If you are making soap at home, it is helpful to know how to test your product’s pH at various stages, especially if you plan to give or sell to others.

First of all, what is pH?  pH is defined as the “potential of Hydrogen” and is a measurement of free hydrogen.  Put simply, it measures how acidic (low pH) or basic/alkaline (high pH) a substance is.  Either extreme can cause skin irritations and/ or burns.

Here is a scale with a few commonly known substances:

Measuring the pH of soap is tricky without a pH electrode, results can vary due to interferences with surfactants. pH electrodes are worth the cost if you plan to sell your soap.  However, if you prefer the simplicity pH strips, I highly recommend the brand Macherey Nagel.  They are known “in the biz” to be robust enough to test soap accurately and also the color interferences of lotions are minimal compared to other brands.  Look for strips in the full 0-14 range or if they are just for soap, 7-14 will do.


There are several stages in the soap-making process that you may be curious about the pH of so I’ve included a picture to demonstrate the progression of pH through saponification.

When the fats and oils are initially combined with the lye and the soap thickens, the pH is quite high and the soap is still caustic.  If you are making soap by “cold process,” this is the pH of the soap that goes into the mold and is why it needs to be insulated for 24 hours and 4-6 weeks to cure.  The majority of saponification will happen in the first 2 weeks, but the soap will also remain soft and dissolve quickly if used before the 4-6 week cure time.  It will continue to get mild over the next several weeks and even months.

If you are making soap by “hot process,” you are heating the soap and causing it to saponify rapidly, within roughly 1 hour.   As you heat the soap, it reaches a stage where it looks like Vaseline and is runny again.  The pH is still high, but you can see that it’s dropping.  This stage takes about 1 hour to reach.

In the final stage the soap is more taffy-like in consistency and has a thicker texture.  The soap has finished and the pH should be < 10.  It is now time to put the heat processed soap in the mold.  The pH may continue to drop a little over time, but most likely will hover above 9.  Depending on the recipe, some handmade soaps may have a lower pH, but typically they are in the 8.5-10 range.

So, how exactly am I measuring the pH?

While the soap is still liquid, I make a 10% soap solution (1 g soap + 9 g water works).  Dip the pH strip and compare to the color chart.

Finished bar soap is most accurately tested by cutting the bar first, adding a few drops of water to the freshly cut piece and dipping the pH strip in the water on the soap.  You can do this to monitor the pH progress of curing soap and to be able to accurately determine when it’s safe to sell or give away (<pH 10) You can alternatively make a 10% solution but make sure that the soap is well dissolved before reading or you will get a falsely low pH reading.