Compare whole-house and under-sink filter costs over your chosen time using your own prices and replacement schedules.
Whole-house option
Under-sink option
Advanced options
Optional gallon limits can shorten the replacement interval. Enter both fields for an option, or leave both blank.
Whole-house gallon limit
Under-sink gallon limit
Table of contents
How to use our Whole-House vs Under-Sink Water Filter Lifetime Cost Calculator
- Choose the number of years to compare in Compare over.
- Enter the equipment price, installation cost, replacement cost, and replacement interval for each system from your quote, receipt, or product manual.
- Open Advanced options only if you know both the gallon capacity and water use for an option; enter both fields so capacity can shorten the replacement interval.
- Select Calculate to see the lower-cost option, cost gap, lifetime totals, yearly costs, and replacement counts.
- Sanity-check that whole-house water use reflects water that actually passes through that filter and that the two systems have the treatment coverage you need.
Definitions
Whole-house filter: A treatment system installed where water enters the home, so it generally treats water used throughout the home. [2]
Under-sink filter: A point-of-use system that usually treats water at one sink or dedicated faucet. [2]
Replacement interval: The maximum number of months a filter or media can stay in use according to its product instructions.
Filter capacity: The rated number of gallons a filter can treat before it should be replaced.
Time between replacements: The interval used for cost math. If both a month limit and gallon capacity are entered, the earlier one controls.
Lifetime cost: Equipment price plus installation cost plus scheduled replacement costs during the comparison period.
Common mistakes and quick fixes
Mistake: Using the same water-use estimate for Water passing through whole-house filter and Under-sink drinking and cooking water.
Fix: Enter all water treated by the whole-house unit, but only drinking and cooking water drawn from the under-sink filtered tap.
Mistake: Entering only Whole-house filter capacity without Water passing through whole-house filter.
Fix: Enter both fields, or leave both blank and use the Whole-house replacement interval from the manual.
Mistake: Leaving out plumber labor, permits, or connection parts from Whole-house installation or Under-sink installation.
Fix: Add costs not already included in the system price. Enter $0 only when installation really has no extra cost.
Mistake: Treating the first filter included with a new system as a replacement.
Fix: Put the purchase price in Whole-house system price or Under-sink system price ; Whole-house replacements during the selected period and under-sink replacements start after purchase.
Mistake: Picking the option with the lower Cost gap between the two options without checking treatment scope.
Fix: Compare the products' stated contaminant reduction and where they treat water. An under-sink unit usually serves one tap, while a whole-house unit treats incoming household water.
Limitations & Key Assumptions / Boundary Conditions
- This is a cash-cost comparison, not a comparison of contaminant reduction, certifications, flow rate, water quality, or health protection. Check the exact product claims and replacement instructions.
- A whole-house system and an under-sink system usually treat different parts of the water supply. A lower cost does not mean equal treatment coverage.
- The calculation assumes the initial filter is included in the system price. It counts a replacement due exactly at the end of the selected period.
- Capacity-based timing is an estimate based on entered water use. Source-water conditions, sediment, pressure, and actual use can cause filters to need replacement earlier.
- Fixed water and sewer fees, electricity, salt, UV lamps, service visits, and reverse-osmosis reject-water charges are excluded unless you included them in a replacement cost.
- For Water passing through whole-house filter, exclude irrigation when that water bypasses the filter.
Methodology
Cost calculation
The calculator changes the selected years into months, then counts replacement dates that occur on or before the final month.
comparison_months = comparison_years * 12
If you enter both optional capacity and water-use fields for an option, capacity creates a second possible replacement interval. The calculator uses the shorter interval. For under-sink use, daily gallons are converted to average monthly gallons using 365.25 days per year.
effective_life_months = min(calendar_life_months, capacity_gallons / average_gallons_per_month)
replacement_count = floor(comparison_months / effective_life_months)
Each lifetime total includes the equipment price, installation, and the cost of each scheduled replacement.
lifetime_cost = equipment_cost + installation_cost + replacement_count * replacement_cost
annual_cost = lifetime_cost / comparison_years
cost_gap = abs(whole_total_cost - under_total_cost)
Worked example
For five years, a whole-house system priced at $1,200 with $0 installation, $100 replacements, and a 12-month interval has five replacements. Its lifetime cost is $1,700 and its average cost is $340 per year. An under-sink system priced at $800 with $500 installation and five $100 replacements costs $1,800. The whole-house option costs $100 less in this example.
Replacement guidance
Use the replacement timing and capacity listed for the exact filters or media being compared. Products can have different replacement parts and schedules, so one combined replacement cost should include every part changed at that scheduled event. EPA guidance also distinguishes whole-home treatment from point-of-use treatment. [1] [2]