Question 1

Why is the factor 17.118?

Accepted Answer

1 grain = 64.8 mg, 1 US gallon = 3.785 L, so 64.8/3.785 = 17.118 mg per liter per grain per gallon.

Question 2

Is mg/L the same as ppm?

Accepted Answer

For dilute aqueous solutions yes — 1 mg/L ≈ 1 ppm because 1 L of water weighs ~1 kg.

Question 3

What hardness is 'hard' water?

Accepted Answer

USGS calls >7 gpg (>120 mg/L) hard, >10.5 gpg (>180 mg/L) very hard. Most softener manufacturers size starting at 5 gpg.

Question 4

Does this factor work for other constituents?

Accepted Answer

Yes — the math is identical for any mg/L ↔ gpg conversion. It is most often applied to hardness.

mg/L to Grains per Gallon (gpg) Hardness Converter