Water hardness is measured in grains per gallon (gpg) or milligrams per liter (mg/L). One gpg equals 17.1 mg/L. Both units appear on test results, so it helps to know the conversion before you start.
Why Test Before Treating
Testing first tells you three things: whether you actually have a hard water problem, how severe it is, and what size softener makes sense if you need one. Buying a softener without testing is guessing. Hardness varies dramatically by region, and even within the same city, source water changes seasonally.
If you already have a softener, testing the output confirms it’s working. Softeners that look fine on the outside can quietly stop conditioning water, especially if the resin is fouled with iron.
The Four Ways to Test
Test strips are the right choice for most people. Dip a strip into a glass of cold water for 1-2 seconds, shake off the excess, and wait 30-60 seconds. Compare the color block to the chart on the package. Total time: under 2 minutes. Cost: $8-15 for 100 strips, which is a few years of quarterly testing. Accuracy is within 1-2 gpg, which is sufficient for any home treatment decision.
Liquid titration kits work by adding drops of a reagent to a water sample until the color changes from red to blue. You count the drops. Each drop equals a defined amount of hardness. Slightly more accurate than strips, around $10-20 per kit. Worth it if you’re troubleshooting a softener or want a more precise baseline number.
TDS meters are not hardness tests. A TDS meter reads all dissolved solids, not just hardness minerals. High TDS often correlates with hard water, but a well could have high TDS from sodium or other minerals and still test as soft. Don’t use a TDS meter for hardness.
Certified lab testing gives you the most accurate result and tells you exactly what’s contributing to the hardness reading. Most labs charge $20-50 for a basic mineral panel. This is the right choice if you’re sizing commercial equipment, buying a home with unknown water quality, or troubleshooting persistent scale despite having a softener.
How to Take a Good Sample
Run cold water for 30 seconds before collecting your sample. This flushes the standing water from the pipes, which may have a different mineral concentration than the main supply.
Collect from the cold tap, not the hot. Water heaters precipitate calcium carbonate out of the water as a byproduct of heating. Hot water will test lower than your actual incoming hardness because some minerals have already dropped out as scale.
Fill a clean glass mid-stream. Don’t test from the first trickle.
Reading the Results
| Range | Hardness | What You’ll Notice |
|---|---|---|
| 0-60 mg/L (0-3.5 gpg) | Soft | No visible effects |
| 61-120 mg/L (3.5-7 gpg) | Moderately hard | Minor spotting, slight scale |
| 121-180 mg/L (7-10.5 gpg) | Hard | Visible scale on fixtures, reduced soap lather |
| Above 180 mg/L (above 10.5 gpg) | Very hard | Heavy scale, appliance impact, notable spotting |
The EPA doesn’t set a Maximum Contaminant Level for hardness because it’s not a health issue. The effects are practical: scale buildup in water heaters (which reduces efficiency and lifespan), spots on dishes and glassware, reduced effectiveness of soap and detergent, and shortened appliance life.
Testing for Specific Purposes
Testing whether your softener works: Measure your incoming hard water (before the softener) and then measure at a post-softener tap. Softened output should read 0-1 gpg. If it’s reading 3+ gpg, the softener isn’t performing. Check the salt level first. If salt is adequate, consider a resin cleaning cycle.
Testing RO output: A working RO system removes 95%+ of hardness. If your source water is 15 gpg (255 mg/L), your RO output should be under 1 gpg (17 mg/L). A rising TDS or hardness reading from your RO output over time indicates membrane degradation.
After a whole-house filter install: Whole-house carbon or sediment filters don’t remove hardness. If you installed one and hardness testing shows no change, that’s expected. Only softeners and RO systems reduce hardness.
What Hardness Testing Can’t Tell You
A hardness test tells you calcium and magnesium levels. It says nothing about lead, arsenic, nitrates, bacteria, or other contaminants that actually affect health. Don’t substitute a hardness test for a comprehensive water test if you’re on a private well or have reason to suspect contamination.
If your hardness test comes back in the hard or very hard range, cross-reference it against your region. Phoenix, Las Vegas, Denver, San Antonio, and Indianapolis are all known for very hard water, often 15-25 gpg. If your test result matches what’s expected for your area, you have a confident baseline.
The test takes 60 seconds. There’s no good reason not to know your number before spending money on treatment equipment. Check out the best water test kits if you want a kit that covers hardness alongside other parameters.