Whole-House Water Filter Size Calculator

Target {{ resultsReady ? formatFlow(computation.values.service_target_gpm) : '—' }} Port {{ resultsReady ? selectedCandidate.portSize : '—' }} Pressure {{ resultsReady ? pressureStatusLabel : '—' }}

The service-path visual is unavailable. Sizing results remain available below.

Household profile:

Presets keep your selected display units and replace the planning values.

Source-water load:

Use current water-test or utility information when available.

Treatment train:

Confirm the exact cartridge or media against its manufacturer data sheet.

Fixture-flow unit:

One column-level unit keeps the repeated worksheet compact and auditable.

Peak fixture worksheet:

Format: fixture, simultaneous count, flow, priority. Priorities: critical, essential, comfort, optional, shed.

Use product labels or measured flow when available.

Daily water use:

Use a meter or water-bill average when possible.

Inlet pressure:

Accepted range equals 25-90 PSI.

Flow headroom:

From 0% to 50% above the worksheet peak.

Maximum clean pressure drop:

Accepted range equals 2-15 PSI.

Filter stages in series:

Use the number of housings or treatment stages installed in the same service path.

Manufacturer capacity override:

The neutral default is 0, which applies no manufacturer override.

Display precision:

One decimal is the default.

Sizing plan

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Calculation trace:

The hydraulic and maintenance factors are planning assumptions, not a certified product curve.

target = Σ(count × fixture flow) × (1 + headroom)

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The chart renderer is unavailable. The same values remain in the sizing plan.

Fixture	Simultaneous	Fixture flow	Peak flow	Priority	Copy
{{ row.fixture }}	{{ row.count }}	{{ row.flow }}	{{ row.peak }}	{{ row.priority }}

Check	Status	Action	Reason	Copy
{{ row.check }}	{{ row.status }}	{{ row.action }}	{{ row.reason }}

Whole-house filtration has to satisfy two different clocks. During a busy few minutes, the filter must pass the combined flow from showers, faucets, toilet refills, appliances, and any filtered outdoor tap. Across weeks and months, its media or cartridge must cope with the total water volume and the material being removed.

Daily use cannot stand in for peak flow. A household may average a few hundred gallons per day yet briefly need several fixtures at once. Choosing a filter from the daily total alone can produce weak showers and slow fills; choosing only by a large port size can ignore the cartridge's restriction and contaminant claim.

Service-flow rating: The flow a housing, cartridge, or media system can pass under stated conditions.
Pressure drop: The pressure lost as water crosses the treatment path. Loss normally rises with flow and as a filter becomes loaded.
Capacity: A volume or time estimate for maintenance. It does not prove that peak household demand can pass through the system.

Water quality determines what treatment belongs in the line. Sediment, chlorine taste, chloramine, iron, turbidity, microorganisms, and dissolved chemicals call for different media and certifications. A larger filter does not automatically remove more kinds of contaminants, and a certified reduction claim applies only to the contaminants and conditions named for that product.

Private wells need particular care because appearance and taste do not establish safety. Current laboratory results and local health guidance should lead the treatment decision. Media tanks also need a separate backwash-flow check, while cartridge systems need manufacturer pressure-drop curves at the intended flow.

A sizing estimate is useful for comparing housing classes and spotting hydraulic risk before shopping. Final selection still depends on measured flowing pressure, actual fixture demand, water analysis, pipe size, bypass and drain requirements, and the exact manufacturer's rated performance.

How to Use This Tool:

Build the service-flow estimate from simultaneous fixtures, then check pressure and maintenance separately.

Choose a household profile as a starting point, then select the source-water load and treatment train that match current evidence.
Edit the Peak fixture worksheet with one fixture, simultaneous count, flow, and priority per row. Use measured or rated fixture flow where possible.
Enter daily water use and the inlet pressure measured while water is flowing. Set headroom from 0% to 50% and a clean pressure-drop limit from 2 to 15 PSI.
Select the number of filter stages in the same series path. Apply a manufacturer capacity override only when a product data sheet supplies a relevant gallon rating.
Review service-flow fit, estimated outlet pressure, and replacement timing. Treat any undersized or pressure-risk result as a reason to measure again or obtain a product-specific design.

Interpreting Results:

The selected class is the first planning class that clears both the service-flow target and the entered clean pressure-drop limit. If none clears both, the estimate falls back to the first class that clears flow; if even that is unavailable, it shows the largest class and marks the flow as undersized.

Sized means rated service flow is at least the headroom-adjusted fixture peak. It is not a guarantee that a particular cartridge will meet that rating.
Healthy pressure requires estimated outlet pressure of at least 40 PSI and pressure drop no greater than the selected limit.
Watch allows at least 35 PSI outlet pressure and drop no greater than 135% of the limit. Anything lower is Risk.
Normal replacement timing reaches the floor assigned to the source-water load. Below that floor, at least 0.75 month is Short; less is Very short.

Technical Details:

The estimate combines a hydraulic screen with a capacity screen. Fixture rows create the simultaneous peak. Headroom raises that peak to a shopping target, while a nonlinear pressure model estimates clean-filter loss. Daily gallons then convert adjusted capacity into a maintenance interval.

Formula Core:

All flows are converted to US gallons per minute (GPM), pressure to pounds per square inch (PSI), and daily volume to gallons per day before calculation.

\begin{array}{lcl} Q_{peak} & = & \sum (n_{i} \times q_{i}) \\ Q_{target} & = & Q_{peak} \times (1 + \frac{h}{100}) \\ r & = & \max (0.05, \frac{Q_{target}}{Q_{rated}}) \\ Δp & = & {Δp}_{base} \times r^{1.85} \times f_{source} \times f_{treatment} \times (1 + 0.38 (s - 1)) \\ C_{adjusted} & = & \max (100, C_{base} \times f_{source capacity} \times f_{treatment capacity} \times f_{utilization}) \\ M & = & \max (0.25, \min (\frac{C_{adjusted}}{D \times 30.44}, M_{source cap}, M_{treatment cap})) \end{array}

n is simultaneous fixture count, q is fixture flow, h is headroom percent, s is the number of stages in series, C is adjusted gallon capacity, D is daily use, and M is the interval in months. The stage multiplier is 1.00, 1.38, or 1.76 for one, two, or three stages.

Lookup Core:

The following are fixed planning classes, not a catalog of certified products. Each class supplies the service rating, base capacity, and base clean drop used by the formulas.

Whole-house filter planning classes
Planning class	Service GPM	Base capacity	Base drop
10 in standard housing	5	12,000 gal	4.8 PSI
10 in large-diameter housing	10	30,000 gal	3.7 PSI
20 in large-diameter housing	15	65,000 gal	3.1 PSI
Dual 20 in parallel housings	25	125,000 gal	2.6 PSI
10 x 54 media tank class	9	220,000 gal	2.8 PSI
12 x 52 media tank class	12	320,000 gal	2.6 PSI
14 x 65 media tank class	18	520,000 gal	2.4 PSI
Parallel media tank class	28	900,000 gal	2.2 PSI

Source-water and treatment factors deliberately make the planning result more conservative as loading or restriction increases. The capacity calculation also applies 0.92 when target flow exceeds 75% of the selected service rating and 0.84 when it exceeds 90%.

Water source and treatment factors
Condition	Capacity factor	Pressure factor	Interval cap	Normal floor
Municipal, low sediment	1.00	1.00	12 mo	3 mo
Municipal chloramine or taste/odor load	0.72	1.08	9 mo	2 mo
Well water, visible sediment	0.50	1.28	4 mo	1 mo
Well water, iron or heavy turbidity	0.36	1.48	2.5 mo	0.75 mo
Seasonal or intermittent home	0.70	1.12	6 mo	1 mo
Sediment prefilter	1.12	0.90	12 mo	—
Sediment plus carbon	0.86	1.18	12 mo	—
Carbon block or fine filtration	0.62	1.42	9 mo	—
Backwashing media tank	3.40	0.82	60 mo	—

Rule Core:

Fixture rows accept zero to 20 simultaneous uses and zero to 50 GPM per fixture, with no more than 30 rows and a positive combined peak. Daily use must equal 25 to 1,500 gallons per day, inlet pressure 25 to 90 PSI, and capacity override 0 to 1,000,000 gallons. One US gallon equals 3.785411784 litres, and one PSI equals 6.894757293168 kPa for the displayed conversions.

Limitations:

The pressure equation, capacity factors, class values, and interval caps are planning assumptions rather than certified product curves. Dirty-filter loss, media chemistry, water temperature, pipe fittings, valves, and actual contaminant loading can move the result substantially.

Verify the exact cartridge or media's service flow and pressure-drop curve at the intended flow.
Match third-party certification to the contaminant of concern; a certification number is not a universal removal claim.
Test private well water and confirm treatment with qualified local guidance. Do not infer drinking-water safety from this size estimate.
Check backwash flow, drain capacity, pump cycling, and pressure-tank behavior separately for regenerating or backwashing systems.

References:

NSF Standards for Water Treatment Systems, NSF.
Guidelines for Testing Well Water, Centers for Disease Control and Prevention, July 1, 2024.
Pentek Big Blue Specification Sheet 310053, Pentair.