Concentration? Percentage? How do I figure the water amount for my lye solution?

When it comes to cold process soapmaking, some things are absolutely necessary and rigid, like how much sodium hydroxide (NaOH) is needed for a given amount of oils. It’s a property of the oils and can’t be changed without affecting the outcome of the soap, sometimes in really bad ways.

Other things are much more flexible, like the amount of water that is used to make the lye solution. The water is only there to dissolve the NaOH. Using more or less may affect how the soap mixes up, but in the end the water evaporates out and the finished, cured soap is what’s left. In fact, many experienced soapmakers use lye solutions that are quite concentrated so that they don’t have to wait so long for their bars to get dried-out and hard. It’s a real time-saver!

If you’re ready to start using your lye solution at a higher concentration, I can help you figure out how much water you will need to get there. The math can be a bit confusing for some. But I have a little formula that should do the trick.

So first, let’s explain percentage strength of a solution.

When we talk about a solution being, for example, at a 25% concentration, we are saying that it is 25% solute (the thing you are making the solution out of) and 75% diluent (the thing that you are dissolving the solute in).

In our case NaOH is the solute and water is the diluent. So a 25% lye solution is 25% NaOH and 75% water. “Per cent” means “per 100”. So the total percentage always adds up to 100.

Want another way to think about it? Imagine you have a 25% lye solution in a bowl. If you have 100 gr of lye solution in that bowl, there is 25 gr of NaOH and 75 gr of water.

OK, got the concept? Now how do you get your lye solution to be the concentration you want?

Well, you could use a lye calculator that can figure it out. The one in Soapmaker 3 software will do the math for you. So will the Soap Calc online.

But what happens when you’re not near your computer? Or you just like doing the calculations yourself. (Yay for math geeks!)

I’ll show you how to work it out. Now don’t get scared off by the math. Just think of it as a simple ratio. All we have to figure out is how many parts of water do we need for the amount of NaOH in our recipe to get a given percentage.

Here’s the formula:

(100 minus “percentage you want”) divided by “percentage you want” = the multiplication factor that you use to calculate the water

Huh? Yeah. Let’s plug in some numbers.

Let’s keep using our 25% solution example. And let’s say that I need 72g of NaOH for the recipe that I’m going to make (I used a lye calculator to figure that out for me.) I need to know how much water to use to make my solution 25%.

Here’s that formula with the numbers plugged in:

(100 minus 25) divided by 25 = 75 divided by 25 = 3.00  <– that is the ratio of NaOH to water

So now we multiply the amount of NaOH times the ratio:

72g X 3.00 = 216g <– the amount of water you need

Mix 72g of NaOH and 216g of water and you get 288g of a lye solution that is at 25% concentration.

Easy, right?

It works for whatever concentration you want. Just make sure your ratio answer has at least 2 decimal places to be more accurate.

Now, I’ll show you what happens if I changed my mind and I want my solution to be 34% concentration. It’s going to be more concentrated, so I’m going to need less water. How much less? Let’s find out:

(100 minus 34) divided by 34 = 66 divided by 34 = 1.94

So, 72g X 1.94 = 140 gr of water, which is what you need to make a 34% solution. Wow! That’s quite a bit less water!

Remember to use the higher concentration of NaOH only when you are comfortable with soapmaking, and are familiar with the formula that you will be using. You don’t want to do this with a brand new fragrance for example, or you may end up reading about soap that seizes!

40 thoughts on “Concentration? Percentage? How do I figure the water amount for my lye solution?

  1. Ruth thank you for spelling this out so clearly! Now even I understand! To rely on soapcalc is so easy until you don’t have it and now I don’t have to it is an option!

  2. Hi Ruth, I attended my first HSMG conference last week and heard you speak on alternative liquids. Thanks so much for sharing your knowledge. You are a good speaker, I like how simple you make it. I’d like to try this method, does it shorten curing times at all? For a first timer how much of a solution would you suggest? I am currently using a 28% solution (if I did the math right).


    • Hi Naomi! I’m glad you enjoyed my talk!
      Yes, using less water to make your lye solution does mean that your soap will be dryer sooner. So it will probably cut a week or so from curing time, depending on where you live. For a beginner, 28% is a great concentration. Once you get comfortable with soapmaking, and are familiar with making a few batches with a given fragrance, then you can try 30% or 33%. The highest I’ve ever heard of using is 40%, but that was from some very experienced soapmakers!
      Good luck with yours!

  3. Ruth, I just wanted to give you a quick update. I tried using 34% lye solution and I’m thrilled with the results! Thanks so much again for sharing this information. I found my soap is drier much sooner than usual. I found I can cut 24 hours off my waiting time before cutting my soap. I used to wait until my soap is 4 weeks old to stamp it because it was so soft, but now I am able to cut that down to just 2 weeks. In addition to that I’ve cut one whole week off of my curing time, yah! I’ve discovered that I must work even quicker with certain FOs that are prone to seizing, though. I’ve made 15 small batches with this new formula and I’m making it a permanent change. Many thanks to you!

  4. Ruth thank u for yuors willingness .i tried to make with 28% concentration but i have not understand how much fatty acid used for 72 gm of NaOH ? And when i tried 500gm fatty the lather of soap is not enough so what shall i do for better lather formation thank u for yours time spending

  5. How can I make my bar soap harder and prevent it from sweating. I’m in Ghana can u assist me to get get titanium dioxide 1kg to buy? Can you help me get it through post since i can get here, by letting me know mode of payment?

    • Hi! Are you asking about cold process soap? Oils that help with making a harder bar are palm oil, cocoa butter and rice bran oil. Try including some of those in your recipe. The sweating comes from the glycerin in the soap. Depending on your environment and how humid the air is, it may be hard to stop that. Try storing the soap in as dry a place as you can. And I don’t sell supplies, but some of my favorite US suppliers are Bramble Berry, TKB Trading and Soap Making Resource. Good luck!

  6. Great article Ruth – I’m not sure what I was doing during maths at high school, but it wasn’t maths. My burning question is this. I’m upscaling the size of my batches so the slushy method of goat milk soap making is getting harder to manage. I want to make a very concentrated lye and then use goat milk to make the balance. I’ve always made 100% goat milk so I’m not keen on going to 50/50 style. I’d like to know how concentrated a lye solution I can make without the sodium hydroxide saturating the solution and precipitating.

    • Thanks, Melissa. I’m afraid that the highest concentration you can go with sodium hydroxide is 50% before it just won’t go into solution anymore. So that means 50% goat milk is the best you can do, unless you use evaporated goat milk. But I’m guessing that you have your own fresh goat milk, so that wouldn’t be very appealing to you. But I think 50% goat milk is still pretty wonderful! :)

  7. I have been making soap for 3 years using the water to oil % and have just learned about the lye concentration, dont know how i missed it before. I’ve been using 38% water to oils, so if i change to using the lye concentration method ( I understand how to work it out I’m not sure what concentration i should use. I have been playing around with soapcalc and notice that the amount of water changes with the oils used so it seems that this is a better method as its targeted more to the oils you are using but I’m still unsure what concentration to start at. I thought that 38% water to oils was a bit much but i noticed that if i did a 100% coconut oil soap using 30% lye concebtration the water works out to be 40.619 % to oils, so I guess I’m still a little confused.

    • Hi Kerrie,
      Funny: I’m just the opposite. I’ve never used the water to oil % to calculate the water, only lye concentration. The water amount changes with a change in oils because it’s based on the amount of sodium hydroxide. So if you change the oils, that will change the amount of sodium hydroxide you need, and that will change the amount of water you need if you are going to have the same concentration. I like to use 29% lye solution in general. It’s great when I’m using new fragrances (that might seize or accelerate trace) and gives me plenty of time to do swirling techniques. But for recipes that I am familiar with and don’t need to stay at a thin trace, you can go up to 30-33%. Experienced soapmakers even go up at high as 35-40%. The higher the concentration, the less water, and the sooner your bars will be dry (since they have less water that needs to evaporate out). Soapmaking can be very flexible, so just use what works for you!

  8. Thank you ruth, your article surely educated to me about lye, lye solution and water. But I still have question that is lye used in hot process? and is there any difference between hot process & melt and pour process?

    • I’m glad you found it helpful, Wajid. Yes, lye is still necessary for hot process soapmaking. The only difference between hot process and cold process is that you heat up the soap mixture before putting it in the mold. That way you speed up the process and saponification is complete before pouring. Melt and pour soap is ready-to-use soap that is meltable so that you can add color and scent to it. No lye is needed because it has already been turned into soap.

  9. Hi I am new here, my Q is as per your explanations in the earlier post, to make a 50% concentration of NaOH solution, i should add 500gm of pellets and 500 ml of water. Is this correct.
    I also need to know , if i heat this liquid to (1) 100 deg C or (2) 160 deg C what will happen.
    Will the concentration change due to loss of water vapour.
    What would be the max recommended temp that i could heat the Naoh of 50% conc too.
    I tried ot find answers all over the internet , but i have got more confused.

    • Rajiv, yes, to make a 50% solution you would mix equal amounts of water and sodium hydroxide. But there is no need to heat lye solution. Even if it gets cool, you can still use it as is. Lye solution does not need to be warm to work. I hope that helps answer your question!

  10. Thanks Ruth, What troubles me , if when i start with say 100 gm of NaOH pellets and 100 gm ( 100 Ml) of water, will i get 100 ml of lye or will the volume /wt increase.

    • Mass (or weight) never disappears. So if you mix 100gm NaOH with 100gm (remember to also weigh your water) of water, you will have a solution that weighs 200gm. Now, in reality some of that water will evaporate as the solution cools. A surprising amount actually. You can weigh it yourself after it cools and see the difference. I recommend when you are making a 50% solution to weigh your solution and replace the evaporated water with more fresh water. In other words, right before using it, weigh the solution and add enough water to bring it back up to 200gm.

  11. Thanks Ruth, this is very helpful.
    I plan to leach some SiO2 with NaOH, do I need to heat the reactants for them to react.Pls advise.

  12. Hi Ruth, is it still possible to make cold process soap with a 16% lye solution? E.g 160 gram of sodium hydroxide and 840 gram of distilled water? Thank you for your article, I tried calculating using soap calc and understand that a change in the lye concentration (e.g. 16%) does not affect the amount of actual lye computed to saponify oil and it only changes the amount of water ratio. But would a lye solution of 16% be too much water? Would it take many days to cure? now that the total soap weight is much larger with more than normal amount of water, would the soap cure and become much smaller than the mold? I am working on such a low concentration because I’m only able to get a lye solution maximum at 16% in my country. Appreciate your advice please.

    • Hi Raymond! While it IS possible to make soap with a solution that is 16%, I’ve never done that before. And my concern is that with so much water, it may take a lot of blending to get the mixture to trace, and it may be weeks before the soap is ready to cut. If you have no other choice, then maybe hot process might be a better technique for you. That way some of the water would evaporate off during the cooking stage, and the soap would be dry sooner. While I’ve used the hot process a few times, I’m no expert. If you are looking for instruction, The Nova Studio has a great class handout for sale. But I teach there, so I’m biased. ;)

  13. Thanks Ruth. I am a beginner in soap making and your article solved the problem of making lye solution . Is there any ratio of amount of oils to lye solution to be used?

  14. IN Arabia we grown up with decimal system
    a simpler way solving your example is:

    for 1st example:

    You stated 72g and 25% so:
    72 / 0.25 = 288 total volume
    288-72=216 water

    for the 2nd example:
    72/0.34=211.76 total volume
    211.76-72 =139.76 water

    good luck

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