PPM ↔ GPG Converter
Enter a value in either field and the other converts automatically. The color-coded result shows your hardness classification.
PPM to GPG Converter
Formula: PPM ÷ 17.1 = GPG. Based on the USGS water hardness classification scale.
Don't know your water hardness? Look up your zip code for a free estimate based on EPA and USGS data.
Water Hardness Scale
The USGS (United States Geological Survey) defines five hardness classifications. This is the standard used by water treatment professionals and the scale we use across WaterHardness.org:
| Classification | PPM (mg/L) | GPG | What It Means |
|---|---|---|---|
| Soft | 0 – 60 | 0 – 3.5 | No treatment needed |
| Slightly Hard | 61 – 120 | 3.5 – 7.0 | Minor spotting, treatment optional |
| Moderately Hard | 121 – 180 | 7.0 – 10.5 | Consider a conditioner or softener |
| Hard | 181 – 250 | 10.5 – 14.6 | Softener recommended |
| Very Hard | 250+ | 14.6+ | Softener strongly recommended |
Common Conversions Reference
Quick reference for the most commonly searched conversions:
| PPM | GPG | Classification |
|---|---|---|
| 25 PPM | 1.5 GPG | Soft |
| 50 PPM | 2.9 GPG | Soft |
| 75 PPM | 4.4 GPG | Slightly Hard |
| 100 PPM | 5.8 GPG | Slightly Hard |
| 120 PPM | 7.0 GPG | Slightly Hard |
| 150 PPM | 8.8 GPG | Moderately Hard |
| 180 PPM | 10.5 GPG | Moderately Hard |
| 200 PPM | 11.7 GPG | Hard |
| 250 PPM | 14.6 GPG | Hard |
| 300 PPM | 17.5 GPG | Very Hard |
| 400 PPM | 23.4 GPG | Very Hard |
| 500 PPM | 29.2 GPG | Very Hard |
What Is PPM?
PPM stands for parts per million. For water hardness, it measures milligrams of calcium carbonate (CaCO₃) equivalent per liter of water. It's the same as mg/L — the units are interchangeable.
PPM is the standard unit used by the EPA, USGS, and most water quality reports. When your city's Consumer Confidence Report (CCR) lists water hardness, it's almost always in PPM or mg/L.
What Is GPG?
GPG stands for grains per gallon. It's the unit used by the water softener industry. One grain equals approximately 64.8 milligrams of calcium carbonate dissolved in one US gallon of water — which works out to 17.1 mg/L (PPM).
Water softener capacity is measured in “grains” — a 32,000-grain softener can remove 32,000 grains of hardness before it needs to regenerate. Knowing your GPG lets you calculate how long a softener will last between cycles:
Daily grains = GPG × gallons used per day
Days between regeneration = softener capacity ÷ daily grains
Example: 10 GPG × 300 gal/day = 3,000 grains/day. A 32,000-grain softener lasts ~10 days.
Real City Examples
What to Do About Your Number
- 0-60 PPM (0-3.5 GPG): Your water is soft. No treatment needed.
- 61-120 PPM (3.5-7 GPG): Slightly hard. A shower filter may help with skin and hair if you're sensitive.
- 121-180 PPM (7-10.5 GPG): Moderately hard. Consider a salt-free conditioner for scale prevention.
- 181-250 PPM (10.5-14.6 GPG): Hard. A water softener is recommended.
- 250+ PPM (14.6+ GPG): Very hard. A water softener is strongly recommended. See our decision guide.
Check your city's water hardness
Look up real hardness data for your zip code or city - free, instant, and based on EPA & USGS sources.
Frequently Asked Questions
How do you convert PPM to GPG?
Divide PPM by 17.1 to get GPG. For example, 200 PPM ÷ 17.1 = 11.7 GPG. This conversion is based on the definition that 1 grain per gallon equals 17.1 milligrams per liter (mg/L) of calcium carbonate.
How do you convert GPG to PPM?
Multiply GPG by 17.1 to get PPM. For example, 10 GPG × 17.1 = 171 PPM. Water softener companies typically use GPG, while water quality reports and test kits often report in PPM or mg/L.
What PPM is considered hard water?
According to the USGS scale: 0-60 PPM is soft, 61-120 PPM is slightly hard, 121-180 PPM is moderately hard, 181-250 PPM is hard, and above 250 PPM is very hard. Most experts recommend a water softener starting at 120 PPM (7 GPG).
Is PPM the same as mg/L for water hardness?
Yes, for practical purposes PPM and mg/L are interchangeable when measuring water hardness. Both express the concentration of calcium carbonate (CaCO₃) equivalents in water. A reading of 200 PPM is the same as 200 mg/L.
Why do water softener companies use GPG instead of PPM?
Water softener capacity is measured in 'grains' — the amount of hardness minerals the resin can absorb before regeneration. Using GPG makes it easy to calculate how long a softener will last: multiply GPG × daily gallons used to get daily grains consumed, then compare to the softener's total grain capacity.