Hard Water and Handmade Soap: Lather, Soap Scum, and What Actually Helps

If your handmade soap produces beautiful fluffy lather in one house and seems downright stingy with the bubbles somewhere else, your recipe may not be the problem at all.

It might be the water.

Hard water can affect how traditional soap lathers, how much soap you need to use, what gets left behind on your skin and shower, and even how you think a particular soap recipe performs. That makes water hardness one of those sneaky little variables in soapmaking that we don’t always think about until something suddenly seems different.

And if you sell soap, it gets even more interesting because your customer may be using your soap in completely different water than you are.

So let’s talk about what hard water actually is, why it behaves the way it does with handmade soap, and what you can realistically do about it.

What is hard water?

Hard water is simply water that contains relatively high levels of dissolved minerals, primarily calcium and magnesium. Water picks up these minerals as it moves through soil and rock before eventually reaching your tap.

Water hardness is commonly reported as milligrams per liter, or mg/L, of calcium carbonate equivalent. The U.S. Geological Survey generally classifies water this way:

• 0–60 mg/L: soft
• 61–120 mg/L: moderately hard
• 121–180 mg/L: hard
• More than 180 mg/L: very hard

If you live in an area with very hard water, you probably already know it even if you’ve never looked at a water report. You may see mineral deposits around faucets and showerheads, spots on glassware, buildup in kettles or appliances, and that lovely film on the shower that seems to reappear approximately five minutes after you cleaned it.

I live in Tucson, Arizona, so hard water and I are already very well acquainted. 😂

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Why does hard water affect handmade soap?

Traditional soap is made from the salts of fatty acids. In bar soap, those are primarily sodium fatty acid salts created when fats and oils react with sodium hydroxide.

When soap enters soft water, those soap molecules can dissolve and do all the things we want them to do: help surround oils and dirt, disperse them into the water, and form lather.

Hard water introduces calcium and magnesium ions into that equation.

Those minerals can react with the soap molecules and form insoluble calcium and magnesium soaps. Because those new compounds don’t readily dissolve in water, they precipitate out of the solution.

And voilà: soap scum.

The American Chemical Society demonstrates this beautifully using magnesium salts and soap. Once the magnesium reacts with the soap, the resulting material doesn’t dissolve well in water and is no longer available to form bubbles.

So when we say that hard water “doesn’t work well with soap,” there is actual chemistry happening. It isn’t simply a matter of personal preference or how the water feels.

Why does soap lather less in hard water?

This is probably the thing soapmakers notice first.

Some of the soap that would normally be available for cleaning and lathering gets used up reacting with calcium and magnesium instead. Until enough of those minerals have reacted, less soap remains available to form the films that create bubbles.

That’s why you may need more soap to produce the same amount of lather in hard water than you would in soft water. It’s also why comparing the lather of two recipes without considering the water can be misleading.

A bar that gives me perfectly respectable bubbles here in Tucson might explode into lather when someone uses it somewhere with softer water. Likewise, a recipe developed in soft water may seem surprisingly underwhelming when someone tries it in very hard water.

The recipe didn’t suddenly change. The environment in which we’re asking that soap to perform did.

Does less lather mean the soap isn’t cleaning?

Not necessarily.

Lather and cleaning are related, but bubbles are not a direct measurement of how much cleaning is happening. We humans just happen to really enjoy bubbles and tend to associate more lather with a more effective cleanser.

Hard water does make traditional soap less efficient because some of the soap is lost to those insoluble reactions with calcium and magnesium. That means you may need to use more soap to accomplish the same job. But a bar that gives you less dramatic lather in hard water isn’t automatically a bad soap.

This is one reason I’m cautious about changing an otherwise good formulation simply because somebody says, “It doesn’t bubble enough.” My first question would be: What kind of water are you using it in?

Why does hard water cause soap scum?

Soap scum is essentially the visible evidence of the reaction we’ve just been talking about. Calcium and magnesium in the water react with traditional soap and create insoluble compounds. Those deposits can then combine with skin oils, dirt, hair products, and whatever else happens to be hanging around your tub or shower.

Eventually you get that familiar white, gray, or cloudy film.

If soap scum is the problem you’re actually trying to solve, I have a whole separate guide on what soap scum is and how to clean it. I separated that topic from this one because understanding hard water and cleaning the resulting mess are two slightly different problems.

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Does hard water leave soap on your skin?

This gets a little tricky because people often judge whether they are “rinsed clean” entirely by how their skin feels.

Hard water tends to make skin feel less slippery because calcium and magnesium react with the soap. That can create the familiar squeaky-clean sensation many of us grew up thinking meant all the soap was gone. But some insoluble soap residue can still remain on the skin.

Soft water behaves differently. Because there is less calcium and magnesium available to react with the soap, the soap continues to remain soluble and slippery longer. That can make people feel as though they simply cannot rinse it away.

The U.S. Geological Survey notes that soap lathers more easily in soft water, so people accustomed to hard water may actually use more soap than they need when washing with softened water. More soap naturally requires more rinsing.

So that slippery feeling in soft water does not necessarily mean your soap is defective or permanently glued to your body. It may simply mean that your usual visual and tactile cues are different.

Should soapmakers use distilled water instead of tap water?

For making soap, yes, I strongly prefer distilled water. That recommendation is different from the problem of what kind of water someone will eventually use to wash with the finished bar.

Your lye solution is part of the actual formulation, so using distilled water removes a variable. Tap water may contain calcium, magnesium, iron, copper, chlorine, and other dissolved substances, and the exact composition varies tremendously from one location to another. I don’t see much upside in bringing all of those unknowns into your soap pot when distilled water is inexpensive and readily available.

Hard tap water and trace metals may also contribute to problems such as oxidation and Dreaded Orange Spots, which is another reason I recommend starting your formulation with distilled water rather than whatever happens to come out of your faucet.

But there’s an important distinction here:
Making your soap with distilled water does not make the finished soap immune to hard water later.

Once your customer takes that bar into a shower full of calcium and magnesium, the same hard-water chemistry happens.

Can you formulate soap to work better in hard water?

To a point.

There are formulation choices that may help a bar perform more pleasantly in hard water, but there is no soap recipe that can magically remove all of the calcium and magnesium from someone’s water supply.

The biggest strategies are:

• Use a chelator, such as sodium citrate, to bind some of the hard-water minerals.
• Formulate a soap that naturally produces plentiful lather so some loss of lather is less noticeable.
• Make sure the soap is well cured and performs well before you start trying to solve a water problem with recipe changes.
• Test the finished soap in the type of water your customers are actually likely to use.

The chelator piece is where citric acid usually enters the conversation.

Does citric acid help soap perform in hard water?

Yes, but with a giant little asterisk.

When you add citric acid to a bar soap formulation and properly compensate for the sodium hydroxide it consumes, the citric acid reacts with NaOH and becomes sodium citrate. Sodium citrate is a chelator. It can bind with some calcium and magnesium ions, leaving fewer of those ions available to react with the soap. That can reduce some soap scum and may allow more of the soap to remain available for lather.

What it does not do is turn hard water into soft water.

I’ve personally tested citric acid for soap-scum reduction and wasn’t impressed enough with the results to make it a standard addition to all of my soap. That doesn’t mean it wasn’t chemically doing anything. My water was simply hard enough that I still had plenty of soap scum.

I go into the chemistry, usage rates, NaOH adjustment, sodium citrate, lather, and all of my other nitpicky details in Citric Acid in Soap: What Does It Actually Do?.

Should you change your oils to get more lather in hard water?

Maybe, but I wouldn’t make that my first move.

A recipe containing oils that contribute abundant bubbly lather may appear to perform better in hard water simply because it starts with more lather to lose. Coconut oil is the obvious example. But increasing coconut oil also changes other qualities of the finished soap, including cleansing and the overall fatty acid profile. I wouldn’t start changing a balanced recipe just to chase bubbles before confirming that the actual issue is formulation rather than water hardness.

If the soap performs beautifully in distilled or soft water but poorly in hard water, the water has already given you a pretty big clue. That doesn’t mean you can’t formulate with your local conditions in mind. It just means I’d want to know which problem I’m solving before changing the recipe.

What about sugar for more bubbles?

Sugar can increase the amount of lather a soap produces, and that can make a noticeable difference in a bubbly-lather formulation.

But sugar and citrate are addressing two different things. Sugar helps the soap produce more lather. Citrate helps reduce some of the interference from hard-water minerals.

Adding sugar doesn’t prevent calcium and magnesium from reacting with your soap. You may simply be starting with more potential bubbles. Those two strategies can certainly exist in the same recipe, but I wouldn’t describe them as interchangeable.

Will salt soften hard water in soap?

No—and this is a particularly confusing one because water softeners use salt.

Adding regular sodium chloride to your soap does not function like a household ion-exchange water softener.

A whole-house water softener contains resin that exchanges calcium and magnesium ions in the water for sodium or potassium ions. The salt you put into the softener is used to regenerate that resin. Simply adding sodium chloride to handmade soap does not recreate that process.

Salt can absolutely change the physical qualities of a soap bar, including hardness and lather, which is why we use it intentionally in salt bars and other formulations. But it is not a substitute for an actual water-softening system.

Will a water softener make handmade soap work better?

Absolutely. If calcium and magnesium are removed or exchanged before the water ever reaches your shower, there are fewer hard-water minerals available to react with your soap.

That generally means:

• Soap lathers more easily.
• You need less soap.
• Less soap scum forms.
• Mineral scale around fixtures is reduced.

People switching from hard water to softened water sometimes initially dislike the slippery feeling because they’re accustomed to the way soap behaves in hard water. They may also continue using the same large amount of soap they needed before and wind up using far too much.

If you install a water softener, give yourself a little time to relearn how much soap you actually need.

Is hard water bad for handmade soap?

I wouldn’t describe hard water itself as “bad for soap.” I’d say traditional soap and hard water have an inconvenient chemical relationship.

Hard water won’t ruin a properly made bar sitting on your soap dish. The issue begins once the soap dissolves into the water and encounters calcium and magnesium.

Hard water can:

• Reduce apparent lather.
• Increase the amount of soap you need to use.
• Create soap scum.
• Leave insoluble residue on surfaces.
• Make the same soap seem to perform differently from one location to another.

That final point is especially important for soapmakers.

Your customer’s water may be completely different from yours

This may be the most useful part of this entire conversation if you sell soap.

We spend enormous amounts of time testing formulations, adjusting superfat, comparing oils, measuring cure times, and analyzing lather. Then we send that bar hundreds or thousands of miles away and forget that the customer may be washing with completely different water.

Someone with soft water may tell you your bar creates mountains of bubbles.

Someone else may tell you the exact same batch barely lathers.

Before assuming something is wrong with the soap, ask where they live and whether they have hard water.

You don’t need to interrogate customers about municipal water chemistry every time someone buys a bar. 😂 But water hardness is worth remembering when you get conflicting feedback about lather, residue, or soap scum.

How do you know if you have hard water?

The most reliable answer is to actually check.

If you’re on municipal water, your water provider may publish a water-quality report that includes hardness. You can also purchase an inexpensive water-hardness test strip or test kit.

You may already have plenty of clues around the house, including:

• White mineral crust around faucets or showerheads
• Spots or cloudy film on glassware
• Scale in kettles or coffee makers
• Persistent soap scum in the shower
• Soap that seems reluctant to lather
• Needing more soap than expected for washing

Those signs don’t tell you an exact hardness number, but taken together they’re pretty persuasive.

If you’re doing a serious soap experiment, though, I’d much rather know the number.

A very simple hard-water soap experiment

Take one cured bar from the same batch and test it in two different water samples:

• Distilled or known soft water
• Your regular tap water

Use the same amount of soap, the same amount of water, the same temperature, and approximately the same amount of agitation.

Then compare:

• How quickly lather begins
• How much lather develops
• Bubble size and stability
• Cloudiness or precipitate in the water
• Residue left on the container
• How much soap you needed to get the result

You can make this as scientific or delightfully kitchen-counter-ish as you want. Either way, actually seeing the difference makes the chemistry considerably more memorable than simply reading that “hard water reduces lather.”

And if you try it, I absolutely want to hear what happens.

What actually helps handmade soap in hard water?

If you’ve made it this far, here’s the practical version.

• For making your soap: Use distilled water so you aren’t adding unnecessary minerals or metals to the formulation itself.
• For soap scum: A chelator such as sodium citrate may reduce some of the mineral interaction, but don’t expect it to completely overcome very hard water.
• For lather: Make sure you’re starting with a well-balanced formulation. Ingredients that increase lather may make the effects of hard water less noticeable, but they don’t remove the minerals causing the problem.
• For the biggest improvement: Soften the actual water. Formulation tweaks can help around the edges, but removing calcium and magnesium before they ever meet the soap addresses the problem at its source.

And perhaps most importantly, don’t automatically blame the recipe. Sometimes a perfectly lovely soap is just having a really lousy relationship with somebody’s water.

Frequently Asked Questions About Hard Water and Handmade Soap

Continue Learning with Soapy Friends

Water is one of those ingredients we tend to take for granted because it looks so simple. But once you start paying attention to what is dissolved in it, suddenly soap scum, lather, citrate, DOS, and even customer feedback start making a whole lot more sense.

If you’d like to continue learning and soaping along with me, here are a few great next steps:

• Soap Scum 101: What Is Soap Scum and How to Clean It — See what those hard-water minerals create when they react with your handmade soap and what you can do about the resulting buildup.
• Citric Acid in Soap: What Does It Actually Do? — Learn how citric acid becomes sodium citrate, why soapmakers use it as a chelator, and why you need to adjust your sodium hydroxide when using it.
• DOS Soap: Dealing with Dreaded Orange Spots in Handmade Soap — Explore the bigger picture behind rancidity, including why I recommend distilled water when making your soap.
• Get the Free Soapy Friends Starter Kit — Get free soapmaking classes, reference charts, worksheets, labels, and tools to help you understand not just what to do, but why your soap behaves the way it does.