Hong Life CalculatorsUnderstanding the bottled vs filtered water cost calculator — concept and principle
Drinking water is a small daily purchase that becomes a large yearly one. This calculator compares the yearly cost of buying bottled water with filtering tap water at home, using the amount you drink, the bottled price per litre, the filter cartridge price and capacity, and any yearly cost for the filter unit or rental. It shows both totals and the yearly saving from filtering.
How it is calculated
- Litres per year = litres per day × 365
- Bottled per year = litres per year × bottled price per litre
- Cartridges per year = litres per year ÷ litres per cartridge, rounded up to whole cartridges
- Filtered per year = cartridges × cartridge price + unit or rental per year + tap water (litres per year × tap price per litre)
- Saving per year = bottled per year − filtered per year
Litres per day should cover all the water your household drinks or cooks with that you would otherwise buy. Litres per cartridge is the rated capacity from the manufacturer. The unit or rental figure is a yearly amount, for example a jug or tap filter's purchase price spread over its life, or a monthly rental × 12.
The principle
Both options are linear in volume: bottled water costs the same per litre however much you drink, while a filter's cartridge cost per litre is its price divided by its capacity. Filtering adds a fixed yearly cost for the unit, so for very small volumes bottled water can be cheaper, but as daily litres grow the per-litre difference dominates. The tap water that goes through the filter is included too, although it is usually tiny: the default $0.002 per litre is $2 per m³.
Worked example
- Inputs: 3 L per day, bottled $0.50/L, cartridge $10 for 150 L, unit $40 per year, tap water $0.002/L.
- Litres per year: 3 × 365 = 1,095 L.
- Bottled: 1,095 × $0.50 = $547.50.
- Cartridges: 1,095 ÷ 150 = 7.3, rounded up to 8; 8 × $10 = $80. Tap water: 1,095 × $0.002 = $2.19. Filtered: $80 + $40 + $2.19 = $122.19.
- Saving: $547.50 − $122.19 = $425.31.
| Litres per day | Bottled per year |
|---|---|
| 0.75 | $136.88 |
| 1.5 | $273.75 |
| 2.25 | $410.63 |
| 3 | $547.50 |
| 3.75 | $684.38 |
| 4.5 | $821.25 |
| 6 | $1,095.00 |
How to read the result
A large saving means filtering pays off quickly; divide the up-front cost of a filter system by the monthly saving to see how soon. If the saving is small or negative, check whether a cheaper cartridge or a plain tap would do. In many places tap water already meets drinking-water standards, and the main reasons to filter are taste and odour.
Common mistakes
- Using cartridge capacity when the maker also sets a time limit; replace at whichever comes first.
- Leaving out the rental fee or installation cost.
Frequently asked questions
Is filtered tap water safe to drink?
Where the tap supply meets national standards it is already safe; a filter mainly improves taste. Neglected cartridges can harbour bacteria, so change them on time.
What would plain tap water cost?
Only the tap water row: $2.19 a year here, against $547.50 for bottled water.
Why whole cartridges?
You cannot buy 7.3 cartridges; the eighth is needed before the year ends, so it is counted in full.
Sources: World Health Organization — Guidelines for Drinking-water Quality · NSF/ANSI 42 and 53 — drinking water treatment units