Water filter cartridge life calculator: how many days a filter lasts from its gallon rating, your daily use and the maker's month cap
Days = rated gallons / gallons per day, capped by the maker's months. A 40-gallon, 2-month Brita Standard at 0.5 gal/day lasts 60 days, not 80.
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Short answer
A water filter cartridge lasts until the earlier of two limits the maker prints: the rated capacity in gallons divided by the gallons that pass through it each day, or the flat month cap. Days to capacity = rated gallons / gallons per day; days to the time cap = months x 30 (a rounding this page assumes, not a maker figure); replace at whichever is smaller. Brita’s Standard filter data sheet rates the OB03 at 40 gallons or about 2 months, so a household filling one pitcher a day (0.5 gallons) reaches 40 / 0.5 = 80 days on gallons but hits the 2 x 30 = 60-day cap first, and the answer is 60 days. A GE RPWFE, rated 170 gallons and 6 months on GE’s performance data sheet, flips the other way at 1.5 gallons a day: 170 / 1.5 = 113 days, ahead of the 180-day cap. Every capacity below is the maker’s published figure, not a tested one.
If you own a pitcher filter and have no idea how many gallons you pour, close this page: the maker’s month cap already covers you, and the calculator only earns its place once you can put a number on daily use.
Days until the cartridge is spent: gallon capacity against the month cap
Formula: days = the smaller of (rated gallons / gallons per day) and (month cap x 30). Capacities and month caps are the maker's published figures (Brita OB03 and OB06 data sheets, PUR product page, GE RPWFE data sheet, Waterdrop WD-RF10 page). The 30-day month is this page's rounding, not a maker number. If JavaScript is off, the worked examples below give the same answer.
What is the formula?
Days until replacement = the smaller of rated gallons / gallons per day and month cap x 30. The cartridge is spent by whichever limit it reaches first, and the maker prints both limits because a heavy user hits gallons before the calendar and a light user hits the calendar before gallons.
| Variable | What it is | Where the figure comes from |
|---|---|---|
| Rated capacity | gallons the maker rates the cartridge for, per its data sheet | Brita OB03 and OB06 sheets, PUR product page, GE RPWFE sheet, Waterdrop WD-RF10 page |
| Gallons per day | water that actually passes through this filter, not the whole house | you: pitcher fills, dispenser draws, or a meter on the filtered line |
| Month cap | the maker’s flat replacement interval in months | the same maker documents |
| 30 days per month | the conversion from months to days | this page’s rounding convention, not a maker figure |
Rated capacity is the maker’s figure: the volume its data sheet says the cartridge processes while holding its stated reductions. Brita’s OB03 and OB06 sheets add a daily ceiling, no more than 2 gallons a day, that the calculator does not enforce, so keep pitcher use under it.
What does a worked example look like?
The three cases below use the maker figures in the table further down and the same arithmetic as the calculator.
| Case | Days to capacity | Days to time cap | Answer | Which limit won |
|---|---|---|---|---|
| Brita Standard OB03, 0.5 gal/day (a couple, one pitcher a day) | 40 / 0.5 = 80 | 2 x 30 = 60 | 60 days | time cap |
| Waterdrop WD-RF10 in a 10UA, 3 gal/day (drinking and cooking at a dedicated faucet) | 11,000 / 3 = 3,667 | 12 x 30 = 360 | 360 days | time cap |
| GE RPWFE, 1.5 gal/day (dispenser plus ice maker, family of four) | 170 / 1.5 = 113 | 6 x 30 = 180 | 113 days (about 3.8 months) | gallon capacity |
The first row matches Brita’s own “about every 2 months” note, so the calculator adds nothing there. The third row is the reason the page exists: a fridge family drawing 1.5 gallons a day spends the RPWFE two months before GE’s six-month cap, and a calendar reminder alone would run it 67 days past its rating.
What capacity and month cap does each cartridge carry?
Every figure is the maker’s published number, listed per the maker’s page. No database listing was checked and nothing was tested.
| Cartridge | Rated capacity | Month cap | Fit and cross-fit page |
|---|---|---|---|
| Brita Standard (OB03) | 40 gal / 151 L | 2 months | Brita pitcher filter compatibility |
| Brita Elite (OB06) | 120 gal / 454 L | 6 months | same page |
| PUR PLUS (PPF951K) | 40 gal | 2 months | PUR pitcher filter compatibility |
| GE RPWFE | 170 gal / 643.5 L | 6 months | GE RPWFE refrigerator filter |
| Waterdrop WD-RF10 (10UA, 10UB) | 11,000 gal | 12 months | Waterdrop 10UA, 15UA, 10UB replacement filters |
| ZeroWater | not published | not published; change at a TDS reading of 006 | ZeroWater filter compatibility |
The Waterdrop 15UA takes a different cartridge with its own capacity; the linked parts page carries that figure. PUR’s product page states 40 gallons or 2 months but, unlike Brita, publishes no daily-gallon ceiling for the PLUS filter.
Why does ZeroWater not fit this calculator?
ZeroWater’s FAQ publishes neither a gallon capacity nor a month cap. Its replacement rule is a reading: change the filter when the supplied meter shows 006 on filtered water, which is total dissolved solids in mg/L as the TDS meter reading guide explains.
A ZeroWater owner therefore skips both inputs above and watches the meter. High-TDS water reaches 006 sooner than low-TDS water, and the maker does not publish a curve for that.
Who should not rely on this calculator?
- A pitcher or fridge owner with no estimate of daily gallons: the maker’s month cap already covers that household, and the calculator only refines it once daily use is known.
- A ZeroWater owner: no gallon or month figure exists, so the TDS meter decides.
- A filter that sees large intermittent draws (a batch-cooking day, a party): the daily average understates use on that day, and the gallon capacity can be reached before the average predicts.
- A whole-house cartridge sized by flow rather than volume: the whole-house filter flow rate calculator and the micron rating picker answer that question.
Before you buy the next cartridge
Do not buy a bigger-capacity cartridge until the calculator says the gallon limit is the one you hit. In the Brita example above, 0.5 gallons a day never reaches 40 gallons inside 2 months, so a 120-gallon Elite bought for capacity alone still comes out on its 6-month cap; only a household nearer Brita’s 2-gallon daily ceiling converts the extra gallons into extra days. The parts pages linked in the table show which cartridge fits which housing; check fit there before comparing capacity here.
Where do the numbers come from?
- Brita Standard OB03, 40 gallons or about 2 months, no more than 2 gallons a day: Brita’s OB03 performance data sheet.
- Brita Elite OB06, 120 gallons or about 6 months, the same 2-gallon daily ceiling: Brita’s OB06 performance data sheet.
- PUR PLUS PPF951K, 40 gallons or 2 months: PUR’s product page for the PLUS pitcher filter.
- GE RPWFE, 170 gallons, change every six months or sooner if flow drops: GE’s RPWFE performance data sheet.
- Waterdrop WD-RF10, 11,000 gallons or 12 months: Waterdrop’s WD-RF10/RF15/RF17 replacement filter page.
- ZeroWater, change at a meter reading of 006: ZeroWater’s FAQ.
- 30 days per month: this page’s assumption, stated so the result can be redone with a real calendar.
Frequently asked questions
How often should I replace my water filter cartridge?
At whichever limit arrives first: the maker's rated gallon capacity divided by the gallons you pass through it each day, or the maker's flat month cap. Brita's performance data sheets print both figures for this reason, 40 gallons or about 2 months for the Standard OB03 and 120 gallons or about 6 months for the Elite OB06.
How many gallons does a Brita filter last?
Brita's performance data sheets rate the Standard filter (OB03) at 40 gallons, about 2 months for an average family, and the Elite filter (OB06) at 120 gallons, about 6 months. Both sheets also say to process no more than 2 gallons a day for maximum efficiency, so a 40-gallon Standard filter should never see fewer than 20 days of use.
How do I know when to change a ZeroWater filter?
ZeroWater's FAQ gives no gallon rating and no month cap. Its instruction is to change the filter when the TDS meter supplied with the pitcher reads 006 on filtered water. That is a reading, not a schedule, so this calculator's two inputs do not apply to ZeroWater; the meter decides on its own.
How long does a GE RPWFE refrigerator filter last?
GE's RPWFE performance data sheet rates the cartridge at 170 gallons and says to change it every six months, or sooner if flow drops noticeably. A fridge drawing 1.5 gallons a day for drinking and ice reaches 170 gallons in 113 days, so that household replaces on gallons at about 3.8 months, well before the six-month cap.
How long does a Waterdrop 10UA replacement filter last?
The Waterdrop 10UA and 10UB under-sink units take the WD-RF10 cartridge, which Waterdrop's replacement filter page rates at 11,000 gallons or 12 months. At 3 gallons a day the gallon path would run 3,667 days, so the 12-month cap is the limit for any household drawing dedicated-faucet water for drinking and cooking.
Fact sheet and how to cite this page
- Answer
- A cartridge is spent at the earlier of rated gallons divided by daily gallons or the maker’s month cap. Brita’s Standard data sheet rates the OB03 at 40 gallons or 2 months: at 0.5 gallons a day the gallon path allows 80 days but the 60-day cap wins. GE’s RPWFE sheet rates 170 gallons or 6 months: at 1.5 gallons a day, 113 days on gallons beats the 180-day cap.
- Formula
days = min(rated gallons / gallons per day, month cap x 30)- Inputs
- Rated capacity: 40 gallons (Brita Standard OB03)
- Gallons per day: 0.5
- Month cap: 2
- Output
- 80 days to capacity, 60 days to the time cap; replace at 60 days
- Assumptions
- A month is 30 days: a rounding convention on this page, not a manufacturer figure
- Daily gallons are a steady average; a large one-day draw can reach the gallon capacity earlier than the average predicts
- Capacities and month caps are the maker’s published figures, listed per the maker’s page; no NSF, WQA or IAPMO database listing was checked and nothing was tested
- Method
- The calculator divides the maker’s rated capacity by the gallons you pass through the filter each day, converts the maker’s month cap to days at 30 days a month, and reports the smaller of the two as the replacement interval.
- Primary source
- Brita, Standard filter (OB03) Performance Data Sheet (PDF) (manufacturer or vendor document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Water filter cartridge life calculator: how many days a filter lasts from its gallon rating, your daily use and the maker's month cap", https://ownerspec.com/water/calculators/water-filter-cartridge-life-calculator/, facts verified 15 September 2026.
Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers
Sources
- Brita, Standard filter (OB03) Performance Data Sheet (PDF) (manufacturer or vendor document)
- Brita, Elite filter (OB06) Performance Data Sheet (PDF) (manufacturer or vendor document)
- PUR, PUR PLUS Pitcher Filter product page (manufacturer or vendor document)
- GE Appliances, RPWFE Performance Data Sheet (PDF) (manufacturer or vendor document)
- Waterdrop, WD-RF10 / WD-RF15 / WD-RF17 replacement filter page (manufacturer or vendor document)
- ZeroWater, Frequently Asked Questions (manufacturer or vendor document)