Water hardness in Foster City
3.6 gpg moderately hard
Tap water in Foster City is moderately hard: 3.6 gpg (62 mg/L · 3.5 °dH · 6.2 °fH).
Source: City of Foster City 2025 Water Quality Report (2025) · data checked 2026-11-02
Range across supply zones: 8–112 mg/L.
Foster City is supplied by the Estero Municipal Improvement District, which buys water from the SFPUC Regional Water System. The 2025 city report gives a median of 62 mg/L as CaCO₃, with a range of 8.1 to 112 mg/L depending on the SFPUC source blend at the time. That is moderately hard water, so kettles and showerheads may show some scale, but a softener is rarely needed.
USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
Water data
| Total hardness | 3.6 gpg |
|---|---|
| Utility | Estero Municipal Improvement District (water purchased from SFPUC Regional Water System) |
How Foster City compares
- Tap water in Foster City is 60 % softer than the median of the 1372 cities we track in the United States (9.1 gpg).
- By hardness it ranks 1169 of 1372 in the United States (1 = hardest); 14 % of these cities have softer water.
- In California it is number 287 of 336 cities; the regional median is 9.5 gpg.
Ranking of the hardest and softest water in the United States →
What it means at home
- Visible kettle scale every few weeks; vinegar or citric acid is enough.
- Water heater and washing machine: some long-term scale; a scale inhibitor slows it.
- Dishwasher: medium salt setting.
- Softener optional; pays off mainly with a gas boiler or tankless heater.
How much salt will you use for softening?
Optional. A scale inhibitor or a small softener protects the heater; soap savings are modest.
Descale a coffee machine or kettle about every 3 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Foster City
| Brand | Setting for this water |
|---|---|
| Bosch | H00 |
| Miele | 3 °dH |
| AEG / Electrolux | 1 |
| Beko | 1st |
| Hoover | S0 |
| LG | H0 |
From each brand's manual for the model family named in the source; your model may differ, so check its manual. All brands and how to change the setting →
FAQ
Is Foster City water hard or soft?
It is moderately hard: 62 mg/L as CaCO₃ (3.6 gpg, 3.5 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Foster City safe to drink?
Hardness is not a health issue; it is about limescale, taste and appliances. Mains water must meet national drinking-water standards. USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
How many grains per gallon is Foster City water?
3.6 gpg, equal to 3.5 °dH, 6.2 °fH or 62 mg/L calcium carbonate.
What dishwasher salt setting should I use in Foster City?
For 62 mg/L (3.5 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Foster City?
Not essential; a scale inhibitor protects the heater cheaply.
Source
Source: City of Foster City 2025 Water Quality Report (2025). Method: figure published by the water supplier. City of Foster City 2025 Water Quality Report · Outdated figure? Tell us