Shampoo bars work in hard water, but the experience and the economics change. The minerals in hard water react with bar soap-based formulas to leave a dull, waxy residue - which is why so many people quit bars before giving the workaround a try.
The vinegar-rinse workaround
A diluted apple cider vinegar rinse (about one tablespoon per cup of water) dissolves the mineral residue and restores shine. It is cheap - a bottle of ACV is a few dollars and lasts months - so the added cost per wash is usually pennies.
The hidden cost: fewer washes per bar
The real expense in hard water is reduced bar longevity. Residue tempts you to use more product, so a bar that should last 80 washes might give you 50-60. That alone can raise your cost per wash more than the vinegar ever will.
Factor it into the calculator
If you have hard water, lower the washes-per-bar number in the tool to reflect your real usage, and treat the vinegar rinse as a small recurring extra. You may find a synthetic-base (syndet) bar, which resists hard-water residue, is the cheaper long-run choice.