
In-ground lined store
A note on terms: a supported flexible tank (self-supporting, above ground, minimal groundwork) and an in-ground lined store (excavated, bunded, lined) are different products, even though both are sometimes loosely called "bags". The distinction matters for groundwork, secondary containment and grant rules.
Slurry storage, in plain English
How much storage do I actually need?
Enough for all the slurry, washings and rainfall entering the store over the required period, plus freeboard. The calculator above does the sum — enter your stock, washings and any uncovered yard area.
What's the legal minimum?
England: 4 months for every farm, rising to 5 months (cattle) or 6 months (pigs/poultry) inside an NVZ; the EA recommends 6. Scotland: 22 weeks (housed cattle) / 26 weeks (housed pigs). Wales: 5 months (cattle) / 6 months (pigs and poultry). Northern Ireland: 22 weeks, or 26 with a pig or poultry enterprise. The "6 cattle / 8 pigs" figure you'll see is the grant target, not the legal floor.
Does the store have to be covered?
For permitted pig/poultry and grant-funded stores, yes, already. For everyone else it's coming — proposed for all stores under the Clean Air Strategy, expected to bite later this decade. Building covered now is the safe bet.
Figures & sources
Slurry production uses standard per-head values; rainfall and washings are added. Dairy is on the Defra standard value; other livestock are working estimates pending confirmation against the Defra NVZ guidance (Table A), flagged est.
Sources: Defra/GOV.UK SSAFO guidance; Defra NVZ guidance (standard excreta values); AHDB Slurry Wizard methodology; Environment Agency Farming Rules for Water; SEPA and the Environmental Authorisations (Scotland) general binding rules; The Water Resources (Control of Agricultural Pollution) (Wales) Regulations 2021, regulation 29, and Natural Resources Wales; the Nutrients Action Programme Regulations (Northern Ireland) 2019, regulations 17 and 18; Clean Air Strategy (covers). Storage periods for all four nations are now confirmed against the regulations (updated August 2026). Conversions: 1 m³ ≈ 220 imperial gallons.
For guidance only. An estimate to help you plan — not a substitute for the AHDB Slurry Wizard (required for Slurry Infrastructure Grant applications) or professional advice. Regulations are changing; confirm the current rules for your nation before relying on these figures. Staging build: livestock figures marked "est" await sign-off.