Grains per gallon ↔ mg/L (ppm) converter

Water reports use mg/L (same as ppm). Softeners and dishwashers often use grains per gallon. One grain per gallon equals 17.1 mg/L as calcium carbonate.

Grains per gallon (gpg)

mg/L (ppm) as CaCO₃

Class: Moderately hard · 5.2 °dH

Formulas

mg/L = gpg × 17.1
gpg = mg/L ÷ 17.1
ppm and mg/L are the same for drinking water.
German degrees: 1 °dH = 17.8 mg/L, so °dH = mg/L ÷ 17.8.

Why 17.1? One grain is 64.8 mg, and one US gallon is 3.785 L. 64.8 ÷ 3.785 ≈ 17.1. Austin Water's report uses the same factor (“approximately 17.1 ppm”).

Hardness scale

Class (USGS)	mg/L (ppm) as CaCO₃	Grains per gallon (÷ 17.1)
Soft	0–60	0–3.5
Moderately hard	61–120	3.6–7.0
Hard	121–180	7.1–10.5
Very hard	more than 180	more than 10.5

Class limits: USGS Water Science School, “Hardness of Water”. Grain values are our conversion at 1 gpg = 17.1 mg/L, rounded to 0.1.

Worked examples from utility reports

Austin, TX: 93 mg/L ÷ 17.1 = 5.4 gpg (moderately hard).
Las Vegas, NV: 280 mg/L ÷ 17.1 = 16.4 gpg; the utility rounds to 16 (very hard).

Sources: USGS, Hardness of Water (scale); Austin Water 2025 report (17.1 factor). Last checked 2026-10-26.