Hard water is a dosing factor, not a fixed percentage
Hard water contains dissolved minerals such as calcium and magnesium. The American Cleaning Institute notes that hard water can reduce cleaning performance and that some laundry situations call for slightly more detergent than a baseline label dose. The important word is product-specific: concentration and formulation differ.
Find your water-hardness information
Your local water utility may publish hardness data. If you use a private well, a water test is more useful than guessing from soap feel or mineral spots. Record the result in the same units used by your detergent guidance when possible.
Start with the package directions
Use the dose printed for your washer type, load size and soil level. If the product provides a hard-water dose, that instruction takes priority. WashCareLab’s calculator can scale a known reference dose by load weight, but it deliberately does not add a universal hard-water percentage.
Watch the outcome, not just the measuring cup
Persistent soil, residue or oversudsing can have multiple causes: overloading, an unsuitable cycle, too much product, too little product or poor rinsing. Change one variable at a time and keep the appliance manual in the decision.
Garment care labels, appliance manuals and laundry-product directions can be more specific than a general guide. Use them when they set a tighter limit.