We use cookies to allow us and selected partners to improve your experience and our advertising. By continuing to browse you consent to our use of cookies. You can understand more and change your cookies preferences here.

Coronavirus Read our latest advice

Dishwasher salt and rinse aid explained

By Aaron West

How does dishwasher salt and rinse aid work, what does it do and how much do you need to use? Read our guide for all the answers.

Put us to the test

Our Test Labs compare features and prices on a range of products. Try Which? to unlock our reviews. You'll instantly be able to compare our test scores, so you can make sure you don't get stuck with a Don't Buy.

In order to achieve good cleaning results, dishwashers need to use very soft water. This is where dishwasher salt comes in. Rinse aid is also key to help good drying.

In this guide we answer the following key questions:

Read on to find out what dishwasher salt is, how to use it, how it works, and which brand to buy.

How to use and where to put dishwasher salt 

The salt compartment of your dishwasher is normally at the bottom of the machine, near the filter.

You'll need to pull out the lower rack to access it, and you should have a small funnel that came with your dishwasher to help make sure all the salt goes in the hole.

You only need to fill up the salt to the water level inside the salt compartment.

You can often adjust the salt setting on your dishwasher to suit the hardness of the water in your area.

To find out how hard the water in your home is, consult your water supplier – you can usually find the information online. You'll be able to find the concentration of calcium and magnesium ions, usually in millimole per litre (mmol/l). 

Then consult your dishwasher’s manual to find out which setting will work best.

Most dishwashers also now have a salt refill light. If your dishwasher doesn’t have indicator lights, it’s a good idea to refill the salt about once per month.

And only put salt into the softener unit. The salt you pour in the softener unit never actually touches your dishes; it just stays in the unit itself. If you leave loose salt in the dishwasher, not only will it not work properly, but it may also give you slightly dirty (or salty) dishes. 

Also don’t put salt into other spots marked for normal detergent, dishwasher cleaner or rinse aid, as this could easily break your appliance.

How much salt to put in dishwasher 

Different dishwashers will have differently sized units that take various amounts of salt, so there’s no precise measurement for this process.

Pour salt into the unit until it’s completely filled up, but not over the waterline.

Since you also have water in the unit, you’re creating a saltwater brine.

What does dishwasher salt do and why do dishwashers need it? 

The sodium in dishwasher salt combines with resin balls in your dishwasher’s workings to absorb the hardness in the water. 

The resin balls have a negative atomic charge and the dishwasher salt has a positive atomic charge.

The calcium and magnesium swap with the sodium ions, removing the ‘hardness’ and softening the water.

When the resin balls’ sodium stores are exhausted, adding dishwasher salt resets them, so they can absorb more.

This is important for two reasons:

  1. Dishwasher detergent works best in soft water, so salt can help you get better cleaning.
  2. Hard water can cause glasses to become cloudy over time, as well as clog up the insides of your dishwasher with limescale.

What does rinse aid do? 

Rinse aid, as the name suggests, will help water rinse off of your dishes and glasses.

It does this by reducing the surface tension of water, meaning it sticks to surfaces less easily and can drain and evaporate away. As such it also helps prevent water marks, to help your glasses come out sparkling and crystal clear.

Rinse aid is key if you want great drying results, although not as fundamental as dishwasher salt is for general upkeep. 

How to use rinse aid 

The rinse aid compartment is usually right next to the detergent compartment.

Depending on how many washes you do you might have to fill up the rinse aid every few weeks. When doing so it can sometimes have a maximum fill line, which you should not go over.

If you are using rinse aid but are still not happy with your dishwasher's drying you might have the option to turn up the rinse aid setting. This is often just a dial under the rinse aid lid that you can twist to a higher setting.

Dishwasher salt brands compared 

On the supermarket shelves, you'll be faced with little choice of dishwasher salt. There will likely just two options, Finish and the supermarket own-brand version.

The Finish salt could cost around £2 to £2.50 for 3kg. The own-brand salt could be cheaper, at £1.70 for 3kg in Tesco and £1.68 for 6kg in Asda.

If you look at the ingredients it will just say ‘salt’. So how you can tell which is better?

On a box of Finish dishwasher salt you’ll see ‘100% better protection’, but if you read further you’ll see this isn’t compared to other dishwasher salts; it’s only compared to not using salt at all.

In reality, they should all be the same. All of them will be 100% salt, with no added anti-caking agents.

The only difference could be in the size of the granules or the packaging. Some could come in a recyclable cardboard box, which is more sustainable than a plastic bag.

Dishwasher salt alternatives 

You could be tempted when you run-out to use table salt or Kosher salt. But you shouldn't.

It needs to be 100% NaCl, and cooking salts often have anti-caking agents to stop them from clumping together.

If the salt you use doesn't have anti-caking agents, such as calcium carbonate or magnesium carbonate, then you can use it. Although there would be little benefit as it would likely be more expensive than regular dishwasher salt.

We wouldn't recommend you use dishwasher salt for cooking, either. It doesn't have the same food and health safety regulations on it, so could contain trace amounts of toxins.

Do I need dishwasher salt in soft water area? 

It’s a good idea when your dishwasher is new, or if you’ve never topped up the salt, to do so.

You can adjust the water hardness setting in your dishwasher to be suitable for soft water.

You’re unlikely to have to top up the salt often, if ever, but it is a good failsafe in any case to avoid cloudy glasses.

Dishwasher salt composition 

Dishwasher salt is different from table salt in that it is pure, 100% NaCl, with no anti-caking agents.

It needs to be this because the sodium in dishwasher salt combines with resin balls in your dishwasher’s workings to absorb the hardness. The resin balls have a negative atomic charge and the dishwasher salt has a positive atomic charge.

The calcium and magnesium swap with the sodium ions, removing the ‘hardness’ and softening the water. So any anti-caking agents or magnesium salts will just defeat the purpose of removing magnesium.

Do you need dishwasher salt if you use all-in-one tablets? 

All-in-one tablets will have some salt in to help replenish the water softener, although we still recommend that you top up with dishwasher salt if you want the best results.

If you always use tablets you'll probably have to top up the salt less often, but it's still a good idea to.

Some dishwashers have an all-in-one tablet option, but we don't recommend you use it. This is because the salt in the tablet often isn't enough to protect dishes from cloudiness over many washes.

Dishwasher salt problems 

Is there too much salt in my dishwasher?

Whenever you're topping up the salt in your dishwasher you're aiming to make a saltwater brine, so you don't want to fill it up to the point that the water inside is overflowing.

If you've done this, then rinsing it with water can help make sure the softener doesn't get clogged with salt.

Why is the dishwasher salt reservoir full of water?

After the first time you use it, your softener unit should always have a bit of water in it. You won’t need to refill it. 

Why does the dishwasher keep asking for salt?

You could have to refill the salt every month if you live in a hard water area and run your dishwasher often.

If you think you're having to refill it too often then you could try lowering the water hardness setting on your dishwasher.

Instructions on how to do this should be in the manual.

SHARE THIS PAGE

Related products

See all dishwashers