Consumer Reports has found in its tests that liquid detergent measuring cups are often impossible to read, leading to overdosing of detergents, which itself can leave detergent residues that make your clothes look dingy.
They suggest marking the fill line with a permanent marker so it’s easy to see. And when it comes to actual amounts, follow your machine manufacturer’s recommendations, not the detergent manufacturer’s. “The manufacturer wants you to use as much detergent as possible,” he says, but they don’t know what kind of washing machine you have.
Also, the softness or hardness of your water affects how much detergent you need to use. Soft water usually requires less detergent, Green says, while hard water usually requires the full amount. If you have hard water, Green also suggests adding a water softener, such as baking soda, to help your soap dissolve. Start with equal parts detergent and baking soda, and then experiment from there.