Why do you have to wash before getting into the bath?
In a Japanese bath, onsen, or sento, you scrub and rinse fully at a separate washing station first, then soak in the tub only to warm up and relax. At home, family members share the same clean, unchanged bathwater.
The tub is for soaking, not for washing—so the water stays clean enough to share and reuse.
Why — five layers
Bathing is framed as a purification and relaxation ritual as much as hygiene; entering clean water while still dirty feels wrong.
Baths are shared—family reuses one tub of water, and onsen/sento are communal—so washing first protects others (meiwaku avoidance).
Heating a full tub is costly, so keeping the same water usable across several people and evenings has long made practical sense.
International view
Many cultures treat the bath itself as where you get clean and drain it after one person. Wash-first-then-soak communal bathing is closer to Korean and some other Asian bathing customs than to Western tub bathing.
But not always
Plenty of people, especially those living alone, just shower and skip the soak entirely. And a quick pre-onsen rinse (kakeyu) is fine—you don't need a full scrub every time.
What to do
At an onsen: rinse or wash at the seated shower first, keep your small towel out of the water, and don't drain or add cold to a shared tub.
Sources
- Common onsen/sento etiquette guidance; resident testimony