Water hardness converter: mg/L to grains per gallon
Labs report hardness in mg/L. Softeners are sold in grains. This is the conversion between them, and what the number means once you have it.
The short answer
One grain per gallon of hardness equals 17.1 mg/L as calcium carbonate. To convert mg/L (or ppm — they are the same) to grains per gallon, divide by 17.1. To go the other way, multiply by 17.1. So 171 mg/L is 10 grains per gallon, which is hard water.
Converted
10.0gpg
- Conversion usedOne grain per gallon as calcium carbonate.
- 1 gpg = 17.1 mg/L
- In mg/L (ppm)
- 171.0 mg/L
- In grains per gallon
- 10.00 gpg
- ClassificationUSGS / Water Quality Association bands.
- Hard
Does this need a softener?
Above about 7 grains the arithmetic starts to work — scale shortens water heater and appliance life, and you use noticeably less soap and detergent.
Hardness alone does not size a softener. Iron and manganese consume the same resin capacity, so both have to be added in before you pick a grain rating.
Why there are two units at all
Laboratories report hardness in milligrams per litre, because that is how every other analyte on the report is expressed. The water treatment industry sizes and sells softeners in grains per gallon, because a softener’s capacity is rated in grains it can remove between regenerations.
So the one number you need to size a softener arrives in the wrong unit, and the conversion factor is an awkward 17.1 rather than something memorable. A factor-of-17 error is the commonest arithmetic mistake a well owner makes reading their own test — and it produces a softener either seventeen times too big or seventeen times too small.
mg/L and ppm are interchangeable for drinking water. One milligram per litre is one part per million, near enough, because a litre of water weighs about a kilogram. If your report uses ppm, treat it as mg/L and convert the same way.
Conversion chart
Both directions, with the classification each figure falls into.
mg/L (ppm) to grains per gallon
| mg/L | gpg | Class |
|---|---|---|
| 17 | 1.0 | Soft |
| 34 | 2.0 | Slightly hard |
| 50 | 2.9 | Slightly hard |
| 68 | 4.0 | Moderately hard |
| 85 | 5.0 | Moderately hard |
| 100 | 5.8 | Moderately hard |
| 120 | 7.0 | Hard |
| 150 | 8.8 | Hard |
| 171 | 10.0 | Hard |
| 200 | 11.7 | Very hard |
| 250 | 14.6 | Very hard |
| 300 | 17.5 | Very hard |
| 400 | 23.4 | Very hard |
| 500 | 29.2 | Very hard |
Grains per gallon to mg/L (ppm)
| gpg | mg/L | Class |
|---|---|---|
| 1 | 17 | Soft |
| 2 | 34 | Slightly hard |
| 3 | 51 | Slightly hard |
| 5 | 86 | Moderately hard |
| 7 | 120 | Moderately hard |
| 10 | 171 | Hard |
| 12 | 205 | Very hard |
| 15 | 257 | Very hard |
| 20 | 342 | Very hard |
| 25 | 428 | Very hard |
| 30 | 513 | Very hard |
What the number actually means
The classification bands come from the USGS scale, which the Water Quality Association also uses. They describe behaviour, not safety — hardness is not a health contaminant, and there is no EPA health limit for it.
| Class | gpg | mg/L | What you notice |
|---|---|---|---|
| Soft | 0–1 | 0–17 | Nothing. Soap lathers freely. |
| Slightly hard | 1–3.5 | 17–60 | Faint spotting on glassware at most. |
| Moderately hard | 3.5–7 | 60–120 | Visible spotting, slight soap scum. A comfort decision. |
| Hard | 7–10.5 | 120–180 | Scale in kettles and heaters, stiff laundry. Softening starts to pay. |
| Very hard | 10.5+ | 180+ | Persistent scale, shortened appliance life, heavy soap use. |
Hardness alone does not size a softener
Once you have grains per gallon, the temptation is to take it straight to a sizing chart. Do not — not on a well.
Iron and manganese are removed by the same ion exchange resin, and they consume its capacity just as hardness does. The industry rule is roughly 4 grains of extra load per ppm of either. Water at 10 gpg with 2 ppm of iron behaves like 18 gpg water, and a softener sized for 10 will regenerate constantly and still deliver hard water.
Above about 3 ppm of iron the softener is the wrong machine altogether and needs an iron filter ahead of it.
Frequently asked questions
- How do I convert ppm to grains per gallon?
- Divide by 17.1. One grain per gallon equals 17.1 mg/L of hardness as calcium carbonate, and ppm and mg/L are the same thing in drinking water. So 200 ppm divided by 17.1 is about 11.7 grains per gallon.
- Is ppm the same as mg/L for water hardness?
- Yes, for practical purposes. A litre of water weighs approximately one kilogram, so one milligram per litre is one part per million. Lab reports use mg/L; test strips and softener dealers often say ppm. Convert them identically.
- What is considered hard water in ppm?
- Above about 120 mg/L is classed as hard, and above 180 mg/L as very hard, on the USGS scale. Below 60 mg/L is soft to slightly hard. The bands describe how the water behaves, not whether it is safe — hardness has no health limit.
- How many grains of hardness do I need a softener for?
- Below about 3 grains per gallon a softener is difficult to justify. Between 3 and 7 it is a comfort purchase. Above roughly 7 grains the economics start to work, because scale shortens water heater and appliance life and you use measurably less soap.
- Why is my hardness reported as calcium carbonate when my water has no calcium carbonate in it?
- Hardness comes mostly from calcium and magnesium in various forms. Rather than report each separately, labs express the total as the equivalent amount of calcium carbonate, so one number can be compared across any water. It is a convention, not a claim about what is dissolved.