Water filter cost per gallon: how to compare pitchers and under-sink systems
Filters are a classic razor-and-blades purchase: the unit can be cheap while the replacement cartridges are where the money goes. Convert everything to cost per gallon.
The formula
Cost per gallon = cartridge price / gallons the cartridge treats
Worked example
| Pitcher (assumed) | Under-sink (assumed) | |
|---|---|---|
| Unit price | $25 | $150 |
| Cartridge | $18 per 40 gal | $60 per 1,000 gal |
| Cost per gallon (cartridge) | $0.45 | $0.06 |
| Cartridges per year | 730 / 40 = 18.25, about $328 | 730 / 1,000 = 0.73, about $44 |
| Unit spread over 5 years | $5 a year | $30 a year |
| Approx. cost per year | about $333 | about $74 |
With these made-up numbers the pricier system is much cheaper per year because its cartridges treat far more water. With low usage the difference shrinks, so use your own volumes.
What to check before buying
- Rated capacity per cartridge and how often it must be replaced by time, not only by volume
- Which contaminants the filter is certified to reduce (look for independent certification)
- Space under the sink and whether installation needs a plumber
- Whether replacement cartridges are sold long term