Soft water is water with very low levels of dissolved minerals, specifically calcium and magnesium. Water is generally considered soft when it contains less than 75 milligrams per liter (mg/L) of these minerals, while water above 150 mg/L is classified as hard. The difference affects everything from how your soap lathers to what builds up inside your pipes.
What Makes Water Soft or Hard
All water picks up minerals as it travels through soil and rock. Water that flows over or through limestone, chalk, and other calcium-rich geology absorbs more calcium and magnesium, making it harder. Water that passes through granite, sandstone, or other mineral-poor rock stays soft. This is why high mountain lakes and streams tend to have naturally soft water, while groundwater in limestone regions can be very hard.
The threshold most often used breaks down like this:
- Soft: below 75 mg/L
- Moderately hard: 76 to 150 mg/L
- Hard: 151 to 300 mg/L
- Very hard: above 300 mg/L
WebMD places the soft water cutoff even lower, at under 17 parts per million (essentially the same as mg/L). In practice, any water below about 75 mg/L will behave like soft water in your home. The water industry generally recommends keeping hardness below 120 to 170 mg/L, or roughly 7 to 10 grains per gallon.
How Water Softeners Work
If your water isn’t naturally soft, the most common way to soften it is with an ion exchange system. These devices contain a bed of resin beads loaded with sodium ions. As hard water flows through, calcium and magnesium ions stick to the resin and sodium ions are released into the water in their place. The result is water that’s free of the minerals that cause hardness but contains more sodium than it did before.
The sodium increase is roughly 50 mg/L for every 100 mg/L of hardness removed. So if your water starts at 200 mg/L of hardness, you can expect roughly 100 mg/L of sodium to be added. One study of households using softeners on well water found an average sodium concentration of 278 mg/L in the softened water, compared to 110 mg/L in untreated municipal water. For most people, this added sodium is insignificant relative to what they consume through food. For anyone on a severely sodium-restricted diet, it’s worth considering a potassium-based softener or keeping an unsoftened tap for drinking water.
Why Soft Water Feels Different
The first thing most people notice about soft water is the slippery feeling when washing their hands or showering. According to the U.S. Geological Survey, this happens because soap lathers much more easily in soft water, so a thin film of soap stays on your skin longer during rinsing. In hard water, calcium and magnesium react with soap to form a sticky residue called soap scum. That scum actually makes you feel like the soap rinsed off faster, but it hasn’t. It’s just been converted into a film you can’t feel as easily. Soft water, by contrast, lets you rinse away actual soap rather than creating scum, which is why it feels slick even though your skin ends up cleaner.
Effects on Cleaning and Energy Use
Soft water’s biggest practical advantage is how much less detergent you need. Independent testing has shown that switching to softened water lets you cut laundry and dish detergent use by 50 percent or more while getting the same or better cleaning results. In areas with very hard water (around 25 grains per gallon), the savings can reach nearly 70 percent less detergent compared to washing in unsoftened water.
The energy savings are notable too. Because soft water cleans effectively at lower temperatures, you can wash laundry in cold water (around 60°F) instead of hot water (100°F) and still achieve comparable stain removal and whiteness. Over time, soft water also prevents the mineral scale that builds up inside water heaters, dishwashers, and washing machines, which helps those appliances run more efficiently and last longer.
Soft Water and Your Plumbing
Hard water is notorious for leaving mineral deposits inside pipes, faucets, and appliances. Soft water eliminates that problem entirely. However, naturally soft water with low alkalinity and a lower pH can actually be more corrosive to certain plumbing materials, particularly copper pipes. Research published in Water Research found that soft, low-alkalinity water sustains higher levels of copper corrosion, especially at lower pH levels and higher water temperatures. The corrosion is worst when water temperatures exceed about 113°F (45°C) and becomes most severe near 170°F (77°C).
This is primarily a concern with naturally soft water sources rather than water treated by a home softener, since softeners don’t typically change the water’s pH or alkalinity. If you live in an area with naturally soft, acidic water and have copper plumbing, a neutralizing filter can raise the pH enough to reduce corrosion.
Soft Water, Skin, and Eczema
There are several reasons soft water may be gentler on skin. Hard water increases the amount of detergent residue (like sodium lauryl sulfate, a common ingredient in body washes) that deposits on the skin’s surface. It also raises the skin’s surface pH and can alter calcium signaling in the outer layers of skin, both of which may weaken the skin’s natural barrier function.
Despite the plausible mechanisms, clinical trials haven’t confirmed that water softeners improve eczema. A major multicenter trial called SWET found no overall benefit in eczema severity for children with moderate to severe cases who used softened water. A smaller pilot trial of children with mild to moderate eczema also found no significant difference after six weeks, though itching did improve. No published studies have yet tested whether softening water from birth could prevent eczema from developing in the first place. So while soft water reduces soap residue on skin and may feel more comfortable, it’s not a proven treatment for skin conditions.
Is Soft Water Safe to Drink
Naturally soft water is perfectly safe to drink. Softened water is also safe for the vast majority of people, though it does contain extra sodium. A typical glass of softened water might contribute a small fraction of your daily sodium intake, far less than a slice of bread. The concern is limited to people following very strict low-sodium diets for conditions like heart failure or kidney disease. In those cases, many households simply bypass the softener for the kitchen cold water tap, keeping softened water for bathing, laundry, and dishes while drinking unsoftened water.
One thing soft water lacks is the calcium and magnesium that hard water provides. These minerals do contribute to your daily intake, but the amounts are small compared to what you get from food. Drinking soft water won’t create a mineral deficiency if your diet is reasonably balanced.