Best Small Water Softeners for Apartments, Condos and Tight Spaces
Quick answer: the smallest whole-house softeners we found are compact cabinets that hold the resin and the salt in one box. Rheem’s Preferred Compact RHS18 is 30¼ inches tall, and A. O. Smith’s 35,000-grain compact is 32 inches tall on a 15 × 19-inch footprint. A standard Whirlpool cabinet is 48 inches tall. Both still need a drain and an outlet nearby. If you have no drain, Whirlpool’s salt-free anti-scale system needs neither, but it reduces scale rather than softening the water. Renting? Ask your landlord first, or use a portable softener on your washing-machine tap, which needs no plumbing changes.
Our picks:
- Shortest cabinet: Rheem Preferred Compact RHS18
- Smallest footprint for 2–4 people: A. O. Smith AO-WH-SOFT-350C
- Best small softener on Amazon: Fleck 5600SXT 24,000-grain
- Best with no drain: Whirlpool WHASC25 anti-scale system (salt-free)
- Best for renters: On The Go Standard portable softener
How we chose: we haven’t installed or tested these softeners ourselves. We looked for softeners that are short or have a small footprint, compared measurements and limits from the makers’ spec sheets and manuals, and sized them for small households with the same math as our size calculator. Where a figure comes only from a seller’s listing, we say so.
Measurements: how much space each one needs
| Softener | Type | Height | Footprint | Capacity | Needs |
|---|---|---|---|---|---|
| Rheem Preferred Compact RHS18 | Cabinet | 30¼ in | 14½ × 21¼ in | 18,500 grains | Drain and outlet |
| A. O. Smith AO-WH-SOFT-350C | Cabinet | 32 in | 15 × 19 in | 35,000 grains max | Drain and outlet |
| Whirlpool WHES30 (standard size, for comparison) | Cabinet | 48 in | 15 × 20½ in | 30,000 grains | Drain and outlet |
| Fleck 5600SXT 24,000-grain | Two tanks | 52 in (whole system)* | 22 × 11 in (whole system)* | 24,000 grains* | Drain and outlet |
| Fleck 5600SXT 48,000-grain (full size, for comparison) | Two tanks | 62 in overall* | 10-in resin tank plus 15-in brine tank* | 48,000 grains* | Drain and outlet |
| Whirlpool WHASC25 anti-scale | Salt-free | 33¾ in | 13 7/16 × 10⅛ in | Doesn’t soften | Nothing |
| On The Go Standard | Portable | 22 in* | 6¾ in across* | 8,000 grains* | A tub or drain while recharging |
From Rheem, A. O. Smith and Whirlpool product pages and manuals. *From seller listings (DuraWater, AFWFilters and On The Go on Amazon). Checked October 2026.
On The Go Standard (portable)
Rheem Preferred Compact RHS18
A. O. Smith AO-WH-SOFT-350C
Whirlpool WHASC25 (salt-free)
Whirlpool WHES30 (standard cabinet)
Fleck 5600SXT 24,000-grain
Fleck 5600SXT 48,000-grain
Blue: the two compact cabinet softeners. Heights are for the unit itself; leave room above the lid to pour in salt.
Measure your space before you shop, then add room for the pipes and bypass valve and space above the lid to add salt. Cabinet softeners are short but fairly wide and deep. A two-tank softener is taller, but each tank is narrow, so it can suit a tall, narrow closet.
What size softener does a small household need?
Size a small softener the same way as any other: people × 75 gallons a day × hardness (in grains per gallon, gpg) × 7 days, divided by 0.75 so you use about 75% of its capacity between weekly regenerations. That’s the method behind our size calculator. For a quick check, it works out to people × hardness × 700. If your water has iron, add 4 gpg for each ppm of iron first.
Here’s the hardest water each compact softener can handle on that schedule:
| People | Rheem RHS18 (18,500) | A. O. Smith 350C (26,300) | Fleck 24,000 |
|---|---|---|---|
| 1 | Up to 26 gpg | Up to 37 gpg | Up to 34 gpg |
| 2 | Up to 13 gpg | Up to 18 gpg | Up to 17 gpg |
| 3 | Up to 8 gpg | Up to 12 gpg | Up to 11 gpg |
| 4 | Up to 6 gpg | Up to 9 gpg | Up to 8 gpg |
Each softener’s capacity ÷ (people × 700), rounded down. We use 26,300 grains for the A. O. Smith, the most its manual lists for a salt setting, rather than its 35,000-grain maximum.
For reference, the U.S. Geological Survey calls water above 7 gpg hard and above 10.5 gpg very hard. If your household is beyond what a compact model covers, it will regenerate more than once a week, which is more often than our size calculator aims for. To regenerate less often, step up to a larger softener from our best water softeners guide.
Will a compact softener work for you? Fit checker
Enter your household and your space. The checker sizes your softener the way our size calculator does, then checks each compact pick’s height, limits and hookups.
Enter your household and hardness to check the fit.
How it works: people × 75 gallons a day × hardness (plus 4 gpg per ppm of iron) gives the hardness to remove each day. Days between regenerations assume you use 75% of each softener’s capacity: 18,500 grains for the Rheem, 26,300 for the A. O. Smith and 24,000 for the Fleck (seller listing). Height, hardness and iron limits come from each maker; the Fleck’s seller doesn’t publish hardness or iron limits.
The best small water softeners
Rheem Preferred Compact RHS18 (18,000 grains)
At 30¼ inches tall, this is the shortest whole-house cabinet softener we found that’s still sold. Rheem says its space-saving design is ideal for smaller spaces, condos and townhomes. It’s also certified to NSF/ANSI 44 for hardness reduction and salt efficiency, according to its manual.
- Size: 30¼ in tall, 14½ in wide, 21¼ in deep
- Capacity: 7,600 grains on 1.8 lb of salt, 11,000 on 3.1 lb, 18,500 on 10.8 lb
- Efficiency: rated 4,100 grains per pound of salt at its lowest setting
- Limits: up to 65 gpg hardness and 3 ppm clear-water iron; 20–125 psi; 7.0 gpm rated flow
- Warranty: 1 year parts and labor (5 years if you use Rheem’s cleaner), 3 years electronics, 10 years tanks
Good to know: Rheem rates it for 1 to 4 people, but by the sizing table above it best suits one or two people on most hard water. Rheem’s site doesn’t link to retailers; Home Depot lists it. Whirlpool’s WHES18 comes from the same maker with the same model code and specs, if that’s easier to find. GE’s 18,000-grain compact (GXSF18G) is discontinued.
See it at RheemA. O. Smith AO-WH-SOFT-350C (35,000 grains)
Only 1¾ inches taller than the Rheem, with more capacity and the smallest footprint of the cabinets here: 15 by 19 inches. A. O. Smith rates it for two to four people and describes it as a slim, single-unit design.
- Size: 32 in tall, 15 in wide, 19 in deep
- Capacity: 35,000 grains maximum; the manual lists 13,500 grains on 3 lb of salt (high-efficiency setting) and 26,300 on 7 lb (high-capacity setting)
- Limits: up to 90 gpg hardness and 10 ppm ferrous iron; pH 7 or higher; 20–80 psi; 7.5 gpm service flow
- Salt storage: 105 lb
- Warranty: 3 years mechanical and electronic parts; 10 years media tank, control valve and brine cabinet
Good to know: its efficiency, about 4,500 grains per pound of salt on the lowest setting, is A. O. Smith’s own figure; we didn’t find an NSF/ANSI 44 certification. Its maximum pressure is 80 psi, so you’ll need a pressure-reducing valve if yours runs higher. The spec table also lists a maximum chlorine of 0 ppm without explaining it, so if you’re on chlorinated city water, ask A. O. Smith what that means for you. It’s sold at Lowe’s.
See it at A. O. SmithFleck 5600SXT 24,000-grain softener (DuraWater)
A conventional two-tank softener: a narrow resin tank topped with Pentair’s Fleck 5600SXT metered valve, plus a separate brine tank. It’s taller than the cabinets, but each tank is narrow, so it suits a tall closet better than a low cabinet.
- Resin: 0.75 cubic feet in an 8 × 44-inch tank (seller listing)
- Brine tank: 11 × 11 × 38 inches (seller listing)
- Whole system: about 22 × 11 × 52 inches (seller listing)
- Valve: Fleck 5600SXT, which regenerates based on the water you use; the listing says it keeps its programming in a power outage
Good to know: these figures come from the seller’s listing, which doesn’t give iron or hardness limits, so ask before buying for well water. The same listing offers 32,000-, 40,000- and 48,000-grain sizes if you need more capacity.
Check price on AmazonWhirlpool WHASC25 Anti-Scale System (salt-free)
If there’s no drain or outlet where your water comes in, a salt-based softener won’t work. This salt-free system needs no salt, electricity or drain. Whirlpool says it changes hardness minerals into a microscopic crystal form so they’re less likely to stick to pipes and fixtures, and it’s clear that the system doesn’t soften water.
- Size: 33¾ in tall, 13 7/16 in wide, 10⅛ in deep; 30 lb
- Flow: 10 gpm service flow, 15 gpm maximum
- Upkeep: no regeneration; the manual says it may benefit from added media every few years
- Warranty: 1 year parts, 3 years in/out head, 10 years tank
Good to know: Whirlpool calls it ideal for homes with low levels of hardness but doesn’t publish a maximum. Because the minerals stay in the water, test strips can’t show whether it’s working. For the evidence on salt-free systems, see our salt-free guide.
See it at WhirlpoolOn The Go Standard portable softener (8,000 grains)
It’s made for RVs, but it connects anywhere there’s a garden-hose thread, including a washing-machine tap. It needs no tools, electricity or plumbing changes, so you can take it with you when you move.
- Capacity: 8,000 grains (seller listing)
- Size: 22 in tall, 6.75 in across (seller listing)
- Recharge: one box of table salt, in under 15 minutes (seller listing)
Good to know: on a washing machine, one unit softens only the cold supply. Recharge it in a tub or shower so the salty rinse goes down a drain. To see how often you’ll recharge it, use the calculator in our portable softener guide.
Check price on AmazonCan you put a water softener in an apartment or condo?
Often, yes. Check these six things first:
- Permission. Renters need the landlord’s OK, and condo owners may need their HOA’s. Get it in writing.
- A drain within reach. Salt-based softeners flush salty water when they regenerate. Rheem’s manual lists a floor drain, laundry tub, sump or standpipe, with a 1½-inch air gap above it. A. O. Smith’s manual also allows a washer drain, and says the drain line can rise up to 8 feet if your water pressure is at least 40 psi, with 3/4-inch tubing for runs of 25 feet or more. If you have laundry in your unit, its standpipe may be your best option.
- An outlet. Rheem’s manual calls for a 120-volt outlet nearby.
- A spot where the water comes in. Rheem’s manual says to always install the softener between the water inlet and the water heater. If your hot water comes from the building’s central boiler, a softener in your unit can only treat the cold water.
- A solid floor. Rheem says to set it on a solid, level surface and warns that the weight of the tank full of water and salt can crack it if it sits on shims. Whirlpool’s version of the same softener holds up to 125 lb of salt.
- Local rules. California requires salt-based softeners to remove at least 4,000 grains of hardness per pound of salt; Rheem’s manual says to turn on its salt-efficiency setting there. Some California communities restrict salt-based softeners, so check yours; our salt-free guide explains why.
Ready to plumb one in? Our guide to installing a water softener walks through it.
Renting? Options that don’t change the plumbing
- A portable softener on the washing machine. Washing-machine taps commonly have a 3/4-inch garden-hose-thread outlet (Watts’ LF4751 valve is one example), the same connection portable RV softeners use. Put the softener between the cold tap and the washer’s cold hose.
- Reverse osmosis for drinking water. An under-sink RO system reduces hardness minerals at one tap, with no change to the main line. See water softener vs reverse osmosis.
- A salt-free system on the main line. It needs no drain or outlet, but it still needs plumbing work, so you’ll still need permission.
Under-sink and inline water softeners: what’s really out there
When we searched Amazon for “under sink water softener” in October 2026, nearly every result was a drinking-water filter or a reverse osmosis system. Carbon filters don’t remove hardness. RO does reduce hardness minerals, but only at one faucet. And a whole-house cabinet is unlikely to fit under a sink: even the shortest one here is 30¼ inches tall, so measure inside the cabinet before you buy anything.
“Inline water softener” usually means one that connects in a water line, like a portable softener on a hose. Inline salt-free cartridges, such as Watts’ OneFlow for tankless water heaters, protect one appliance rather than the whole home; see our salt-free guide.
Water softeners for apartment showers
Most shower filters are made to reduce chlorine, not hardness. NSF/ANSI 177, the standard for shower filters, tests how well they reduce free chlorine, so a certified shower filter tells you about chlorine, not hardness. Some shower units contain softening resin. If you’re considering one, check how many gallons the maker says each cartridge treats at your water’s hardness before you buy.
Frequently asked questions
What is the smallest water softener?
Of the whole-house softeners we found still on sale, Rheem’s Preferred Compact RHS18 is the shortest at 30¼ inches tall, and A. O. Smith’s AO-WH-SOFT-350C has the smallest footprint at 15 × 19 inches. Portable softeners are smaller still; the On The Go Standard is 22 inches tall, according to its listing.
What size water softener do I need for one or two people?
Multiply people × hardness (gpg) × 700. Two people with 10 gpg water need about 14,000 grains, so an 18,000-grain compact like the Rheem covers them. Use our size calculator for a detailed answer.
Can I install a water softener in an apartment?
Yes, if your landlord or HOA agrees and you have a drain, an outlet and access to the water line where it enters your unit. If not, a portable softener on the washing machine or an under-sink RO system for drinking water doesn’t change the plumbing.
Do I need a drain for a water softener?
A salt-based softener does, because it flushes salty water when it regenerates. A salt-free system doesn’t, and neither does a portable softener in use, though you’ll rinse it over a drain when you recharge it.
Do under-sink water softeners work?
True under-sink softeners are hard to find; most products sold that way are drinking-water filters. If you want soft drinking water at one tap, reverse osmosis reduces hardness minerals there.
Related guides
- What size water softener do I need? (calculator)
- Best water softeners (all types)
- Best portable water softeners for RVs
- Best salt-free water softeners
- Water softener vs reverse osmosis
Sources
- Rheem, Preferred Compact RHS18 product page, and the RHS18 installation manual (rev. G, 2025): NSF/ANSI 44 certification, drain, outlet and installation requirements.
- Whirlpool, WHES18 installation manual (2021, copy hosted by InspectAPedia): same model code as the RHS18, 125 lb salt storage.
- A. O. Smith, AO-WH-SOFT-350C product page and owner’s manual: specifications and drain line limits.
- Whirlpool, WHES30 and WHASC25 anti-scale system product pages, and the WHASC manual.
- GE Appliances, GXSF18G (marked no longer manufactured).
- U.S. Geological Survey, Hardness of Water.
- California Legislative Counsel’s Digest, AB 460 (2009), summarizing existing law on softener salt efficiency.
- Watts, LF4751 washing machine valve (3/4-inch GHT outlet).
- Rick Andrew, New Standard for Shower Filters, Water Conditioning & Purification, December 2004 (NSF/ANSI 177).
- Seller listings on Amazon for the DuraWater Fleck 5600SXT 24,000-grain, AFWFilters Fleck 5600SXT 48,000-grain and On The Go Standard softeners, checked October 2026.