Everything about water in soap making

Originally posted on 25 February 2018. Updated on 5 October 2019.

One of the most common questions I get asked here in New Zealand is “Do I need to use distilled water when I make soap?” To answer this, we need to take a closer look at the water you use. Where does your water come from? When rain water falls, it is mostly free from impurities, but as it makes its way through the soil and rocks, the water dissolves and picks up minerals, such as calcium and magnesium. Water containing minerals is called hard water, whereas rain water and treated water very low in dissolved minerals is called soft water.

In your home, hard water is the cause of lime scale and soap scum, which is when the minerals in the water bond with the soap forming a greasy film. Another effect that hard water has on soap is that it tends to reduce the quality and amount of lather in your soap.

In soap making, hard water can also affect the saponification process, i.e. when the dissolved minerals in the water react with the other chemicals, leading to inconsistencies  and poorer quality of your soaps. However the most common problem with hard water is the increased chance of getting DOS (= Dreaded Orange Spots) in your soaps, a term used in the soap making community to describe the localised oxidation (orange spots) of unsaturated soft oils, such as olive oil, sunflower oils and rice bran oil. These DOS not only affect the appearance of the soap, but will also cause the oxidised oils in the soap to go rancid.

glass of water

However, the good news is that most of the tap water here in New Zealand is rain water or comes from a rainwater reservoir. So we can, with ease of mind, use tap water to make soap. For those on bore water (water from underground aquifers), don’t despair! You can either use distilled water or you can add chelators to your soap. Distilled water is available from your supermarket or hardware store, and is fairly cheap.

To find out if you are on bore or reservoir water, check with your water supplier or local council!

If you want to use chelators to soften your water, the most common chelators used in soap making are citric acid, sodium citrate and EDTA (ethylenediaminetetraacetic acid). They work by binding the minerals, basically rendering them inactive, and thus improving the lather and effectiveness of the soap. Citric acid is probably the easiest to get hold of here in New Zealand, with the added bonus that it is also considered a natural additive. If you intend to use citric acid to soften your hard water, usage is 10 g of citric acid for every 200 to 300 ml of water (roughly the amount of water used in a 1000 g oil recipe). Mix the citric acid into the water before you add the caustic soda. However, because citric acid also neutralises, you will need to add extra caustic soda to compensate. For each 10 g of citric acid used, you will need to add an extra 6.24 g NaOH to your recipe.


Another dilemma many beginner soap makers have is should you weigh or measure your water? Going by volume, for example using a measuring jug, will never be as accurate as weighing using scales. On the other hand, water is used as a medium to dissolve the caustic soda, because if I were to add the caustic soda pellets or flakes directly to my oils and fats, nothing would happen. For the chemical reaction between the oil and caustic soda to occur, we need to use aqueous lye (caustic soda that has been dissolved in water).

Always remember to calculate water with respect to the amount of lye!

The amount of water you need, depends on the amount of caustic soda in your recipe. For the chemical reaction to happen, you will need equal amounts of caustic soda and water. However, this is a very strong solution, a 50% lye solution or also known as 1:1 lye solution. To slow down the chemical reaction and give you time to work with the soap, we add additional water. However, this extra water will not be used by the chemical reaction and will need to cure out again. Why the extra water gives us time, imagine the caustic soda molecules floating around in the water. The more water, the more diluted the lye solution is, and the more distance there will be between the individual caustic soda molecules. Adding a more diluted lye solution to the oils makes the caustic soda molecules swim further to the oil molecules, hence giving us more time to work with the soap. However, too much water can slow down the chemical reaction too much, even to the point that it may stop altogether. Too much water can also create these so-called glycerin rivers in soap. They’re completely harmless, but often an undesired look in a soap.

The rule of thumb is to use twice as much water as caustic soda (= 33% lye solution), and you can round it up or down. So for example, if your recipe calls for 114 g of caustic soda, you can. use 220 g or 230 g of water.

Don’t go lower than 1.5 times the amount of caustic soda (40% lye solution), because a strong lye solution like that will speed up the saponification significantly.

Also don’t add more than 3 times the amount of water (25% lye solution), because as mentioned earlier, you’ll start risking a reaction that is too slow and glycerin rivers. Extra water will extend the curing time, because all that water has to evaporate again. It can also make the soap softer and stickier, and take longer to unmould, but you can counter that by adding salt or sodium lactate (1 teaspoon per 500 g of soap).

How to calculate your water:

25% lye solution
weight of caustic soda (g) x 3 = water (g)

33% lye solution
weight of caustic soda (g) x 2 = water (g)

40% lye solution
weight of caustic soda (g) x 1.5 = water (g)

Note: 1 gram of water equals 1 ml water

Using less water than your 2:1 lye solution (33% lye solution) is also known as water discount. You use less water in recipes where you use a high proportion of slow moving oils, such as olive oil, or where the chemical reaction is too slow, or when using fragrances which slow down the chemical reaction. You may also add a water discount when you want to prevent glycerin rivers, or when you are adding other liquid ingredients or fresh ingredients.

On the other hand, if you have a recipe that moves too fast and you want to slow it down, you can add more water. For example, when you want to do a swirl design and need more time. You also add extra water when using certain ingredients, such as clays and flours. Some ingredients heat up the process and extra water can also help reduce temperature and prevent overheating.

Unfortunately I see many soap makers using a set amount of water, regardless of how much caustic soda is used. For example, 300 ml of water per 1000 g of oils. This is not only inaccurate, you also risk very variable results in soap making.

Not so long ago, I was contacted by someone who asked me why her soap wasn’t thickening, even after two weeeks! After a short discussion, I found out that she always uses the same amount of water, namely 350 g water per 1000 g of oils. For her normal recipe, this would have been ok apparently. But because she was making a pure olive oil soap, this amount was way too high, and the chemical reaction had stopped completely.

To understand how this can impact soap making, I’ll explain it with two different soaps: a pure olive oil soap and a pure coconut oil soap and both with 5% superfat.

A 100% coconut oil soap with a 5% SF needs 169 g of caustic soda.
A 100% olive oil soap with a 5% SF needs 127 g of caustic soda.

For the caustic soda to work in the chemical reaction, it will need an equal amount of water. For the coconut oil, this would be 169 g of water and for the olive oil this would be 127 g of water.

the coconut oil soap will have 350 – 169 = 181 g of water left, and
the olive oil soap will have 350 – 127 = 223 g of water left.

This might not seem like a huge amount, but in the case of the olive oil soap, it was able to stop the chemical reaction. You’ll see it better if we calculate the water amount with respect to the caustic soda.

The correct amounts would have been twice the amount of caustic soda. For the coconut oil soap, the water amount would have been 2x 169 g = 338 g of water. For the olive oil soap the amount of water would be 2x 127 g = 254 g.

So 350 g of water is slightly more than twice the amount of caustic soda in the coconut oil soap, and would work fine. But for the olive oil soap it’s nearly 100 g extra water, which diluted the lye/oil mixture to the extend that it slowed down the reaction so much that it had stopped. Or if you like to imagine the caustic soda and oil molecules swimming in the soap soup: the distance between them was too great for them to find each other, and so they just stopped swimming.

In all my workshops, I always keep telling the students that water is only there for the caustic soda, and oils have nothing to do with the water.

Never calculate your water as a percentage of the oils!

To understand and see the effects different strengths of lye solutions can have on soap, check out this blog post by Auntie Clara, who created a stunning Ghost Swirl soap by using only different strengths of lye solutions. And while you’re there, have a browse through her soap gallery. Her soaps are absolutely amazing!

Lastly, a friendly word of caution: Adding a water discount will increase the strength of your lye solution, making your lye more potent and more dangerous! Be careful when working with lye. Always wear protective gear, such as safety goggles and disposable gloves to protect your eyes and skin. Stay safe!


  1. I have 500gof caustic soda, how many liters of water will i use to dissolve it. How many liters of oil will i use to make the soap

    • That depends on many things. For your first question, I like using a 2:1 solution (33.3% solution), which is one part sodium hydroxide to two parts water. So if you have 500 grams of caustic soda, I would dissolve it in 1000 grams of distilled water. Please note that if you plan on doing this, use high grade alkaline and heat proof bottles, such as polypropylene PP plastic, and that you label them clearly as a caustic and dangerous substance and store them out of reach of children. To your second question, that I cannot answer, because it depends on the soap recipe that you are using. Each oil is different and requires different amounts of sodium hydroxide to turn that particular oil into soap. But as a rough guideline, you use about 130-140 grams of sodium hydroxide for 1000 grams of oils. If you have 500 grams, that will make about 4.5 kg of soap. Also note everything is weighed (in grams or ounces) and not measure in volume. Good luck!

  2. What about when you’re done making soap and you wash all your soap making dishes? Will washing them with hard tap water leave residue, causing DOS in the next batch of soap you make??

    • No, you’ll be fine. Usually your dishes will be dry before you use them again, and even if there is a little hard water left, it’s only drops…

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