Treatment devices are rated on flow, so start with flow
A softener, a carbon tank, a cartridge housing and a UV chamber all carry a service flow rating in gallons per minute. Exceed it and three separate things go wrong at once: the pressure loss climbs steeply, the contact time falls below what the media needs to do its job, and in a bed of granular media a high enough flow starts to channel and lift it. None of those announce themselves. The water still comes out of the tap.
So the first number in any treatment sizing job is the peak simultaneous flow, and the important word is simultaneous. Not the daily total, not an average, not the flow of the largest fixture. The realistic worst case is somebody in the shower while a toilet refills and the kitchen tap is running — around 7 gpm in an ordinary house, and considerably more if a hose is open or a tub is filling.
Why this is not a fixture unit calculation
Fixture units exist to size pipe for a whole building, and they carry a probability weighting: the assumption baked into them is that in a hundred-fixture building only a fraction run at once, and the fraction falls as the building grows. That weighting is right for its purpose and wrong for this one. A treatment device sits on one line and sees whatever passes through it, and a single house is small enough that the coincidence assumption has nothing to average over. Two showers in a three-bathroom house genuinely do run together.
So this page does the plainer thing: you decide what can run at the same time, you enter what each of those draws, and it adds them. The fixture unit calculator does the other job, producing a WSFU total for a code sizing table, and the two answers are not interchangeable. Use flow for equipment and fixture units for pipe.
| Fixture | Typical flow | How long it lasts |
|---|---|---|
| Shower head, current standard | 1.8 - 2.0 gpm | Five to fifteen minutes |
| Older shower head | 2.5 gpm and up | Same |
| Toilet refilling | around 3 gpm | Under a minute, and it overlaps everything |
| Kitchen faucet | 1.8 - 2.2 gpm | Seconds to minutes |
| Clothes washer filling | 3 - 5 gpm | A minute or two, several times per cycle |
| Tub filler | 4 - 7 gpm | Several minutes, continuous |
| Hose bib, wide open | 5 - 8 gpm | As long as somebody forgets it |
Those are starting points, not your numbers. Fill a one gallon container from the fixture and time it: 60 divided by the seconds is the flow in gpm. It takes a minute per fixture and it routinely surprises people by twenty or thirty percent in either direction.
The teaching point: loss goes as roughly the square of flow
Manufacturers publish a pressure loss at a stated flow. That single point is not the whole story, because loss is not proportional to flow. For a cartridge, a screen or any fitting where the loss is dominated by turbulence, it scales close to the square:
Loss at your flow ≈ rated loss × (your flow ÷ rated flow)²
Run a device at 1.4 times its rating and it costs twice its rated loss. At twice its rating it costs four times. At three times, nine. This is why a treatment train that behaves perfectly on a single tap becomes unusable the moment two fixtures open — the loss is not creeping up, it is accelerating. And it is why oversizing a housing is unusually good value: a device at half its rated flow costs a quarter of its rated loss, so the pressure penalty of a bigger unit is close to nothing.
Deep granular beds behave differently. Flow through a packed bed at low velocity is closer to proportional to flow than to its square, because the loss is dominated by viscous drag rather than turbulence. A real train is a mixture, which is why the exponent is a field on this page rather than a constant. Where a manufacturer publishes a loss curve rather than a single point, read the curve — it beats any exponent.
Two housings in parallel is the underrated fix
Put two identical cartridge housings side by side, each taking half the flow, and the arithmetic is better than it looks. Capacity doubles, which is the obvious part. But the loss through each one falls by a factor of four, because each sees half the flow and the loss goes as the square. So a pair costs roughly a quarter of the pressure that a single housing did at the same total flow, and lasts twice as long between changes.
The same logic explains why a bigger tank on a media filter fixes a pressure complaint that a finer media never will. Loss depends on the velocity through the bed, velocity is flow divided by cross-sectional area, and area goes as the square of the tank diameter. A 13 inch tank has 1.7 times the area of a 10 inch one for the same media volume.
What is left for the house
Start with the pressure at the head of the train. On a well that is the cut-in pressure, not the cut-out — the switch spends most of its time near the bottom of the band and that is the condition to design against. The well pressure tank calculator works out the band and the drawdown behind it. Subtract the train loss and what remains is the budget for everything downstream: pipe friction, every fitting, and 0.433 psi for each vertical foot of climb.
The pressure drop calculator handles that side, and it has a field for a fixed device loss in psi. The total from this page is exactly what belongs in it. Running the two together is how you find out whether a proposed train leaves an upstairs shower with anything usable, before it is installed rather than after.
Every water quality number on this page — hardness, iron, solids, transmittance, a concentration, a target dose — is something you enter from a laboratory test of your own water. Use a state-certified laboratory; your state or county health department maintains the list and will usually tell you what a well in your area is commonly tested for. This calculator has no idea what is in your water, will not tell you whether a result is acceptable, and does not select equipment. Treatment follows the report, and a device that removes one thing does not remove another.
Questions people ask
What flow rate should a whole house filter be rated for?
Whatever the peak simultaneous draw of the house is, with room on top rather than exactly. Add the fixtures that can genuinely run together — a shower, a toilet refilling and a kitchen tap is a common worst case at around 7 gpm — and choose equipment rated comfortably above it. The reason to leave headroom is that pressure loss climbs as roughly the square of flow, so a device sized at its exact rating is already at its full rated loss on an ordinary morning and has nowhere to go. Sizing at perhaps two thirds of a device rating costs very little extra and roughly halves the pressure penalty.
Why does my water pressure drop when I add a filter?
Because every device in the line takes a share of the pressure, and the share grows faster than the flow does. A housing rated at 2 psi at 10 gpm is not costing 2 psi when the house is drawing 14 gpm — at a square-law scaling it is costing about 3.9. Stack three or four devices and the total is often ten to fifteen psi clean and twice that with a loaded cartridge, all of it taken before the pipe run has been paid for. If the well cut-in is 30 psi, that is a large fraction of the whole budget, which is why treatment trains and low pressure complaints turn up together.
Should outdoor taps run through the treatment train?
Usually not, and the reason is this calculation. A hose wide open is 5 to 8 gpm on its own, which is more than most residential treatment equipment is rated for and comparable to everything else in the house combined. Running irrigation through a softener or a cartridge multiplies the media consumption for water that gets poured on a lawn, and it drags the whole train past its rated flow while it does. The usual arrangement takes hose bibs and irrigation off ahead of the treatment equipment. Where that split gets made is a plumbing decision, and moving it afterwards means opening walls.
Is the pressure loss the same when the filter is dirty?
No, and the clean figure is the optimistic end of the range. A sediment cartridge at the point of replacement is commonly at double its clean loss and can be well beyond that, which is why so many installations have a gauge before and after. That rising loss is what most people actually use as their change trigger, rather than a calendar or a capacity calculation. The uplift field on this page lets you see the far end of the interval. If the loaded figure leaves the house short, the honest conclusion is that the housing is too small rather than that the cartridge needs changing sooner.
Can I just use the fixture unit total instead of adding flows?
They answer different questions. A fixture unit total feeds a pipe sizing table and carries a probability weighting designed for whole buildings, where the chance of everything running at once falls sharply with size. Treatment equipment is rated in gallons per minute and sits on one line, so it needs an actual flow, and a single house is too small for the coincidence assumption to help — two showers really do run together. Use the flow sum for equipment, use the fixture unit total for pipe, and do not convert one into the other.