GPG, or Grains Per Gallon, is the standard unit of measurement used primarily across the United States and Canada to quantify the hardness of water. Water hardness is a natural characteristic caused by the presence of dissolved multivalent metallic ions, predominantly calcium and magnesium. These minerals are picked up as water travels through underground geological formations, such as limestone and chalk, before reaching the tap. Understanding the GPG value of your water supply is the first step in assessing its potential impact on your household plumbing and appliances.
Defining GPG and Water Hardness
The term “grain” is an old unit of measurement, dating back to the weight of a single grain of wheat or barley. In the context of water hardness, one grain is equivalent to approximately 64.8 milligrams of dissolved solids. The GPG measurement quantifies the total weight of these dissolved hardness minerals in one U.S. gallon of water.
One GPG means one gallon of water contains one grain of dissolved hardness minerals, chemically expressed as calcium carbonate. Calcium and magnesium ions are responsible for hard water properties. For conversion purposes, one GPG is equal to 17.1 parts per million (PPM) or 17.1 milligrams per liter (mg/L).
Classifying Water Quality by GPG Levels
The Water Quality Association (WQA) provides a widely recognized scale for classifying water based on its GPG measurement. This classification system helps consumers and professionals determine the degree of hardness and the need for water treatment.
The WQA classifies water hardness into five categories:
- Soft: 0 to 1 GPG.
- Slightly hard: 1 to 3.5 GPG.
- Moderately hard: 3.5 GPG to 7 GPG.
- Hard: 7 GPG to 10.5 GPG.
- Very hard: Exceeds 10.5 GPG.
The Impact of High GPG Water
Water with a high GPG rating causes practical problems due to the chemical behavior of calcium and magnesium ions. When water is heated or evaporated, these dissolved minerals precipitate out of the solution to form a chalky solid known as limescale. This scale buildup affects appliances and plumbing, particularly where water flow is restricted or temperature is elevated.
Scale buildup inside water heaters and boilers acts as an insulator, reducing heat transfer from the heating element to the water. The U.S. Department of Energy estimates that even a thin layer of scale can decrease a water heater’s efficiency by up to 12%. This forces the appliance to work harder, leading to increased energy consumption and higher utility bills.
In plumbing, limescale accumulation inside pipes narrows the inner diameter, restricting water flow and reducing water pressure over time. This mineral residue also clogs the internal components of water-using appliances like dishwashers, washing machines, and coffee makers. Deposits can obstruct spray arms or damage pumps and valves, shortening the appliance’s lifespan.
Hard water interferes with the cleaning process by reacting with soaps and detergents. Calcium and magnesium ions bind with the fatty acids in soap, preventing lather formation and creating an insoluble residue called soap scum. This reduces the detergent’s cleaning power, requiring more product, and leaves behind film on fixtures, dishes, and laundry.