A 100g bar of soap is about the size of a deck of cards and weighs slightly more than one. That is the object in question, three and a half ounces of hardened oils and lye, dense in the hand, ready to be worn down by water and use.
How long it lasts depends on how it is used. A standard 100g bar, used by one person for body washing only and dried properly between uses, typically lasts four to six weeks. Used for both face and body, three to four weeks. Shared across a household, multiple people, sometimes left wet, two to three weeks. Those figures are honest averages, not promises. The range is wide because the variables are real.
What the bar is made of changes the maths
The single largest difference between bars is hardness, and hardness comes from curing. A cold-process bar cured for six weeks or more loses most of its water content before it ever reaches a shower. It is denser, harder, and dissolves more slowly than a bar pulled from the mould after three weeks. The well-cured bar resists water; the young bar gives in to it.
This is one reason curing time matters beyond scent and feel. A longer cure produces a bar that simply outlasts a shorter one, sometimes by weeks. The relationship between formula, cure, and lifespan is worth understanding if you want to know why two bars of identical weight behave so differently, and it connects directly to the question of whether bar soap expires at all. A hard, well-cured bar is stable for a long time, both in storage and in use.
Oil composition plays a part too. Bars high in olive oil, true Castile and its relatives, are famously slow to wear. Castile soap is built to last for years in part because the same density that makes it gentle also makes it stubborn against water.
What you do with it matters more
Bar hardness sets the ceiling. Daily habits decide where within that range a bar actually lands.
Storage is the first variable. A bar left sitting in standing water, in a flat dish, in a puddle on a ledge, softens and dissolves quickly. The same bar in a draining dish, lifted off the surface so air can reach it, can rest for months between uses without losing shape. The difference is not marginal. A bar that drains lasts several times longer than one that sits in its own runoff.
Water exposure during washing is the second. A bar dissolves where it meets running water, so holding it directly under the stream wears it down fast. Wetting the bar briefly, then setting it aside while you wash, conserves it.
Use pattern is the third. Lathering with a cloth or hands and applying the lather to skin uses far less soap than rubbing the bar across the body directly. The cloth carries the soap; the bar stays mostly intact. Vigorous direct application, by contrast, grinds the bar down with every wash.
These factors compound. A young bar, held under the stream, left in standing water, applied directly to skin, might disappear in under two weeks. A well-cured bar, drained between uses and lathered with a cloth, might last past six.
What it costs per wash
The upfront price of a 100g craft bar, roughly $12 to $18, looks high beside a bottle of supermarket body wash. Per use, the picture changes. A bar lasting four to six weeks of daily single-person washing covers thirty to forty washes or more. Spread across that, the cost per wash sits comfortably alongside mass-market body wash, often below it. The number on the shelf feels larger; the number per use does not. Liquid formats carry their own considerations, liquid soap expires on a different timeline than bars, and bottled products dispense more product per use than most people notice.
If you want the bar to last, there is one thing worth doing above all others. Let it dry between uses. Lift it out of the water, give it air, and let it firm up before the next wash. Drying is the single biggest factor in how long a bar lasts, larger than the formula, larger than the price, larger than anything else you control. A bar that dries fully between washes will outlast its own averages.