Water Hardness Calculator: PPM to GPG & Softener Sizing
Two free tools: convert water hardness between ppm (mg/L as calcium carbonate) and grains per gallon with an instant hardness classification, then size a water softener for your household. Everything runs in your browser — no signup.
Don't know your number? Check your city's hardness — free ZIP lookup
Jump to: ppm ⇄ gpg converter · softener size calculator
PPM to GPG Converter
Utility reports usually state hardness in mg/L (the same as ppm) as calcium carbonate, while softener control panels use grains per gallon. One gpg equals 17.1 mg/L — enter either value and get the other, plus where it falls on the USGS hardness scale.
1 gpg = 17.1 mg/L (ppm), hardness as calcium carbonate. Enter a value in either box — the other converts instantly.
The Water Hardness Scale
| Classification | mg/L (PPM) | Grains/Gallon |
|---|---|---|
| Soft | 0-60 | 0.0-3.5 |
| Moderately hard | 60-120 | 3.5-7.0 |
| Hard | 120-180 | 7.0-10.5 |
| Very hard | >180 | >10.5 |
Water Softener Size Calculator
Softeners are sized by how many grains of hardness they can remove between regenerations. The standard sizing method:
grains capacity = hardness (gpg) × people × 75 gallons per person per day × days between regenerations (default 7)
Worked example: 10 gpg × 4 people × 75 × 7 = 21,000 grains, so the next common size up — a 24,000-grain unit — is the right fit. The 75 gal/person/day figure and this method follow the SoftPro Water Systems softener sizing guide.
Have mg/L (ppm)? Convert it in the converter above.
Sizing assumes 75 gallons per person per day.
7 days is the standard sizing assumption.
Adds 5 gpg per 1 mg/L of iron. Above about 5 mg/L, a dedicated iron filter is usually needed.
Iron compensation: dissolved iron fouls softener resin, so add 5 gpg to your hardness number for every 1 mg/L (ppm) of iron — the same rule our softener settings guide uses. The iron box above applies it for you.
Shopping for one? See our water softener guide for how ion-exchange softeners work and what to look for.
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Frequently Asked Questions
How do I convert ppm to gpg?
Divide ppm (mg/L as calcium carbonate) by 17.1 — one grain per gallon equals 17.1 mg/L. For example, 171 ppm ÷ 17.1 = 10.0 gpg, which is hard water.
What size water softener do I need for a family of 4?
Multiply your water hardness in gpg by 4 people × 75 gallons per person per day × 7 days between regenerations. At 10 gpg that is 10 × 4 × 75 × 7 = 21,000 grains, so a 24,000-grain softener is the right fit.
Do I need to adjust the softener size for iron?
Yes. Add 5 gpg to your hardness number for every 1 mg/L (ppm) of dissolved iron before sizing. For example, 15 gpg hardness with 3 mg/L iron sizes as 30 gpg. Softeners handle up to about 5 mg/L of dissolved iron — above that you likely need a dedicated iron filter.
What water hardness level needs a softener?
Most water treatment professionals suggest considering an ion-exchange water softener above 120 mg/L (7 gpg). Above 180 mg/L (10.5 gpg) a softener is strongly recommended to protect plumbing and appliances from scale.
Is 25 gpg very hard water?
Yes. 25 gpg is about 428 mg/L (25 × 17.1), well above the very-hard threshold of 180 mg/L (10.5 gpg). Only set a softener to 25 if your water actually tests at 25 gpg — setting it higher than your real hardness wastes salt and water.
More Water Hardness Resources
These tools are part of our water hardness hub — measured hardness for thousands of US cities, with the hardest cities, an interactive map, and a free open dataset. Every city value is sourced and tiered as described in our data & methodology.
Related water hardness pages
By TapWaterData Editorial
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