How to Measure the Hardness of Water
How to test water hardness & how to measure it using test strips, a water hardness soap test solution and other methods.

This article explains how to measure water hardness or grains of hardness: a measure of water mineral content. We provide a table of water hardness measurements and we discuss different methods for measuring water hardness including a simple soap test.

Electronic instruments can also measure water hardness but are a cost that most homeowners can eschew given the very low, even free costs of other water hardness test methods.

We give references on where to buy water low-cost but quite effective water hardness tests and we describe how various water hardness test kits are used and how to choose among them.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to Measure Water Hardness: how many grains of hardness is found in your water?

Definition of hard water

As we explain in more detail at WATER SOFTENERS & CONDITIONERS, Water "hardness" refers to the level of unwanted minerals, principally calcium and magnesium, found in your water supply. Hard water is an aesthetic issue (poor lathering, spots on dishes) and a mechanical issue (clogged pipes, expensive repairs, poor washing machine performance) but it is not a health concern.

Photo at left: lime scale fragments collected from a water heater. 

Hard water is found only in certain neighborhoods and usually only on private water supplies from a local well or wells.

At DETECT HARD WATER we describe how to visually detect that your water is high in mineral content or "hard".

How is water hardness measured?

Water hardness is measured in "grains" per gallon, in milligrams of calcium (Ca) per liter, or water hardness may be expressed in ppm - parts per million, and also in German Degrees of Hardness (dH) [which we drop here as this is for fish tanks].

You can take a water sample to a water test lab to have its hardness measured, but it is also trivially easy to measure the hardness of water yourself, as we explain here.

Readers whose water is acidic or corrosive rather than "hard" and readers needing to know about the Langelier Saturation Index or LSI should

see CORROSIVITY or ACIDITY of WATER.

How to convert among different water hardness measures:

Water with hardness of 25 ppm = 25 mg. of hardness-causing minerals per liter of water.

Table of Degrees of Water Hardness

Soft water

0-17.1 mg/L of minerals

Slightly hard water

16.1-60 mg/L of minerals

Moderately hard water

61-120 mg/L of minerals

Hard water

121-180 mg/L of minerals

Very hard water

more than 180 mg/L of minerals

Notes to the table above

- adapted from web search Wikipedia 01/31/2011

You can compare the water hardness range above with the water hardness test sensitivity of the various water hardness tests listed later in this article.

We repeat this table with more details, below where we show other measures of water hardness..

How to Use A Simple Soap Test to Measure the Grains of Hardness of the Water Supply

You will need:

Water hardness can be easily measured using a simple soap test kit that will measure in "grains of hardness" (a little bottle with a line marked on it which you fill to the line with water, add a drop of soap, and shake to look for suds. More drops of soap - more degrees of hardness).

The very crude do-it-yourself water hardness test method that some folks try is just adding dish soap to 12-oz of tap water (the left two columns in our table below).

In our OPINION this is a very crude test and you'd be better off obtaining a free or low-cost but more accurate water hardness test from one of the sources listed below at Water Test Kits & Guide

Water Hardness Measurements - How to Use the Home Soap Test Method for Water Hardness

Test Water Volume - crude test 

Soap drops - crude test

Grains/gallon GPG of Water Hardness

ppm mg/L

Water hardness in parts per million in milligrams per liter

Softness of the Water

12 oz

10

0 - 1

0-17.1

Very Soft

 

20

1 - 3.5

17.1 - 60

Slightly Hard

 

30

3.5 07/13/2010 - 07.0

60 - 120

Medium Hard

 

40

7.0 - 10.5

120 - 180

Hard

 

50

10.5 & higher

180 & over

Very Hard

Notes to the table above

NOTE: 1 German degree of hardness dH = 17.5 ppm. For fish tanks, 200-300 ppm is considered "hard", 300-500 ppm "very hard" and over 500 ppm "extremely hard" water.

Water Hardness Test Kit - Test Strips

Using simple plastic test strips that carry a chemically-treated pad is an easy and quick way to measure water hardness and / or to confirm that a water softener is working correctly.

LaMotte [cited in detail below] provides a helpful comparison between water hardness measurement using instruments and water hardness measurement using test strips:

Instrument Measurement of Water Hardness: A colorimeter passes a white light beam through an optical filter which transmits only one particular color or band of wavelengths of light to the photodetector, where it is measured.  The difference in the amount of colored light transmitted by a colorless sample (blank), and the amount of colored light transmitted by a colored sample, is a measurement of the amount of colored light absorbed by the sample. In most colorimetric tests, including hardness, the amount of colored light absorbed is directly proportional to the concentration and is reported by the meter.

Test Strip Measurement of Water Hardness:Test strips are typically produced from plastic, with chemically impregnated pads on the end. These pads are designed to react with specific ions and produce a specific color change.  Once a test strip is reacted and a color is developed, the strip is then compared to a printed color chart. The color chart is specifically designed to represent color reactions at various concentrations. Matching the strip to the closest color match produces a concentration reading. - retrieved 12/11/2014, original source:

A step-by-step water hardness test procedure using these test strips and hardness test results are given in detail

at WATER HARDNESS TEST San Miguel de Allende

where we illustrate using the JNW Direct water hardness test kit strips (Amazon.com) to test the hardness level of the water supply in San Miguel de Allende, Guanajuato Mexico (Col. San Antonio) in March of 2019.

What's the difference between using a total water hardness test strip kit and using the "true calcium" liquid reagent drop count titration method?

Ø  Water hardness or other water test kits are used in a very wide variety of applications beyond simply checking a drinking water supply.

Ø  Some other uses include balancing the water hardness of pools and spas, testing the condensate in a steam boiler system or in water operated cooling towers, and checking the proper operation of water softeners.

Ø  A difference between this calcium test using the dropcount titration method and total hardness test kits that use a test strip is that the test strips only measure total water hardness - which is perfectly fine for examining a residential water supply.

Ø  But for measuring water hardness when maintaining a pool or spa, you need to measure calcium hardness specifically. That measurement requires the drop count titration method described next. Quoting from Taylor who explains the difference in exquisite detail:  ... whereas test strips for total hardness have just four or five color blocks to cover a tremendously wide range—from 0 to 1,000 parts per million (ppm) with color blocks for 0, 100, 250, 500, 1,000, for instance—a drop test will allow you to approach the true concentration of calcium hardness in 10-ppm increments.

Ø  This means if you're faced with wanting to balance water in a spa by increasing the calcium hardness from 60 ppm to 150 ppm, the strip can't help but the drop test can.9]

Sources of Water Test Kits & Guide to Using a Water Hardness Test Kit

Water hardness test kits may use water hardness test tablets or paper test strips.

When mixed with (a test tablet) or wet by (a test strip such as those shown at left) water of a specified volume (fill the test bottle with the specified amount dictated by the test kit instructions or to the line marked on the test bottle), the water in the bottle plus test tablet changes color (green or red, for example) or the test strip changes color (on a scale, for example) that is compared to a standard provided with the kit. Other simple test kits provide a small glass bottle with a line marked on it and intended for use with dishwasher soap. One drop dish soap at a time. More soap drops = harder water.

We bought our Total Water Hardness Test Kit at Sears where water softeners were sold. Or the hardness of your water may be measured at no charge if you ask the local water treatment company to take a look. A typical water hardness test kit that uses water strips provides with each strip a color chart. Match the color on the strip (after you wet it with your test water) against the color chart to see the hardness of your water sample, measured in ppm or gains.

A typical water hardness test kit strip measures hardness from 0-1000 ppm or from 0-59 grains of hardness.

Potable Water Hardness Test Kits: sources, features, costs: soap, strips, electronic hardness testers are listed here.

Here we describe different methods & prices for building water supply hardness testing and we include description of the different tests, their technology, and their application.

[These water tests are available at Amazon.com through links given here, from the companies and producers listed here, and from other suppliers]

Watch out: a general search for "water test kit" or "water hardness test kit" will list kits and tests that may not match your application. Don't order an aquarium test kit or a swimming pool test kit if your need is to test building drinking water or potable water supply. The test criteria, methods, and standards may be quite different.

[Disclosure: you can buy these test kits from many online sources including Amazon.com. Amazon pays us a pittance per item if purchased via these links.

Here are our WEBSITE VISITOR PRIVACY POLICY (we do not collect any information about individual website visitors)

Hardness in water was originally defined as a measure of the water's ability to precipitate soap. In current practice it is considered to be the sum of the calcium and magnesium ions in water, expressed as milligrams per liter (mg/L) or parts per million (ppm) as calcium carbonate.

The modern way to test for hardness is by titration with EDTA (ethylenediaminetetraacetic acid), an organic chelating agent which reacts with both calcium and magnesium ions. When all calcium and magnesium ions are complexed, the excess EDTA reacts with an indicator to give a characteristic blue color, which is the endpoint.[9]

For potable water testing Taylor also provides tests for water alkalinity, water pH, Nitrites, Nitrite, Permanganate (testing water treatment equipment that removes sulphur odors), Sulfites, Iron, Copper, Chlorine, Bromine, Acidity, and a nearly endless catalog of additional water tests and water test methods.

How to use the Taylor R-0012 water hardness test kit - drop count titration method

This water hardness test method uses a water hardness reagent, an organic chelant solution whose principal ingredient is Ethylenediaminetetra- 60-00-4 <5 Irritant acetic acid. The reagent is added drop by drop to a quantity of test water and the user looks for a permanent color change in the water test sample.

By using the Taylor R-0012 0.75 oz test bottle and filling the bottle to a mark provided on that container you can be assured that you are using the proper test volume. In discussing water hardness testing (calcium level) in a discussion of pools and spas, Taylor adds these details:

The color progression in a hardness test is from red, to a mixture of red and blue, to blue. To be certain the color change is permanent, indicating you have reached the true endpoint, add one more drop of titrant. If the blue color remains unchanged, do not count this drop.[9]

Taylor also warns about water hardness tests (using a reagent and drop count titration) that produce a "fading endpoint" - the color ends at purple rather than blue due to contamination of the test water by metal ions. The company provides a procedure for adjusting the process if this happens during your test.

Store your reagent between 36-85 degF., keep it out of direct sunlight, and avoid abrupt temperature changes that can cause reagent deterioration. Handle appropriately. Like many acid reagents the liquid can be a skin or eye irritation. Taylor's website provides an MSDS if you need it. More information is available from the manufacturer, Taylor Technologies [9] whose contact information is at our references section below.

Measuring Water Hardness to Test Water Softener / Water Conditioner Operation using Water Hardness Test Tablets

If you need to confirm that your water softener equipment is working properly you will want to measure the hardness or mineral content of your water supply taken at a plumbing tap (sink faucet) located after or "downstream from" the water softener. The following procedure is adapted from IBC [using "water hardness tablets" and other water conditioner installation manuals.[8]

There are three conceptual data points you might want to test:

  1. incoming water hardness - take a water sample from a tap that bypasses or is before the water softener
  2. water hardness after a water softener regeneration or "backwash" cycle
  3. water hardness a day or more after a backwash cycle, or each day of normal water usage in the building between one backwash cycle and the next one - this can confirm that the settings on the water conditioner controls, including regeneration or backwash cycle frequency are adequate.