How it works
Cartridges are rated for a number of gallons, and many also have a time limit. If you use little water, the time limit decides when you change the cartridge, which raises your real cost per gallon.
Lower cost per gallon is only useful if the filter is certified for what you want to reduce. Compare like-for-like certifications.
Default prices and ratings are example assumptions. Use the numbers on your product's performance data sheet.
The formula
- Days per cartridge = min(rated gallons ÷ gallons per day, rated months × 30.44)
- Cartridge cost per year = 365 ÷ days per cartridge × price × cartridges per change
- All-in cost per gallon = (system price + yearly cartridge cost × years) ÷ (gallons per day × 365 × years)
How to use this calculator
- Enter the cartridge price and how many you change at once.
- Enter the rated gallons and any time limit from the data sheet.
- Enter how much filtered water you use per day.
- Read how often you will change cartridges and the cost per gallon.
Frequently asked questions
Why is my cost per gallon higher than the box suggests?
Box claims usually assume you reach the full gallon rating. If the time limit comes first, you pay for capacity you don't use.
Can I use a cartridge past its rating?
Manufacturers rate performance only up to the stated capacity. Past that point, contaminant reduction isn't certified.