Water Hardness PPM Calculator
American water reports say "12 grains per gallon" while everyone else says PPM. Multiply by 17.1 — 12 gpg is about 205 PPM, which is firmly "hard" water that will scale your kettle and water heater.
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Water Hardness PPM Calculator
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American water reports say "12 grains per gallon" while everyone else says PPM. Multiply by 17.1 — 12 gpg is about 205 PPM, which is firmly "hard" water that will scale your kettle and water heater.
How to use this calculator
- Enter hardness in grains per gallon.
- Multiply by 17.1.
- Read PPM as CaCO₃.
- Check the hardness scale below.
The formula
1 grain per US gallon = 17.1 mg/L = 17.1 PPM as CaCO₃.
What the units mean
PPM (parts per million) — one part of solute per million parts of the total mixture. The workhorse unit for trace concentrations in water, air, and food.
Worked example
A well test shows 15 gpg → 15 × 17.1 = 256.5 PPM — very hard water, softener recommended.
Quick reference
| Grains/gallon | PPM | Level |
|---|---|---|
| 3 | 51 | Soft |
| 7 | 120 | Moderate |
| 10 | 171 | Hard |
| 15 | 257 | Very hard |
As CaCO₃; US gallons.
Where you will actually use this
Water softener sizing, appliance protection, and soap efficiency all depend on knowing your real hardness number.
- Water softener sizing
- Appliance protection
- Well water assessment
- Soap and detergent dosing
Mistakes that give wrong answers
- Using UK gallons (different factor)
- Ignoring that "hardness" is reported as CaCO₃ equivalent
Key takeaways
- Formula: PPM (as CaCO₃) = grains per gallon × 17.1
- Remember: As CaCO₃; US gallons.
- Watch out for: Using UK gallons (different factor)
- Next step: run your own numbers in the calculator above, then verify with a measurement — calculated values are only as good as the inputs.
Frequently asked questions
What is considered hard water in PPM?
0–60 soft, 61–120 moderate, 121–180 hard, 180+ very hard.
How many PPM is 10 grains per gallon?
171 PPM.
Does hard water affect appliances?
Yes — scale buildup shortens water heater and dishwasher life.