Water Hardness Calculator

About the Water Hardness Calculator

Water hardness is the concentration of dissolved calcium and magnesium, expressed as mg/L (ppm) of CaCO₃, grains per gallon (gpg), German degrees (°dH) or French degrees (°fH).

How to use this calculator

1. Enter the hardness value you know, such as \(mg/L\) as \(CaCO_3\), \(gpg\), or degrees.
2. Choose the unit you want to convert from and the unit you want to convert to.
3. Use the calculator to convert the value instantly.
4. Check the result and, if needed, use the formula below to verify the conversion by hand.

The formula explained

This conversion uses the fact that \(1\\ gpg = 17.118\\ mg/L\) as \(CaCO_3\). From that base, the calculator converts to other hardness units by scaling up or down.

• gpg = grains per gallon, a common water hardness unit
• mg/L = milligrams per liter, reported here as \(CaCO_3\)
• \(CaCO_3\) = calcium carbonate equivalent used to express hardness
• dH = degrees of hardness, depending on the degree scale used by the calculator

Step by step method

1. Start with the hardness value in your original unit.
2. If converting from \(gpg\) to \(mg/L\), multiply by \(17.118\). If converting from \(mg/L\) to \(gpg\), divide by \(17.118\).
3. Use the same idea for degree units, converting first through \(mg/L\) as the common reference if needed.

Worked example

Problem. Convert \(10\\ gpg\) to \(mg/L\) as \(CaCO_3\).

1. Use the relationship \(1\\ gpg = 17.118\\ mg/L\) as \(CaCO_3\).
2. Multiply \(10\) by \(17.118\): \(10 \\times 17.118 = 171.18\).
3. So the hardness is \(171.18\\ mg/L\) as \(CaCO_3\).

Tips and common mistakes

• Make sure you know whether a value is reported as \(mg/L\) as \(CaCO_3\) or as another substance, because hardness conversions depend on that standard.
• If your result looks a little different from a label, rounding may be the reason, so keep extra decimal places until the final step.

Frequently asked questions