Bottle or filter: what you are really paying for

A household that drinks three litres of water a day gets through more than a thousand litres a year. Buying them in bottles or filtering tap water looks like a matter of taste, but it is also one of the household items with the biggest gap in price per litre. This calculator compares the two annual costs and shows how much you save, how many cartridges you need and how many bottles you avoid.

It works in any currency and with any filtering system: a jug, a tap filter or an under-sink unit.

How the calculation works

You enter the litres drunk per day L, the bottled water price per litre, the price of one cartridge, how many litres it filters before replacement and the tap water price per cubic metre (a thousand litres), as shown on your bill.

• Annual bottled cost: 365 × L × price per litre.
• Annual filtered cost: 365 × L × (cartridge price / litres per cartridge + tap price / 1000).
• Annual saving: the difference between the two.
• Filtered cost as % of bottled: the ratio between the two costs, on which the verdict is based.

The ratio depends neither on the litres you drink nor on the currency: it tells you what each filtered litre costs compared with a bought one, and it is the right measure for comparing your situation with anyone else's.

Tap water costs almost nothing

In many countries tap water costs from one to a few euros, dollars or francs per cubic metre, that is per thousand litres. Per litre that is a fraction of a cent. The table compares the cost per litre of tap water with bottled water at 0.30 per litre.

Even where tap water is expensive, a litre from the tap costs dozens of times less than a bottled one. That is why water barely counts in the filtered bill: almost the whole cost is the cartridge.

The cartridge is the deciding item

The filtering cost per litre is the cartridge price divided by the litres it filters. The table shows some typical combinations, with tap water at 2 per cubic metre and three litres a day.

The difference between one cartridge and another can be worth ten times the cost per litre. The smallest, cheapest cartridges are often the most expensive per litre filtered, while under-sink units cost more to buy but filter thousands of litres. The price that matters is not the one on the shelf, it is the one divided by the litres.

How much the bottled price matters

The table keeps the filter fixed (a cartridge at 6 for 150 litres, tap water at 2 per cubic metre, three litres a day, annual filtered cost 45.99) and varies the bottled price per litre.

With the cheapest discount bottled water the filter still saves money, but the margin narrows; with premium brands or small half-litre bottles the saving far exceeds the cost of the filter itself.

Plastic and transport

Besides money, the calculator estimates the 1.5-litre bottles you do not buy: for three litres a day that is about 730 a year. They are packs to carry home, store and dispose of, and plastic that stays out of the waste stream. Transporting bottled water from source to shelf also weighs far more on the environmental footprint than water that arrives through the tap.

The five bands of the verdict

• The filter costs more: with very cheap bottles and expensive cartridges, bottles remain cheaper.
• Below 10%: the filter wins by a mile.
• 10% to 25%: the filter clearly pays.
• 25% to 50%: it pays, but the cartridges decide the margin.
• Above 50%: the financial advantage is thin, and taste, convenience and plastic decide.

Water quality and safety

In most countries with a regulated water service, tap water is tested frequently, often under rules as strict as or stricter than those for bottled water. Household filters mainly improve taste and smell, for example by reducing chlorine, and some reduce limescale or specific substances. A filter that is not replaced in time can, however, worsen water quality: respecting the manufacturer's stated life is part of the bill. If you have doubts about your supply, your local water utility publishes its test results.

What is not in the bill

The calculator does not include the price of the jug or unit, installation, any energy used by electric or reverse-osmosis systems, or the water rejected by reverse-osmosis units. If your system has a significant purchase cost, you can divide it by its years of life and subtract it from the annual saving for a complete comparison.

Limits of the calculator

The model uses a constant daily consumption, an average bottled water price and the nominal cartridge life. It does not consider offers, different pack sizes, water used for cooking or differences in taste. It is a tool for financial comparison, not an assessment of water quality.

Frequently asked questions

Is it worth filtering tap water?

Financially, almost always: tap water costs dozens of times less than bottled water and the filtered cost depends almost entirely on the cartridge. The calculator tells you by how much in your case.

How is the cost of filtered water calculated?

Add the filtering cost per litre, that is cartridge price divided by the litres it filters, to the tap water cost per litre, that is the price per cubic metre divided by a thousand, then multiply by the litres you drink in a year.

Where do I find the tap water price?

On your water bill, usually expressed per cubic metre, that is per thousand litres. If your bill has tiered rates, use an average or the rate of the tier your consumption falls into.

How many litres does a cartridge filter?

It depends on the system: jug cartridges typically filter about 100 to 200 litres, under-sink units from a few thousand to over ten thousand. Use the figure given by the manufacturer.

Why can cheaper cartridges cost more?

Because the cost that matters is per litre. A cheap cartridge that filters few litres can cost more per litre than a pricier one that filters many more.

What does the filtered cost percentage mean?

It is the cost of filtered water divided by that of bottled water. 14% means each filtered litre costs 14% of a bought litre, about a seventh.

Can a filter cost more than bottled water?

Yes, if bottled water is very cheap and cartridges are expensive or short-lived. In that case the calculator flags it with a dedicated verdict.

Does the calculator include the price of the jug?

No. If the system has a significant purchase cost, divide it by its expected years of life and subtract it from the annual saving for a complete comparison.

Is tap water safe to drink?

In most countries with a regulated water service it is tested frequently. Your local water utility publishes its test results; if in doubt, check the ones for your area.

What happens if I do not change the cartridge?

An exhausted cartridge filters less and can worsen water quality. Respecting the manufacturer's stated life is part of the cost of the filter.

How many plastic bottles do I avoid?

The calculator divides annual litres by 1.5: with three litres a day that is about 730 fewer 1.5-litre bottles a year to buy and dispose of.

Does it work in any currency?

Yes. Enter all prices in the same currency; the verdict is based on a percentage that does not depend on the currency.