How long a shampoo bar lasts is the single biggest lever on its cost per wash, and the numbers brands quote (80-100 washes) rarely match what people actually get. Real-world longevity depends on how you use it, your hair type, and your water.
The honest range
Most users land at 45-80 washes per bar. Brand-ideal conditions push that to 80-100, while heavy-handed everyday use can drop it to 45-55. We default the calculator to 65 raw washes as a fair midpoint before any technique bonus.
Lather in your hands, not on your hair
Rubbing the bar straight onto wet hair wears it down fast. Working a lather in your hands and applying that instead can nearly double the bar's life, which is why the calculator adds roughly a 1.5x bonus when you choose that method.
Hair length and water matter
Long or thick hair uses more product per wash, and hard water can leave residue that tempts you to keep scrubbing. Both shorten bar life. If yours runs out faster than expected, lower the washes-per-bar figure in the tool to see your true cost per wash.