Washing machine filters come in two unrelated kinds, and mixing them up wastes money. Inside the machine are inlet screens and a drain pump trap that catch debris. On the wall behind it, some homes fit a supply filter that treats the water going in. Only the second one changes what the water is doing to your clothes.
The inlet screens on the hoses
Where each fill hose screws onto the machine there is a small mesh screen, sometimes in the valve and sometimes in the hose end. Its job is to stop grit, pipe scale and rubber fragments reaching the solenoid valve, where a single particle can hold the valve open or shut. When a machine fills slowly, or fills on one temperature only, a blocked inlet screen is one of the first things worth checking.
The drain pump filter, or coin trap
Front loaders carry a removable filter at the bottom of the cabinet, ahead of the drain pump. It collects coins, buttons, hair and lint so they cannot jam the impeller. This one is a maintenance item on a schedule: clean it every few months, with a shallow tray ready because water always comes out with it. A neglected trap shows up as slow draining, a machine that stops mid-cycle, or a persistent smell.
Neither of them treats the water
Both filters catch solids. Neither removes anything dissolved, and that is where most laundry complaints actually come from. The USGS defines hardness as dissolved calcium and magnesium, at soft up to 60 mg/L, moderately hard 61 to 120, hard 121 to 180 and very hard above that. Those minerals react with detergent to leave a residue in the fabric, which is why hard-water laundry comes out gray, stiff and stale-smelling no matter how clean the coin trap is.
Filtering the supply instead
If the complaint is staining rather than stiffness, look at iron. Penn State Extension gives recommended maximums of 0.3 mg/L for iron and 0.05 mg/L for manganese as secondary standards, and both stain laundry well below any health concern. A sediment filter on the laundry supply catches rust particles and grit; a whole-house iron filter deals with dissolved iron; a softener deals with hardness. Choose from the water test, not from the symptom in the drum.
Questions people ask about washing machine filters
Where is the filter on a washing machine?
There are two places: mesh screens at the inlet hose connections, and a drain pump filter behind a small door at the bottom front of most front loaders.
How often should I clean the drain filter?
Every few months for most households, and sooner if the machine drains slowly, stops mid-cycle or smells musty between washes.
Will a filter on the supply line soften the water?
No. A cartridge filter catches particles and can address taste and chlorine, but only an ion exchange softener removes the dissolved calcium and magnesium.