To convert grains per gallon to ppm, multiply by 17.118: 1 gpg = 17.1 ppm (mg/L as calcium carbonate), 10 gpg = 171 ppm and 20 gpg = 342 ppm. To go the other way, divide ppm by 17.118, so 120 ppm is 7.0 gpg and 300 ppm is 17.5 gpg. The factor comes straight from the definitions of the grain and the US gallon.
On this page
Below you’ll find where 17.118 comes from, full conversion tables in both directions, worked examples and the mix-ups that catch people out, especially with UK figures.
This converter needs JavaScript. Multiply gpg by 17.118 for ppm; divide ppm by 17.118 for gpg.
Where 17.118 comes from
Both units describe the same thing: how much hardness, counted as calcium carbonate, is dissolved in the water. They just use different weights and volumes.
- A grain is a unit of mass defined as exactly 64.79891 milligrams.
- A US gallon is exactly 3.785411784 litres.
- So 1 grain per US gallon = 64.79891 mg ÷ 3.785411784 L = 17.118 mg/L.
- For water, 1 mg/L is 1 part per million, because a litre of water weighs about a kilogram. That makes 1 gpg = 17.118 ppm.
You’ll often see 17.1 or 17 used as a shortcut. For softener settings that’s close enough: at 300 ppm, dividing by 17 gives 17.6 gpg instead of 17.5.
Grains per gallon to ppm conversion table
| gpg | ppm | gpg | ppm | gpg | ppm |
|---|---|---|---|---|---|
| 1 | 17.1 | 11 | 188.3 | 21 | 359.5 |
| 2 | 34.2 | 12 | 205.4 | 22 | 376.6 |
| 3 | 51.4 | 13 | 222.5 | 23 | 393.7 |
| 4 | 68.5 | 14 | 239.7 | 24 | 410.8 |
| 5 | 85.6 | 15 | 256.8 | 25 | 427.9 |
| 6 | 102.7 | 16 | 273.9 | 26 | 445.1 |
| 7 | 119.8 | 17 | 291.0 | 27 | 462.2 |
| 8 | 136.9 | 18 | 308.1 | 28 | 479.3 |
| 9 | 154.1 | 19 | 325.2 | 29 | 496.4 |
| 10 | 171.2 | 20 | 342.4 | 30 | 513.5 |
ppm to grains per gallon conversion table
| ppm | gpg | ppm | gpg |
|---|---|---|---|
| 10 | 0.58 | 120 | 7.01 |
| 20 | 1.17 | 150 | 8.76 |
| 30 | 1.75 | 180 | 10.52 |
| 40 | 2.34 | 200 | 11.68 |
| 50 | 2.92 | 250 | 14.60 |
| 60 | 3.51 | 300 | 17.53 |
| 70 | 4.09 | 350 | 20.45 |
| 80 | 4.67 | 400 | 23.37 |
| 90 | 5.26 | 450 | 26.29 |
| 100 | 5.84 | 500 | 29.21 |
Worked examples from real reports
Las Vegas: 280 ppm. 280 ÷ 17.118 = 16.4 gpg. The Las Vegas Valley Water District rounds it to 16 and tells customers to set softeners to 16. More in our Las Vegas water hardness guide.
Austin: 70–126 ppm. 70 ÷ 17.118 = 4.1 gpg and 126 ÷ 17.118 = 7.4 gpg, exactly the figures Austin Water prints next to the ppm values. See Austin water hardness.
San Antonio: 15–20 gpg. Going the other way, 15 × 17.118 = 257 ppm and 20 × 17.118 = 342 ppm. See San Antonio water hardness.
Five mix-ups to avoid
US gallons vs imperial gallons. UK sources sometimes quote “grains per gallon” meaning imperial gallons, which are about 20% larger. One grain per imperial gallon is a Clark degree: 64.79891 mg ÷ 4.54609 L = 14.254 mg/L. If a UK figure looks about 20% higher than you expected, that’s probably why.
“As calcium carbonate” vs “as calcium”. The 17.118 factor applies to hardness expressed as calcium carbonate, which is how US reports normally give it. A figure for calcium alone (mg/L Ca) is a different measurement and needs multiplying by about 2.5 before you can compare it.
Grains vs grams. A grain is about 65 milligrams; a gram is 1,000. Nobody mixes them up in a lab, but they turn up in forum posts and product listings.
ppm of TDS. A cheap TDS meter also reads in ppm, but it measures all dissolved salts, not hardness. Don’t convert a TDS reading to gpg for your softener.
Rounding twice. If your report rounds ppm and then you round gpg, the error stacks up. Convert the original number, then round once.
Other units you might meet
European appliances often use German degrees (°dH) or French degrees (°f). From mg/L as calcium carbonate: divide by 17.848 for °dH, by 10 for °f and by 100.09 for mmol/L. The converter at the top of this page does all of them at once, and our page on setting a water softener’s hardness explains which number to enter on the valve.
Frequently asked questions
How many ppm is 1 grain per gallon?
17.118 ppm (mg/L as calcium carbonate), often rounded to 17.1.
Is 10 grains per gallon a lot?
10 gpg is 171 ppm, which the USGS classes as hard. Above 10.5 gpg (180 ppm) it’s very hard.
How do I convert mg/L to gpg?
Exactly as for ppm: divide by 17.118. For water, mg/L and ppm are the same number.
Why does my softener want gpg when my report gives ppm?
US softener valves count capacity in grains, so they need hardness in grains per gallon to work out how many gallons they can soften before regenerating.
