UTILITYMATH

Water-test result converter

Turn the lab number into something useful.

Convert hardness between grains per gallon, ppm, and milligrams per liter—then carry the correct number into your softener planning.

Designed by Gary & Quinnlan Rae

01

Enter your test result

Use total hardness reported “as CaCO₃.” In ordinary water-test reporting, ppm and mg/L are used as equivalent concentration values. The iron factor is an editable equipment-planning assumption.

02

Your converted result

USGS hardness classification

Hard

Grains per gallon10.5 gpg
Milligrams per liter180 mg/L
Parts per million180 ppm
Iron-adjusted planning hardness10.5 gpg

Hard water can contribute to scale and reduced soap performance. Classification describes hardness—not whether the water is safe to drink.

The conversion

One grain per U.S. gallon equals 17.1 milligrams per liter. To convert mg/L or ppm to grains per gallon, divide by 17.1. To convert grains per gallon to mg/L or ppm, multiply by 17.1.

gpg = mg/L ÷ 17.1   |   mg/L = gpg × 17.1

How the classification works

The U.S. Geological Survey’s general classification uses total hardness as calcium carbonate:

Hardness is not the whole water test. This classification covers dissolved hardness minerals, mainly calcium and magnesium. It does not interpret bacteria, nitrate, arsenic, lead, pH, salinity, sulfur, or other health and treatment concerns.

Why iron is shown separately

Iron is measured in ppm, but some softener-sizing methods add a hardness-equivalent planning allowance for each ppm of dissolved iron. The correct allowance and whether a softener should handle the iron depend on the equipment and water chemistry. UtilityMath keeps the original hardness classification separate and shows the adjusted number only for equipment planning.

Take the next step

Water Softener Sizing CalculatorUse your converted result to estimate working capacity. Salt & Operating Cost CalculatorEstimate salt, bags, regeneration water, and annual cost.

Sources

Planning and unit-conversion tool only. Use a certified laboratory or qualified water professional for health-related interpretation and treatment decisions. Spot an issue? Email corrections@utilitymath.com.