How much detergent should I use based on my water hardness?

The amount of detergent depends on both water hardness and soil level. For soft water (0–4 grains per gallon), use 5–8 teaspoons per cup; for medium water (5–9 gpg), use 7–10 teaspoons; and for hard water (10–12 gpg), use 9–12 teaspoons, with higher amounts for heavier soil levels. In extremely hard water (13 grains per gallon or more), a mechanical water softener may be necessary for good results.