Quick Answer: A laundry water softener additive is the cheapest way to make hard water behave in the washer without plumbing in a softener. Our top pick is Charlie's Soap Booster & Hard Water Treatment, which takes only a half to one tablespoon per load. For a phosphate-free liquid, use Calgon Water Softener Liquid. On a tight budget, Arm & Hammer Super Washing Soda works well for towels and work clothes.
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Here's an unpopular opinion: most people with crunchy towels don't need a new detergent. They need a tablespoon of the right powder in the drum. Laundry softening additives are old, cheap and well understood, but they don't all work the same way. Pick the wrong type for your fabrics and you can trade stiff towels for faded darks or residue on your clothes.
So we'll start with the chemistry, which takes two minutes and will save you from buying all five.
In this guide
Two ways to beat hardness in the wash
Sequestering (also called chelating). The additive grabs calcium and magnesium ions and holds them in solution so they can't react with detergent or settle onto fabric. Nothing drops out, and minerals rinse away with the water. Phosphates and sodium citrate work this way. This is the cleaner approach, and it's gentle on fabrics.
Precipitating. The additive reacts with the hardness minerals and forms an insoluble solid, which in the case of washing soda is calcium carbonate. That takes the minerals out of play, but the solid has to go somewhere. Most of it washes out, but in very hard water a little can lodge in fabric, especially dark cottons. Washing soda works this way and also raises the pH, which boosts cleaning on greasy, heavy soil.
Borax sits somewhere in between. It's a mild alkaline booster that helps detergent cope with moderate hardness, but on its own it's a weaker softener.
Rule of thumb: sequestering additives for everyday and delicate loads, precipitating ones for towels, sheets and work clothes.
Which one, at which hardness?
Before you choose, find your hardness in grains per gallon. If your water report gives it in mg/L or ppm, divide by 17.1 to get gpg. There's more on that in water hardness units: gpg vs ppm. The USGS hardness scale puts anything over about 10.5 gpg (180 mg/L) in "very hard."
- Under 7 gpg: you probably need only borax, or nothing at all with a good powder detergent.
- 7 to 12 gpg: Charlie's Booster or Calgon in every load.
- 12+ gpg: Charlie's or Calgon in every load, plus washing soda for towels and heavy loads.
- Softened water: skip the additives. They'll just add residue.
If you haven't tested, a simple strip or drop kit will tell you in a few minutes. Here's how to test water hardness at home.
What made our list
We chose products that are clearly sold as laundry water softeners or hardness boosters, with ingredients or a mechanism we could confirm from the label, retailer pages or the manufacturer's disclosure. We left out "fabric softener" products, since those coat fibers instead of dealing with the minerals, and we left out unbranded bulk chemicals unless they're a household staple. We haven't tested these in a lab. The comments below are based on published ingredients, label directions and patterns in owner reviews.
The five, in order of how often we'd reach for them
1. Charlie's Soap Booster & Hard Water Treatment
Check price on Amazon · Best overall
Charlie's is built only for this job, and the dosing proves it: the brand recommends half a tablespoon for a normal load and a full tablespoon for hard water or heavy soil, added straight into the drum with your regular detergent. A 2.64 lb bag is listed at 160 loads. Retailer pages list potassium diphosphate as its key ingredient, which puts it in the sequestering camp. It works in hot or cold water, and in standard and HE machines.
It's our top pick because it's easy to use, safe for nearly any fabric, and the bag lasts a long time. The brand also says it can take a few washes before you notice softer laundry, since old mineral residue comes out gradually. That matches what owners report.
Skip it if: you need a product that's labeled phosphate-free. Choose Calgon Liquid instead.
2. Calgon Water Softener Liquid
Check price on Amazon · Best liquid / phosphate-free
Calgon is the name your grandmother knew, and the liquid version is the most modern take. The manufacturer's ingredient disclosure lists sodium citrate as the sequestering agent and states it contains no phosphate. The direction is one capful (1/4 cup) per load alongside your detergent. This listing is a three-pack of 32 fl oz bottles.
Pick this if you want something that pours into the dispenser like detergent, or if you live somewhere phosphate-free products are preferred.
Skip it if: you go through a lot of loads. At a quarter cup per load, a 32 oz bottle runs out quickly compared with Charlie's tablespoon dose. It also contains fragrance.
3. Calgon Water Softener Powder
Check price on Amazon · Best classic powder
The 40 oz box of powdered Calgon is the original laundry water softener for many households. Some people simply prefer powder because it's easy to measure and store, and it pairs naturally with a powder detergent. Follow the scoop directions on the box, and increase the dose for harder water within the label's range.
Skip it if: you mostly wash in cold water and use liquid detergent. The liquid version dissolves more easily.
4. Arm & Hammer Super Washing Soda
Check price on Amazon · Best budget, for heavy-duty loads
Washing soda is sodium carbonate, the same builder used in many powder detergents. It precipitates hardness and raises wash pH, which helps cut grease, sweat and ground-in dirt. Pennies per load, and it's one of the strongest options for towels, gym clothes, sheets and work wear.
The downside is real, though. Its high alkalinity is harsh on wool and silk, it can fade some dyes over time, and in very hard water it can leave a faint residue on dark cottons. Don't use it on everything. Use it where you need extra power, and handle it with gloves, since it can irritate skin.
Skip it if: you're washing delicates, wool, silk, or baby clothes. For those, see our guide to washing baby clothes in hard water.
5. 20 Mule Team Borax Laundry Booster
Check price on Amazon · Best gentle booster for moderate hardness
Borax (sodium tetraborate) is a mild alkaline booster that helps detergent cope with moderate hardness and adds a little whitening and deodorizing. This listing is a 65 oz (4 lb) box, unscented. It's gentler than washing soda and works fine for mixed loads in lightly to moderately hard water.
Skip it if: your water is very hard. Borax alone won't keep up there, so use Charlie's or Calgon instead. Also store it out of reach of kids and pets, since it shouldn't be swallowed.
Quick reference
| Product | Mechanism | Dose (per label/retailer) | Best for |
|---|---|---|---|
| Charlie's Booster | Sequestering (phosphate) | ½–1 tbsp | Every load, all fabrics |
| Calgon Liquid | Sequestering (sodium citrate) | 1 capful (¼ cup) | Liquid users, phosphate-free |
| Calgon Powder | Water softener powder | Per box scoop | Powder detergent users |
| Washing Soda | Precipitating, high pH | Per box | Towels, work clothes |
| Borax | Mild alkaline booster | Per box | Moderate hardness, mixed loads |
Can you just mix your own?
You can, up to a point. Plenty of people keep washing soda and borax on the shelf and add a couple of tablespoons of each to heavy loads. That works well for towels and work clothes in moderately hard water, and it's the cheapest route.
Where DIY falls short is everyday mixed loads and delicates. Washing soda's high pH is the problem there, and you can't easily get a sequestering agent like sodium citrate or a phosphate blend in a convenient form at the grocery store. That's what you're paying for with Charlie's or Calgon: chemistry that holds hardness in solution without raising pH much.
A sensible split for most homes:
- Everyday loads: a sequestering booster (Charlie's or Calgon).
- Towels, sheets, gym clothes: add washing soda on top.
- Lightly hard water and mixed loads: borax alone is often enough.
Additive or a whole-house softener?
An additive treats the washer and nothing else. If your hardness is only causing laundry problems, that's the right fix, and it costs very little per load.
But if you're also descaling the coffee maker monthly, scrubbing white crust off faucets, and fighting film in the dishwasher, the laundry additive is one of four separate products you're buying to deal with the same minerals. That's when a softener becomes worth pricing out, since it fixes laundry, dishes, fixtures and water heater scale at once. Once you have softened water, you can drop the laundry additive and cut detergent by up to half.
There's no single breakpoint. As a rough rule: under about 10 gpg with problems mostly in laundry, use an additive. Over about 12 gpg with problems all over the house, a softener usually pays off.
Getting the most out of an additive
- Add it at the start of the wash, not the rinse. It has to deal with the minerals before the detergent does.
- Strip first if your towels are already stiff. An additive prevents new buildup. It won't undo months of residue in one go. Our guide to laundry that feels stiff after washing covers the stripping method.
- Cut back on fabric softener. It coats fibers, traps minerals and makes towels less absorbent. Most people find they don't need it once the hardness is handled.
- Don't stack everything. One sequestering additive plus detergent is plenty for most loads. Add washing soda only for heavy loads.