Free Calculator · Published September 2026
Sediment Filter Sizing Calculator for Whole House and Well Water
Enter the water source, the sediment you are fighting, peak flow and what sits downstream to get the micron staging, whether you need a spin-down filter, the housing size (standard or Big Blue), how many housings and how often the cartridges will need changing.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoWhat Sediment Filter Does This House Need?
The sediment picks the stages. The downstream equipment picks the final micron. Peak flow picks the housing.
Wells carry far more sediment, so the calculator uses Big Blue housings only.
Check what collects in the toilet tank, a jar test, or the old cartridge.
UV, RO and carbon need a 5 micron final stage.
Pick an existing housing to see whether it can carry the flow.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on conservative housing ratings and typical sediment loads, which vary by manufacturer and by well. They are not a substitute for a water test or the equipment spec sheet. Always confirm staging, housing size and change intervals with a licensed installer or certified water treatment professional.
How Sediment Filter Sizing Works
Why a sediment filter comes first
A whole house sediment filter is the first piece of equipment after the pressure tank, and everything behind it depends on it. Sand scores softener valves and fixture cartridges, silt fouls iron and carbon media, and any particle ahead of a UV sterilizer can shade bacteria from the light. On a well, sediment is usually the reason a treatment system underperforms before anyone suspects the equipment itself.
City water is a much lighter load: usually some fine silt and pipe scale, heavier after main work. A single 20 micron cartridge often covers it. Wells range from almost clean to pumping visible sand every morning, which is why the calculator asks what the sediment actually is before it picks a micron rating. Sediment work is one of the most common calls a water treatment dealer or plumbing contractor gets from well owners.
Water filter micron chart
A micron is one millionth of a meter. The smallest particle most people can see is around 40 micron, and a human hair is roughly 50 to 100. Approximate sizes of what shows up in water:
| Particle | Approximate size |
|---|---|
| Sand | About 75 micron and up (coarse sand 500+) |
| Silt | About 2 to 75 micron |
| Clay | Under about 2 micron |
| Rust / oxidized iron | Wide range, sub-micron to tens of microns |
| Giardia cysts / Cryptosporidium | About 8–12 / 4–6 micron |
| Bacteria | About 0.2 to 2 micron |
| Viruses | About 0.02 to 0.3 micron |
| Dissolved iron, hardness, salts | Dissolved: no sediment filter stops them |
And what each rating typically does in a whole house train:
| Rating | Stops | Gets through | Typical position |
|---|---|---|---|
| 60–100 mesh (~250–150 micron) | Coarse and medium sand, grit, pipe scale | Fine sand, silt, clay | Spin-down, first on a sandy well |
| 50 micron | Sand and most visible grit | Most silt, clay | Coarse cartridge, softener protection |
| 20–30 micron | Fine sand, coarse silt, most visible dirt | Fine silt, clay, microbes | General whole house, city water |
| 5 micron | Most silt, fine rust particles | Clay, bacteria, viruses | Final stage before UV, RO or carbon |
| 1 micron (absolute) | Nearly all silt; cysts if certified for it | Bacteria, viruses, some clay | Polishing or point of use, not first stage |
Most whole house cartridges are rated nominal, which means they catch a large share of particles at the rating, not all of them. An absolute rating is tested to a much stricter standard. A sediment cartridge is not a disinfection step: bacteria and viruses are handled by UV, and a cartridge that claims cyst reduction should carry a certification for it.
Step 1: stage it coarse to fine
The most common sediment mistake is putting one fine cartridge on a dirty well. A 1 micron cartridge on a well with heavy sediment can plug in days, because it catches the sand and coarse silt as well as the fine particles it was bought for, and its surface blinds over. The homeowner sees pressure falling at every tap and blames the pump.
Staging spreads the load. A spin-down screen flushes sand out without a cartridge at all. A coarse 50, 30 or 20 micron cartridge takes the bulk of what is left. A 5 micron final stage, when the downstream equipment needs one, only sees fine particles, so it lasts. Each stage ends up doing the job it is suited to, and the total cartridge spend usually goes down even though there are more housings.
Step 2: the downstream equipment sets the final micron
A softener or tankless water heater only needs protection from sand and grit, so a 20 to 50 micron final stage is usually enough. A UV sterilizer is different: most UV manufacturers call for 5 micron prefiltration, because particles shield bacteria from the light and coat the quartz sleeve. Size that stage alongside the lamp with the UV water sterilizer sizing calculator. Carbon and RO also want 5 micron water, since sediment blinds carbon and fouls membranes; the carbon filter sizing calculator covers the stage that follows.
Step 3: peak flow picks the housing
| Housing | Typical published range | Rating used here | Sediment capacity |
|---|---|---|---|
| 10" x 2.5" standard | 2–3 gpm | 2.5 gpm | 0.3x a 10" Big Blue |
| 20" x 2.5" standard | 4–6 gpm | 5 gpm | 0.6x a 10" Big Blue |
| 10" x 4.5" Big Blue | 8–10 gpm | 8 gpm | 1x a 10" Big Blue |
| 20" x 4.5" Big Blue | 15–20 gpm | 15 gpm | 2x a 10" Big Blue |
The calculator uses the conservative end, and derates a 5 micron stage by 20% because a finer cartridge of the same size drops more pressure. When a housing runs over its rating the pressure drop climbs quickly and keeps climbing as the cartridge loads, which shows up as weak showers the moment two fixtures run. Above about 12 to 15 gpm, the answer is two 20 inch Big Blue housings in parallel rather than one overworked housing.
On a well, the calculator only recommends Big Blue (4.5 inch) housings even when a slim one would carry the flow. The fatter cartridge holds several times more sediment, which is the difference between a change every few weeks and one every few months. A gauge on each side of the housing turns change intervals from guesswork into a reading: many dealers change the cartridge when the drop grows about 10 psi over the clean figure, and at least every 6 months regardless.
Clay, rust and the limits of a cartridge
Clay is under about 2 micron, so cloudy water from clay usually comes through a 5 micron cartridge looking the same while the cartridge plugs. Persistent turbidity is normally handled with a backwashing media filter, often with a coagulant fed ahead of it so the fine particles clump together, with cartridges kept as a polishing step.
Rust particles are caught, but dissolved iron is not. Water that comes out clear and turns orange after it sits carries dissolved ferrous iron, which passes every sediment filter and oxidizes downstream. That needs an iron filter; see the iron filter for well water guide, and use the well water treatment system selector to put the full train in the right order.
Worked examples
Each row is the calculator’s own output for a common job, with housings chosen automatically.
| Job | Peak | Stages | Housing | Change (fastest stage) |
|---|---|---|---|---|
| City water, 2 baths, softener | 9 gpm | 20 micron | 1 x 20" Big Blue | 6 months |
| City water, 1 bath, carbon filter | 6 gpm | 5 micron | 1 x 10" Big Blue | about 4 months |
| Sandy well, 3 baths, UV | 12 gpm | spin-down 60 mesh → 50 micron → 5 micron | 2 x 20" Big Blue | about 4 months |
| Well with rust particles, 2 baths, softener | 9 gpm | 20 micron | 1 x 20" Big Blue | about 3 months |
| Cloudy well (clay), 2 baths | 9 gpm | 20 micron → 5 micron | 2 x 20" Big Blue | about 2 months |
The sandy well is the one worth reading twice. It needs 12 gpm through a 5 micron stage for the UV, which is already the full derated rating of a 20 inch Big Blue, and without the spin-down screen and the 50 micron stage ahead of it, that 5 micron cartridge would be doing the sand’s work too. For the equipment and install cost side of the same job, see the whole house water filter cost calculator.
For dealers: sediment filters are the service visit
A sediment filter is the lowest-ticket part of a treatment system and the one the customer touches most often. Cartridges on a well come due every few weeks to few months, the spin-down needs flushing, and a skipped change shows up as a pressure complaint that sounds like an equipment failure.
That makes every install a recurring stop: cartridges in the right microns, a gauge reading, and a look at the softener, iron filter or UV downstream while you are there. Keeping those dates and cartridge specs across a book of wells is what software for water treatment dealers is for.
Frequently Asked Questions
What micron sediment filter is best for well water?
There is rarely one right micron rating for a well, because the job is done in stages. A sandy well usually starts with a 60 to 100 mesh spin-down screen (about 250 to 150 micron), then a 50 or 30 micron cartridge. If the water feeds a softener only, that is often enough. If it feeds a UV sterilizer, an RO system or a carbon filter, add a 5 micron final stage. A well that carries mostly silt usually starts at 20 micron. Going straight to 5 or 1 micron on a dirty well is the most common mistake, because the fine cartridge ends up doing the coarse filter's job and plugs quickly.
What is a spin-down filter and do I need one?
A spin-down filter is a reusable screen filter, usually in a clear housing, that spins the water so heavy sand drops to the bottom, where a valve flushes it out. Screens are sold by mesh: 60 mesh is about 250 micron, 100 mesh about 150 micron and 200 mesh about 75 micron. You need one when a well pumps visible sand or grit, because sand fills a cartridge fast and scores valves and fixtures. On city water it is usually unnecessary unless there has been main work. The screen is flushed rather than replaced, often weekly to monthly on a sandy well.
What size Big Blue filter do I need?
Size the housing on peak flow. This calculator uses conservative clean-cartridge ratings of about 2.5 gpm for a 10 inch by 2.5 inch standard housing, 5 gpm for a 20 inch standard, 8 gpm for a 10 inch by 4.5 inch Big Blue and 15 gpm for a 20 inch Big Blue, and it derates a 5 micron stage by 20%. A two-bathroom home peaking near 9 gpm is past a 10 inch Big Blue, so it lands on a 20 inch Big Blue. On a well, a Big Blue is the better choice even when a slim housing would pass the flow, because the 4.5 inch cartridge holds several times more sediment. Manufacturer ratings vary, so confirm against the spec sheet.
Why does my 5 micron or 1 micron filter clog so fast?
Because it is catching everything. A fine cartridge on a well with real sediment load stops the sand and coarse silt as well as the fine particles it was meant for, so its surface blinds over. A 1 micron cartridge alone on a heavy sediment well can plug in days, with pressure falling at every tap. Staging fixes it: a spin-down or coarse cartridge takes the bulk out first, and the fine cartridge only sees what is left. It also matters whether a rating is nominal or absolute. A nominal 5 micron cartridge lets some 5 micron particles through, while an absolute rating is tested to a much stricter standard.
How often should a whole house sediment filter be changed?
On city water a whole house cartridge often lasts 3 to 6 months. On a well it can be anywhere from a few weeks to a few months, depending on the sediment and the size of the housing. The most reliable signal is pressure drop: many dealers put a gauge on each side of the housing and change the cartridge when the drop grows about 10 psi over the clean reading. Whatever the gauge says, change it at least every 6 months, because a loaded, damp cartridge can grow bacteria. A 20 inch Big Blue holds roughly twice the sediment of a 10 inch Big Blue and several times that of a 2.5 inch standard cartridge.
Does a sediment filter remove iron?
Only iron that has already turned into particles. Rust flakes and oxidized, orange iron will be caught by a sediment cartridge, often at 20 or 5 micron. Dissolved ferrous iron, the kind that comes out of the tap clear and turns orange after it sits, is dissolved in the water and passes straight through any sediment filter. It then oxidizes downstream and stains fixtures anyway. If a test shows dissolved iron, the answer is an iron filter, such as an air injection or greensand system, with the sediment filter ahead of it as protection.
What micron filter do I need before a UV system?
Most UV manufacturers call for 5 micron prefiltration. Particles can shade bacteria from the UV light, and sediment coats the quartz sleeve and cuts the dose. On a clean city supply a single 5 micron stage is usually enough. On a well, put a coarser stage such as 20 or 50 micron ahead of the 5 micron cartridge so the final stage does not plug, and add a spin-down screen if the well pumps sand. Hardness and iron also foul the sleeve, so those are treated ahead of the UV too.
Can a sediment filter fix cloudy well water from clay?
Usually not well. Clay particles are under about 2 micron, smaller than a typical 5 micron cartridge can reliably stop, so cloudy water often comes through looking the same while fine cartridges plug quickly. Persistent clay turbidity is usually treated with a backwashing media filter, often with a coagulant fed ahead of it so the fine particles clump into something the media can hold. A cartridge train still helps as a polishing stage afterward. Sudden cloudiness in a well that used to be clear can also point to a well or pump problem worth checking first.
Every Sediment Filter Install Is a Cartridge Change on the Calendar
Cartridge changes, spin-down flushes and the equipment downstream only become recurring water treatment revenue if something is tracking them. Quote the full treatment train on site and put every install on a service schedule.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. Housing ratings are approximate and vary by manufacturer and cartridge type; confirm flow ratings against the equipment spec sheet and change cartridges by pressure drop.