Short answer
Houston's 2025 water quality report lists Main System hardness from 43.5 to 229 mg/L, with an average of 135.5 mg/L, about 7.9 grains per gallon. That is hard water on the USGS scale. It is enough to build sediment in tanks and scale in tankless units, so a yearly flush is a sound habit.
Key facts
- City of Houston 2025 Water Quality Report, Main System: total hardness 43.5 to 229 ppm as calcium carbonate, average 135.5 (about 7.9 grains per gallon).[1]
- Houston's separate Kingwood system reported hardness of 160 to 167 ppm, average 163.5, in the same 2025 report.[1]
- The U.S. Geological Survey classes water of 121 to 180 mg/L as calcium carbonate as hard, and over 180 mg/L as very hard.[3]
- Sugar Land's 2025 report lists Main System hardness of 104 ppm; League City's lists 124 mg/L.[4]
What is Houston's water hardness?
Houston Public Works runs the city's drinking water systems and publishes a yearly water quality report[2]. The 2025 report, covering water sampled in 2025, lists total hardness as calcium carbonate[1]:
| System | Low | Average | High | Grains per gallon (average) |
|---|---|---|---|---|
| Houston Main System | 43.5 mg/L | 135.5 mg/L | 229 mg/L | about 7.9 |
| Kingwood system | 160 mg/L | 163.5 mg/L | 167 mg/L | about 9.6 |
The report states these in ppm, which is the same as mg/L for water[1]. To convert to grains per gallon, divide mg/L by 17.1.
The Main System range is wide. The low end is soft water and the high end is very hard[1][3]. Where you fall depends on which plant or wells feed your street, and that can change over the year. The average is the best single number, but your tap may differ.
Is that hard or soft?
The U.S. Geological Survey uses these classes[3]:
| mg/L as calcium carbonate | USGS class | Houston area systems in this band |
|---|---|---|
| 0 to 60 | Soft | Houston Main System low reading[1] |
| 61 to 120 | Moderately hard | Sugar Land Main System, 104[4] |
| 121 to 180 | Hard | Houston Main System average 135.5[1]; Kingwood 163.5[1]; League City 124[5] |
| More than 180 | Very hard | Houston Main System high reading, 229[1] |
So Houston's average is hard, not very hard. That is still enough to leave minerals in a water heater.
What about the suburbs?
Each city runs its own water system and decides whether to print hardness. Hardness has no federal health or secondary standard[12], and the federal report rule does not require it[11].
| City | What its utility publishes | Source |
|---|---|---|
| Sugar Land | 104 ppm, a single 2025 measurement for the Main System; the River Park, Greatwood and New Territory systems are separate | [4] |
| League City | 124 mg/L total hardness, 2025 data | [5] |
| Pearland | The 2025 report does not list hardness | [6] |
| Pasadena | The newest report in the city archive is from 2021 and gives no current hardness figure | [7] |
If your city publishes no figure, call the water utility and ask for "total hardness as calcium carbonate," or test at the tap.
What does hard water do to a tank water heater?
Heating hard water drops some of its minerals out as scale. In a tank, most of it settles on the bottom as sediment. On a gas tank, water trapped in that layer flashes to steam under the burner, which is the popping or rumbling sound many owners hear. Sediment also takes up room and insulates the water from the burner, so the heater works longer for the same hot water.
Many manufacturers' manuals call for draining and flushing a tank once a year. At Houston's hardness, that is a sensible schedule. Rheem puts typical tank life at 8 to 12 years[10]. Heavy sediment can push a tank toward the short end of that range. A rumbling tank is worth a flush before it is worth a new heater; see popping and rumbling noises.
What does it mean for a tankless heater?
Tankless units are more sensitive, because scale coats the narrow passages of the heat exchanger. A guide prepared for the U.S. Department of Energy's Building America program says scale can restrict flow and shorten a tankless unit's life. It adds that above about 8 to 14 grains per gallon (136 to 239 mg/L), manufacturers may require a water softener[9]. Houston's 135.5 mg/L average sits right at the bottom of that band[1].
Rinnai's SENSEI manual shows how one maker sets limits[8]:
| Rinnai label | mg/L | Rinnai guidance[8] | Houston area systems in this band |
|---|---|---|---|
| Slightly hard | 61 to 120 | Softener or scale cutter recommended; flush yearly | Sugar Land Main[4] |
| Moderately hard | 121 to 160 | Softener or scale cutter recommended; flush yearly | Houston Main average, League City[1][5] |
| Very hard | 161 to 200 | Strongly recommended | Kingwood average[1] |
| Extremely hard | Over 200 | Softener or scale cutter required | Houston Main high reading[1] |
The same manual lists 200 mg/L as the maximum total hardness and says component damage from water quality is not covered by the warranty[8]. Houston's Main System high reading of 229 mg/L is above that line[1]. Other brands set their own limits, so read the manual for the exact model before you buy. For upkeep, see how to descale a tankless water heater.
When to call a pro
- You plan to buy a tankless heater: ask an independent licensed plumber about scale control and the maker's hardness limit for your address.
- A tank pops or rumbles even after a flush, or the drain valve will not open or will not close.
- A tankless unit shows a scale or service code.
- Water around the base of the tank: a leak from the tank body usually means replacement, not repair.
More questions
Do I need a water softener in Houston?
Not for the water heater alone at the city's average of 135.5 mg/L. It becomes a stronger case if you buy a tankless unit, live where readings run near the top of the 229 mg/L range, or see heavy scale on fixtures. Read your heater maker's water quality limits first.
Why does my neighbor's report show a different number?
Houston runs more than one water system. The 2025 report lists the Main System and the separate Kingwood system with different figures, and suburbs such as Sugar Land and League City run their own systems.
How do I convert mg/L to grains per gallon?
Divide by 17.1. Houston's 135.5 mg/L average is about 7.9 grains per gallon. Softener and tankless manuals usually use grains.
Sources
- City of Houston Water Quality Report 2025, City of Houston, Houston Public Works. Accessed 2026-10-06.
- Drinking water quality report, City of Houston, Houston Public Works. Accessed 2026-10-06.
- Hardness of water, U.S. Geological Survey, Water Science School. Accessed 2026-10-07.
- 2025 Water Quality Report, Main System, City of Sugar Land. Accessed 2026-10-06.
- 2026 Drinking Water Quality Report (2025 data), City of League City. Accessed 2026-10-06.
- 2025 Consumer Confidence Report, City of Pearland. Accessed 2026-10-06.
- Water Quality Reports archive, City of Pasadena, Texas. Accessed 2026-10-06.
- SENSEI RX Residential Installation and Operation Manual, Rinnai America. Accessed 2026-10-07.
- Measure Guideline: Transitioning to a Tankless Water Heater (IBACOS, September 2012), U.S. Department of Energy, Building America program. Accessed 2026-10-07.
- Water heater lifespan: when to repair vs. replace (Mar 25, 2026), Rheem Manufacturing. Accessed 2026-10-06.
- 40 CFR 141.153, Content of the reports (Consumer Confidence Reports), eCFR, U.S. Government. Accessed 2026-10-07.
- 40 CFR 143.3, Secondary maximum contaminant levels, eCFR, U.S. Government. Accessed 2026-10-07.
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