Quick Answer: St. Louis water is Hard, averaging 8–12 gpg (136–205 ppm). That is above the US national average of ~10 gpg. St. Louis draws from Missouri River + Mississippi River. Yes, worth considering. At 8–12 gpg, scale is noticeable. A 32,000-grain softener handles most households. If you have a tankless water heater, softening is especially recommended.
St. Louis Water Hardness at a Glance
St. Louis, MO — Water Hardness Data
Hardness Range8–12 gpg (136–205 ppm)
ClassificationHard
US National Average~10 gpg (171 ppm)
Primary Water SourceMissouri River + Mississippi River
Water ProviderMissouri American Water / City of St. Louis Water Division
Softener Recommended?Yes — consider it
Hardness Scale Position
Soft (0–3.5)Moderate (3.5–7)Hard (7–10.5)Very Hard (10.5+)
This city's hardness sits above the national average.
Why Is St. Louis Water So Hard?
St. Louis draws from the Missouri and Mississippi Rivers — both of which drain extensive limestone and sedimentary terrain across the Midwest before converging near St. Louis. The Missouri River alone drains limestone plains from Montana to Missouri. Combined, these sources deliver water with moderate to hard mineral content.
The USGS classifies St. Louis's 8–12 gpg as Hard.
How St. Louis Compares to Other Major Cities
| City | Hardness (gpg) | Classification |
|---|---|---|
| St. Louis, MO (this city) | 8–12 gpg | Hard |
| Las Vegas, NV | 16–17 gpg | Very Hard |
| Phoenix, AZ | 16–18 gpg | Very Hard |
| Indianapolis, IN | 14–16 gpg | Very Hard |
| Chicago, IL | 8–11 gpg | Hard |
| Atlanta, GA | 7–9 gpg | Moderately Hard |
| Denver, CO | 4–6 gpg | Moderately Hard |
| Seattle, WA | 1–3 gpg | Soft |
| Vancouver, BC | 0.3–0.6 gpg | Soft |
| US National Average | ~10 gpg | Hard |
What Hard Water Does to a St. Louis Home
At 8–12 gpg, scale damage in St. Louis is noticeable. Here's what that looks like in practice:
- Water heaters: The US Department of Energy estimates scale reduces water heater efficiency by up to 30% for every 5 gpg of hardness. Tank and tankless units accumulate scale on heating elements and heat exchangers, forcing them to work harder.
- Dishwashers: White film on glasses and dishes is common. Spray arm ports clog with scale, reducing cleaning performance — see our guide to hard water and dishwashers for solutions short of a full softener.
- Showerheads and faucets: Visible white mineral deposits accumulate on fixtures. Flow rates drop as scale narrows openings. See our guide on hard water and hair for tips on protecting hair and skin.
- Pipes (older homes): Calcium carbonate deposits gradually narrow pipes over years, reducing pressure in homes over 15+ years old.
- Skin and hair: Mineral film on skin after showering and dull, brittle hair are common complaints. Those with eczema may see flare-ups — see our hard water and eczema guide.
- Coffee and tea: High mineral content affects extraction and flavor. Hard water and coffee — what the science says.
💡 Check Your Exact Hardness: Hardness can vary by neighbourhood and distribution zone. Missouri American Water / City of St. Louis Water Division publishes annual Consumer Confidence Reports with exact hardness data. Well water users should test annually — groundwater hardness varies significantly from municipal supply.
Best Water Treatment for St. Louis Homes
At 8–12 gpg, a whole-home water softener is worth considering to protect appliances and plumbing. The EPA's Consumer Confidence Report program requires Missouri American Water / City of St. Louis Water Division to publish annual contaminant data — check it for your specific address.
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St. Louis Metro Area — Water Hardness Nearby
Kansas City Mo10–14 gpg · Very Hard
Chicago Il8–11 gpg · Hard
Nashville Tn10–13 gpg · Very Hard
Indianapolis In14–16 gpg · Very Hard
Memphis Tn5–7 gpg · Moderately Hard
Frequently Asked Questions — St. Louis Water Hardness
Is St. Louis water hard or soft?
St. Louis water is Hard, averaging 8–12 gpg (137–205 ppm). This is near the US national average of ~10 gpg. St. Louis draws from Missouri River + Mississippi River.
What is the water hardness in St. Louis, MO?
St. Louis water hardness is approximately 8–12 gpg (137–205 ppm), classified as Hard. Missouri American Water / City of St. Louis Water Division publishes annual water quality data in its Consumer Confidence Report at www.stlwater.com.
Why is St. Louis water so hard?
St. Louis water is hard because of its source geology: Missouri River + Mississippi River. The rock formations this water travels through dissolve calcium and magnesium carbonates, producing hard water.
Do I need a water softener in St. Louis?
Yes, it's worth considering. At 8–12 gpg, scale builds up noticeably over time. A softener isn't urgent but will extend appliance lifespans and reduce maintenance.
Does Kansas City Mo have similar water hardness to St. Louis?
Kansas City Mo has water hardness of approximately 10–14 gpg — classified as Very Hard. The two cities have different hardness levels due to different water sources and geology. See the Kansas City Mo water hardness guide for details.
How does St. Louis water hardness compare to Chicago Il?
St. Louis averages 8–12 gpg (Hard), while Chicago Il is approximately 8–11 gpg (Hard). They are similar. See the Chicago Il water hardness guide.
Related Guides on HardWaterHQ
Missouri & Regional Water Hardness
- Missouri State Water Hardness Guide — All Cities Ranked
- Kansas City Mo Water Hardness — 10–14 gpg · Very Hard
- Chicago Il Water Hardness — 8–11 gpg · Hard
- Nashville Tn Water Hardness — 10–13 gpg · Very Hard
- Indianapolis In Water Hardness — 14–16 gpg · Very Hard
St. Louis Water Quality & Contaminants
- St. Louis Full Water Quality Guide — PFAS, Contaminants & Treatment
- Best PFAS Water Filters 2026
- Does Brita Remove PFAS?
- Berkey vs Clearly Filtered
Hard Water Problems & Solutions
- Hard Water and Dishwashers — Fixes Without a Softener
- Hard Water and Hair Loss — What the Research Says
- Hard Water and Eczema — Is Your Water Making It Worse?
- Hard Water and Coffee — How Minerals Affect Your Brew
- Well Water Iron Problems — Diagnosis & Treatment
Buyer's Guides
- Best Water Softeners for 2026 — Sized for Every Hardness Level
- Best Whole-House Water Filters 2026
- Best Whole-House Water Softeners 2026
- Reverse Osmosis vs Water Softener — Which Do You Need?
- How to Test Water Hardness at Home
- Salt-Free vs Traditional Water Softeners