A garden hose water filter is an inline cartridge with hose threads on both ends, and what it does depends entirely on what is inside it. Carbon handles chlorine taste and smell, sediment cartridges catch grit, and only a deionizing or reverse osmosis cartridge gives you genuinely spot-free rinse water. Choosing the wrong one is the usual disappointment.
RV and drinking-safe use
The best-known use is on an RV or camper fresh water fill, where the point is to make water from an unfamiliar campground spigot taste acceptable and to keep grit out of the tank and pump. These cartridges are carbon with a sediment element, and the ones to buy are those explicitly rated for potable use with a hose rated for drinking water, since a standard garden hose is not. Note the limits: a carbon cartridge is not a disinfection device, so it does not make questionable water safe. It reduces chlorine, taste and odor, and catches sediment, which is exactly what a campground tap usually needs and no more.
Car washing and the spot problem
Water spots on paint are dried mineral deposits, mostly the calcium and magnesium that make water hard. The USGS classifies hardness as soft at 0 to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180 and very hard above 180 mg/L, and the harder your supply, the worse the spotting. A carbon hose filter does nothing about this, because carbon does not remove minerals. What gives a spot-free rinse is a deionizing resin cartridge or a small reverse osmosis unit, both of which actually remove the dissolved minerals. Deionizing resin exhausts based on how much mineral it has captured, so it lasts far longer on soft water than on hard.
Plants, ponds and pressure washers
For sensitive plants and for filling a pond or a fish tank from a hose, chlorine is the concern and a carbon cartridge is the right and sufficient answer, since chlorine is what harms fish and soil biology. For a pressure washer, the concern is grit, because sand reaching the pump chews the seals and the nozzle, so a sediment cartridge protects the machine. For general garden watering on a municipal supply, a filter is usually unnecessary; plants tolerate normal tap water. On a well with visible sediment, an outdoor hose filter is worth it purely to stop grit blocking drip irrigation emitters.
Fittings, flow and storage
Hose filters use standard garden hose threads and should be fitted with a washer at each end and hand tightened; a wrench splits plastic housings. They restrict flow to some degree, which matters more on a sprinkler system than on a bucket fill, so look at the flow rating if you are running an irrigation zone through one. Do not leave a cartridge sitting full of warm water in the sun between uses, because that is an excellent environment for growth; drain it and store it dry, or keep it in the shade. Replace on the maker's gallon rating, and replace a deionizing cartridge when the rinse water starts leaving spots again.
Questions people ask about garden hose water filter
Will a garden hose filter give me a spot-free car rinse?
Only a deionizing or reverse osmosis cartridge will. Carbon filters remove chlorine and taste but leave the minerals that dry into spots.
Is water from a hose filter safe to drink?
Only if the cartridge and the hose are both rated for potable use, and even then it is a filter, not a disinfection device.
Do I need a hose filter for watering the garden?
Usually not on city water. It is worth it for sensitive plants where chlorine matters, or on a well where grit blocks drip emitters.
How should I store a hose filter between uses?
Drain it and keep it dry and out of the sun. A cartridge left full of warm water is a good environment for microbial growth.