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Use 72 hours as your first practical target. For drinking alone, plan on 3 litres per person per day. That means 9 litres for one person, 18 litres for two people, and 36 litres for a family of four over three days. If you want enough for basic cooking and hygiene as well, a much stronger planning figure is around 10 litres per person per day — so 120 litres for four people over three days.

Build in layers: first, store ready-to-drink water; second, keep clean food-grade containers you can refill; third, have a reliable treatment method in case you need to use an uncertain source. Add extra for babies, medical needs and pets. You do not need to reach the full target in one purchase — but you should know what your target is.

Use 3 litres per person per day as the minimum drinking-water planning figure. For a three-day disruption that means:

• 1 person: 9 litres
• 2 people: 18 litres
• 4 people: 36 litres
• 5 people: 45 litres

That is drinking water only. The UK Government’s Prepare guidance notes that around 10 litres per person per day gives a more comfortable allowance because it also covers some basic cooking and hygiene. On that basis, a family of four would need around 120 litres for three days.

Treat 3 litres as the minimum layer, not the finished plan. Add more if anyone is pregnant, breastfeeding, unwell, using water-dependent medical equipment, if you have pets, or during hot weather.

Bottled water is a very good first layer, but it should not be your only layer. A family of four needs about 36 litres just to cover three days of drinking water at 3 litres per person per day. In 2-litre bottles, that is 18 bottles. If you were aiming for the more comfortable 10-litres-per-person-per-day figure for drinking, basic cooking and hygiene, the same family would need about 120 litres — 60 two-litre bottles.

That is why a practical setup normally combines bottled water with larger food-grade storage containers, collapsible carriers and a treatment method. Bottled water gives you immediate safe water; containers and filtration or purification give you options if the disruption lasts longer than expected.

A filter and a purifier are not automatically the same thing. A filter mainly works by physically trapping contaminants as water passes through it. Depending on the product, that may include sediment, protozoa, bacteria and microplastics. Many common portable filters do not reliably remove viruses.

A purifier is intended to provide a higher level of microbiological protection and may use filtration, chemicals, UV or a combination of methods. Do not buy on the word “purifier” alone: check the manufacturer’s actual test data and exactly what it claims to reduce.

For emergency use, ask four questions: does it deal with bacteria, viruses and protozoa; how many litres can it treat; how fast is it; and can it be cleaned or maintained without mains power? If the answer to those is unclear, keep looking.

Do not assume collected rainwater is safe to drink. The UK Government specifically advises that water from a rain butt should not be drunk because it can contain bacteria or disease. Roof-collected water can also pick up bird droppings, dirt, metals, roofing residues and other contaminants.

For preparedness, rainwater is excellent for uses such as toilet flushing, cleaning and watering plants, which preserves your stored drinking water. If you ever had to consider it for drinking, use the cleanest catchment possible, discard the initial dirty run-off where your system allows, pre-filter visible debris and then use an appropriate treatment method.

Crucially, boiling or disinfecting does not make water contaminated with fuel, toxic chemicals or similar substances safe. If chemical contamination is suspected, use a different source.

Use containers designed for drinking water or food storage, with tight-fitting lids. Clean and sanitise them before filling, label them “drinking water” with the fill date, and keep them somewhere cool, dark and away from petrol, pesticides, cleaning chemicals or anything with a strong odour.

Remember the weight: 1 litre of water weighs roughly 1 kilogram. A 10-litre container is therefore about 10 kg before the container itself; a 20-litre one is about 20 kg. Large containers are efficient for storage but can become awkward to lift, especially for older people or children.

For most homes, a mix works well: larger containers for static storage and several 5–10 litre carriers that are easier to move, refill and pour safely.

For unopened commercially bottled water, follow the storage instructions and date printed by the manufacturer. Keep it cool, out of direct sunlight and away from chemicals.

For water you fill into your own sanitised food-grade containers, a sensible conservative routine is to label the fill date and replace the water every six months. That is also the interval recommended by the US CDC for home-filled emergency water. You can use the old water for plants or cleaning rather than wasting it.

The container matters as much as the water. If a seal is damaged, the container has been contaminated, the water has an unusual smell or appearance, or you are unsure how it was stored, do not rely on it as safe drinking water simply because it is “within date”.