Water Hardness Explained: GPG, TDS, and What It Means for Your Home
What hardness actually measures, how GPG and TDS differ, where Indiana and Michigan cities rank, and what happens at each hardness level.
What water hardness actually measures
Water hardness measures the concentration of dissolved calcium and magnesium ions in water. These minerals are present in almost all groundwater because water naturally dissolves them from limestone, dolomite, and chalk rock as it moves through the ground. The more limestone it contacts, the harder the water. Indiana sits on a particularly thick limestone belt, which is why Indiana water is among the hardest in the country.
GPG vs. ppm vs. mg/L
Hardness is measured in three common units that can cause confusion:
- GPG (grains per gallon): The standard unit in the water treatment industry. 1 grain = 64.8 milligrams. This is the unit you'll see on softener labels.
- ppm (parts per million): Used interchangeably with mg/L. 1 GPG = 17.1 ppm.
- mg/L (milligrams per liter): Same as ppm for water. Used in scientific literature and utility reports.
- To convert: ppm ÷ 17.1 = GPG. A CCR listing 340 ppm hardness = 20 GPG.
Quick reference
0-3 GPG: Soft. 4-7 GPG: Moderately soft. 8-12 GPG: Moderately hard. 13-18 GPG: Hard. 19-25 GPG: Very hard. 25+ GPG: Extremely hard (rare in municipal supply).
Hardness by city — Indiana and Michigan
Based on Aqua Otter's in-home water testing across our service area:
- Fort Wayne, IN: 22 GPG (among the highest in the state)
- Zionsville, IN: 21 GPG
- Noblesville, IN / Westfield, IN: 20 GPG
- Fishers, IN / Carmel, IN: 19 GPG
- Indianapolis, IN: 17 GPG
- South Bend, IN: 16 GPG
- Ann Arbor, MI: 10 GPG
- Grand Rapids, MI: 8 GPG
- Detroit Metro (GLWA): 7 GPG
- Lansing, MI: 6 GPG
What happens at each hardness level
The effects of hard water compound over time. At moderate levels you might notice soap scum and water spots. At high levels (15+ GPG), the damage becomes measurable.
- 8-12 GPG: Visible water spots on glass and fixtures. Soap lather reduced. Minor scale on heating elements.
- 13-18 GPG: Scale building inside pipes and water heater over years. Significant soap film. Skin dryness noticeable.
- 18-22 GPG: Rapid scale accumulation. Water heater loses efficiency measurably within 2-3 years. Appliance lifespan shortened. Showerheads may clog within months.
- 22+ GPG: Severe scale. Fixtures may show white deposits within weeks of cleaning. Softener recommended for both home protection and appliance warranty preservation.
TDS: not the same as hardness
Total dissolved solids (TDS) is a broader measurement of everything dissolved in water — minerals, salts, metals, organic compounds. Hardness (calcium and magnesium) is a component of TDS, but TDS includes much more. A high TDS reading doesn't mean the water is hard, and a softener won't significantly lower TDS because it trades calcium and magnesium for sodium. For low TDS at the kitchen tap, a reverse osmosis system is needed.
Temporary vs. permanent hardness
Temporary hardness is caused by calcium bicarbonate (Ca(HCO₃)₂) and can be removed by boiling — the bicarbonate decomposes and calcium carbonate precipitates out. This is the white scale you see in a kettle. Permanent hardness is caused by calcium sulfate and magnesium sulfate, and boiling doesn't remove it. Most Indiana groundwater contains both forms, but ion exchange softening removes both regardless of form.
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