Water Hardness Calculator
Grains per gallon, USGS band, and softener note from mg/L hardness.
What it does
Water hardness counts dissolved calcium and magnesium as calcium carbonate: soft under 60 mg/L, hard past 120, very hard past 180. Softeners and soap doses think in grains per gallon, seventeen point one mg/L each.
The tool above takes hardness in mg/L. It returns grains per gallon with the USGS band and a softener note beside them.
The default of 150 mg/L gives 8.8 gpg: Hard, softener worth considering. At 30 mg/L the same water reads 1.8 gpg Soft, and 200 reads 11.7 gpg Very hard.
The bands follow geological survey water science with survey water FAQs matching each line. Both divide soft at 60, moderate at 120, hard at 180 mg/L.
How to use
- Read hardness as CaCO3 off a test or report.
- Enter mg/L (ppm reads the same here).
- Read grains for softener and soap math.
- Read the band against the four lines.
- Read the note before shopping softeners.
Formula
- 17.1
- mg/L per grain per gallon.
- CaCO3
- Common hardness yardstick.
- bands
- USGS soft to very hard lines.
ppm equals mg/L for dilute water. Softeners swap calcium for sodium at these grains; very hard water also wants iron and pH checked.
Worked example
Example: 150 mg/L
Default: 150 mg/L.
- Grains: 150 / 17.1 = 8.8 gpg.
- Band: 121 to 180 reads Hard.
- Note: scale builds, softener worth considering.
- Soft check: 60 would read 3.5 gpg Soft.
- Tool shows 8.8 gpg, Hard (150 mg/L), the softener note.
Reference table
| mg/L | Grains | Band |
|---|---|---|
| 30 | 1.8 gpg | Soft |
| 100 | 5.8 gpg | Moderately hard |
| 150 | 8.8 gpg | Hard |
| 200 | 11.7 gpg | Very hard |
Limits
- Hardness only; iron, pH, and TDS need their tests.
- Single sample misses seasonal swings.
- Softened sodium counts for low salt diets.
- EPA sets no hardness limit: nuisance, not poison.
Common mistakes
- Reading ppm iron as hardness grains.
- Softening without testing first.
- Sizing softeners on peak guesses.
- Ignoring iron that fouls resin beds.
- Skipping retests after treatment.
Try it yourself
Quick checks. Answers sit tucked under each question.
30 mg/L?
1.8 gpg: Soft, easy on pipes and soap.
200 mg/L?
11.7 gpg: Very hard, softener territory.
100 mg/L?
5.8 gpg: Moderately hard, usually fine.
Questions
What is hard water?
Dissolved calcium and magnesium as CaCO3: hard past 120 mg/L.
mg/L to grains?
Divide by 17.1: 150 mg/L is 8.8 gpg.
Do I need a softener?
Consider past 120, likely past 180: scaling and soap costs decide.
Is hard water unsafe?
No. EPA sets no limit; it annoys pipes and skin, not health.
Why spots on glasses?
Carbonate scale from hard water drying: vinegar dissolves it.
Soft water slippery feel?
Normal: soap rinses fully without scum instead of squeaking.