What’s the Difference Between Hardness in Mg/L and Gpg?

Written by Craig "The Water Guy" Phillips

• Mg/L measures mineral weight per liter of water, while GPG counts grains of minerals per gallon.
• One grain per gallon (GPG) equals approximately 17.1 milligrams per liter (mg/L).
• Mg/L is the scientific standard used in laboratories, while GPG is America's traditional measurement for water hardness.
• To convert from mg/L to GPG, divide by 17.1; to convert from GPG to mg/L, multiply by 17.1.
• Water softener manufacturers typically specify capacity in GPG, while water quality reports often use mg/L.

Understanding Water Hardness Measurements

When we talk about water hardness, we're actually referring to the concentration of certain minerals—primarily calcium and magnesium—in our water supply.p>

These minerals are measured using two common units: milligrams per liter (mg/L) and grains per gallon (GPG).

The mg/L measurement represents the weight of minerals per volume of water, while GPG has historical roots in the water treatment industry.

Converting between them is straightforward—simply divide the mg/L value by 17.1 to get GPG.

For context, water below 17.1 mg/L (or 1 GPG) is considered soft, while anything above 120 mg/L (7 GPG) is hard.

Understanding both measurements helps you properly assess your water's hardness level and choose appropriate treatment options for your home's specific needs.

The Science Behind Milligrams Per Liter (mg/L)

Milligrams per liter (mg/L) stands as the scientific standard for measuring water hardness in modern water treatment and analysis. This metric represents the precise weight of calcium carbonate (CaCO₃) dissolved in one liter of water, giving us a detailed picture of water quality at the molecular level.

When we're evaluating hardness classifications, we use mg/L as our reference point—water below 17.1 mg/L is considered soft, while levels exceeding 121 mg/L indicate hard or very hard water conditions. The precision of mg/L measurements enables water treatment professionals to develop targeted solutions for specific hardness challenges.

While GPG provides a more consumer-friendly measurement (with 1 GPG equaling approximately 17.1 mg/L), the mg/L standard gives us the scientific accuracy we need for sophisticated water quality assessments.p>

Grains Per Gallon (gpg):

Unlike the scientific standard of mg/L, grains per gallon (gpg) remains America's traditional water hardness measurement, deeply rooted in our water treatment industry. This practical unit tells us exactly how many grains of hardness minerals exist in each gallon of water, with one gpg equaling approximately 17.1 mg/L.p>

When selecting water treatment solutions, we'll typically reference this hardness classification system:

We've found that homeowners appreciate gpg's straightforward nature—it directly correlates to water softener settings and salt requirements in a way that's intuitive for daily use.p>

Converting Between Mg/L and Gpg:

Now that we've explored why Americans favor the gpg measurement, let's tackle the practical challenge many homeowners face: moving between these two common hardness units.p>

The conversion formula is remarkably straightforward: to convert mg/L to gpg, simply divide by 17.1. Going the other direction? Multiply your gpg value by 17.1 to get mg/L.

For example, if your water test shows 100 mg/L hardness, that's approximately 5.84 gpg.

This conversion becomes essential when selecting water treatment equipment, as manufacturers may specify capacity in either unit.

We've found this knowledge particularly valuable when comparing international water quality reports or when discussing treatment options with professionals who might use different measurement standards.

Choosing the Right Measurement for Water Treatment Solutions

When selecting water treatment equipment for your home, understanding which hardness measurement to reference can make the difference between an effective solution and a costly mistake. Many manufacturers specify their system capacities in GPG, while water testing labs often report results in mg/L.

For example, if you're considering a water softener rated for 32,000 grains, you'll need to know if your water's 10 GPG hardness level falls within its capacity. If your water report shows 171 mg/L, you'd convert this to approximately 10 GPG to make an accurate comparison.

We recommend becoming comfortable with both measurements and their conversion (1 GPG ≈ 17.1 mg/L).

This knowledge guarantees you'll select appropriately sized equipment that effectively addresses your specific water hardness issues, avoiding undersized systems that can't handle your water's mineral content.

Frequently Asked Questions

How Do You Convert Water Hardness From Mg L to Gpg?

We divide the mg/L value by 17.1 to convert it to gpg. For example, if your water tests at 100 mg/L hardness, that's about 5.8 gpg (100÷17.1).

What Is the Ideal Water Hardness in Mg/L?

We recommend ideal water hardness between 0-60 mg/L (soft water). This range prevents scale buildup in pipes and appliances while maintaining the beneficial minerals you'll want in your water.

What Does Hardness Gpg Mean?

GPG means "grains per gallon," a unit we use to measure water hardness. One grain equals about 17.1 mg/L of hardness minerals. We'll find this measurement on most water softener systems.

What Should My Water Softener Hardness Be Set at Gpg?

We recommend setting your water softener at 10 GPG for hard water or 3-5 GPG for moderately hard water. You'll want to test your water regularly and adjust as needed for ideal performance.