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Water Hardness Calculator
Convert between GPG, PPM, mg/L, and mmol/L — and calculate your water softener hardness setting
Contents
Exact Conversion Factors and Formulas
Water hardness is expressed in multiple units across different industries and countries. All are measuring the same thing — dissolved calcium and magnesium expressed as calcium carbonate (CaCO₃) equivalent — but in different scales.
| From | To GPG | To mg/L (PPM) | To mmol/L | To °dH | To °fH |
|---|---|---|---|---|---|
| 1 GPG | 1.000 | 17.118 | 0.1711 | 1.0420 | 1.7118 |
| 1 mg/L (PPM) | 0.05842 | 1.000 | 0.01000 | 0.06086 | 0.10000 |
| 1 mmol/L | 5.842 | 100.09 | 1.000 | 6.086 | 10.009 |
| 1 °dH | 0.9591 | 17.848 | 0.17848 | 1.000 | 1.7848 |
| 1 °fH | 0.5842 | 10.000 | 0.10000 | 0.5604 | 1.000 |
| GPG = grains per gallon (US). mg/L = milligrams per liter as CaCO₃ (= PPM for water). mmol/L = millimoles per liter. °dH = German degrees of hardness (Deutsche Härte). °fH = French degrees of hardness. | |||||
The GPG to PPM Formula
The exact conversion is derived from first principles: 1 grain = 64.799 mg, and 1 US gallon = 3,785.41 mL. Therefore 1 grain per gallon = 64.799 ÷ 3,785.41 × 1,000 = 17.118 mg/L. Most water treatment professionals round this to 17.1 for field calculations, which introduces less than 0.1% error — acceptable for all practical applications.
How to Calculate Your Softener Hardness Setting
The hardness setting on a water softener controls when the unit regenerates — it tells the control valve how many grains of hardness the resin is treating per gallon, so it can calculate when the resin bed will be exhausted and schedule regeneration accordingly.
The Iron Compensation Formula
Iron competes with calcium and magnesium for ion exchange sites on softener resin. A standard correction factor accounts for this: add 5 GPG to the hardness setting for every 1 PPM of iron in the feed water.
Softener Setting (GPG) = Total Hardness (GPG) + (Iron PPM × 5)
Example: 15 GPG hardness + 0.8 PPM iron = 15 + (0.8 × 5) = 15 + 4 = 19 GPG softener setting.
Softener Capacity and Regeneration Frequency
Softener capacity is rated in grains — the total grains of hardness the resin can remove before regeneration. Divide capacity by daily grain demand to get days between regenerations:
Daily Grain Demand = Hardness Setting (GPG) × Daily Water Use (gallons)
Days Between Regens = Softener Capacity (grains) ÷ Daily Grain Demand
Target 5–7 days between regenerations for most commercial applications. More frequent than every 3 days increases salt and water use; less frequent than every 10 days risks hardness breakthrough as the resin bed approaches exhaustion.
How to Calculate Hardness from a Titration Test
The Hach 5-B and 5-EP titration kits use EDTA (ethylenediaminetetraacetic acid) to chelate calcium and magnesium ions. Each drop of EDTA titrant neutralizes a fixed amount of hardness ions, and the color change from red to blue marks the endpoint. The number of drops multiplied by the titrant factor gives hardness in GPG.
Standard 5-B Calculation (25 mL sample)
Hardness (GPG) = Number of Drops × Titrant Factor
(For standard 25 mL sample with Hach 5-B: 1 drop = 1 GPG)
If the water is very hard and requires dilution before testing, multiply the result by the dilution factor. For example, if you diluted 5 mL of sample to 25 mL (5× dilution) and used 8 drops, actual hardness = 8 drops × 5 = 40 GPG.
When to Dilute
Dilute the sample when you expect hardness above 25–30 GPG (the upper limit of most titration kits without dilution), or when the endpoint color change is ambiguous because of very high ion concentration. A 2× dilution (12.5 mL sample + 12.5 mL deionized water, brought to 25 mL) doubles the effective range.
Hardness Classification Scale
| Classification | GPG | mg/L (PPM) | °dH | Practical Effect |
|---|---|---|---|---|
| Soft | 0–3.5 | 0–60 | 0–3.7 | No scaling. Soap lathers freely. |
| Moderately Hard | 3.5–7 | 61–120 | 3.7–7.3 | Minor scale on heating elements. Softener beneficial for sensitive equipment. |
| Hard | 7–10.5 | 121–180 | 7.3–10.9 | Visible scale, soap issues, staining. Softener recommended. |
| Very Hard | >10.5 | >180 | >10.9 | Rapid equipment damage. Softener required before RO, boilers, tankless heaters. |
| Classification per USGS Water Science School. Thresholds for equipment protection may be lower than USGS "hard" threshold — confirm with equipment manufacturer specifications. | ||||
FAQ
How do I convert GPG to PPM?
Multiply GPG by 17.118. Example: 10 GPG × 17.118 = 171.2 PPM. For quick field math, multiply by 17. To go the other direction — PPM to GPG — divide by 17.118 (or 17 for quick estimates).
How do I set water softener hardness based on my test result?
Set the hardness setting to your total hardness in GPG, then add 5 GPG per PPM of iron. Use the softener calculator above for the exact figure. The setting controls regeneration timing — if set too low, the resin exhausts before regeneration and hard water breaks through; if set too high, the unit regenerates unnecessarily, wasting salt and water.
What is the difference between total hardness and calcium hardness?
Total hardness = calcium hardness + magnesium hardness, both as CaCO₃ equivalent. Most test kits measure total hardness. Calcium hardness alone matters for pool water Langelier Saturation Index calculations and some industrial processes. For softener sizing and most commercial water treatment purposes, total hardness is the correct value to use.