Due to geographical conditions and local geology, many municipal water supplies and private wells in North America contain significant amounts of calcium and magnesium and are therefore considered hard water. Hard water is generally safe for outdoor and indoor uses, but for household activities such as showering, washing dishes and clothes, and operating water-using appliances, soft water can provide practical benefits by reducing scale and improving soap and detergent performance.

The drawbacks of hard water include the need for more soap or detergent when washing; soap scum on bathroom surfaces, which can be difficult to clean; mineral deposits and spots on cups, dishes, faucets, and other surfaces; and scale buildup in water heaters and other appliances. Scale can reduce the efficiency of water-heating equipment and, over time, contribute to maintenance problems and reduced equipment life. Hard water can also leave mineral deposits and soap residue on the skin and hair, which some people may find unpleasant or drying.

The minerals primarily responsible for water hardness are dissolved calcium and magnesium. Hardness is commonly expressed as grains per gallon (GPG) or milligrams per litre (mg/L) as calcium carbonate. According to the Water Quality Association, water with less than 1 GPG is generally considered soft, while water above 7 GPG is considered hard.

Water Hardness Standards:

Soft water – Less than 1 GPG
Slightly hard water – 1–3.5 GPG
Moderately hard water – 3.5–7 GPG
Hard water – 7–10.5 GPG
Very hard water – Over 10.5 GPG

Water hardness in Ontario varies considerably depending on the local geology and water source. Municipal water supplies in different Ontario communities can have substantially different hardness levels, so residents should consult their local water-quality reports for current measurements rather than relying on a single province-wide figure.

Hard water can be treated in several ways. Chemical additives can help reduce the effects of hardness in some applications, including detergents and dishwashing products. The most common method for producing softened water in homes is an ion-exchange water softener. The softener exchanges calcium and magnesium ions in the water for sodium or potassium ions, reducing the minerals responsible for hardness and helping prevent scale formation.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026