Dōgo Onsen — photo: Arnaud Malon · CC BY 2.0 · Wikimedia Commons
Onsen, sento or super sento? How to tell them apart
You are in Tokyo, you search for an onsen nearby, and the map sends you down a side street to a tiled building with a tall chimney and a curtain over the door. Inside, an older man at a raised desk takes a few coins, and the bath turns out to be one hot tub under a mural of Mt Fuji. It was a lovely bath. Was it an onsen? Probably not. But the answer is less obvious than most guides make it sound.
Three words for three kinds of place
Most English guides settle it in one line: a 銭湯 (sentō) heats tap water, an 温泉 (onsen) uses natural hot spring water, and a スーパー銭湯 (sūpā sentō) is a big sento with extras. That is right often enough to be useful and wrong often enough to confuse you, because the three words describe different things. "Sento" is a type of business. "Super sento" is a style of building. "Onsen" is a description of the water. One place can be two of them at once.
The sento: a neighbourhood bath with a fixed price
A sento is a public bathhouse built for people who live nearby, including, historically, people whose flats had no bath. The standard kind is classed as a general public bath, and its adult entry price is not set by the owner but capped by the prefecture, so every sento in a prefecture charges the same. Across Japan that price sits roughly in the ¥450 to ¥600 range. Children pay less.
What you get for that: a changing room with lockers or baskets, a row of low shower stations, and one to three tubs, often very hot. Many old sento have a painted mountain on the wall and a raised desk, the 番台 (bandai), where the attendant can see into both changing rooms. Towels, soap and shampoo are usually not included; you bring them or buy a small set at the desk. Some sento also have a sauna for a small extra charge.
A sento is a daily routine, not an outing. People come after work, bathe for twenty minutes and go home. They are open in the late afternoon and evening, usually closed one day a week, and they are quiet, local and cheap.
The super sento: a day out, priced like one
A super sento is a large bathing complex, usually in the suburbs or next to a big road, with car parking, many baths indoors and out, saunas, a restaurant, a room with reclining chairs to nap in and, often, massage. Its price is not capped, so it varies: somewhere around ¥900 to ¥3,000 is typical, more on weekends, and towels are either included or rented at the counter. People stay for half a day.
Many super sento pump real hot spring water from a deep well and advertise it as 天然温泉 (tennen onsen), "natural hot spring". Others use heated tap water with bath salts added, and some mix both: one outdoor tub of spring water, the rest ordinary. The sign at the entrance usually tells you which, and the board on the wall inside tells you for certain (more on that below).
What makes water an onsen, and why it can be cold
In Japan, "onsen" is a legal term. Under the Hot Springs Act of 1948, water or gas coming out of the ground counts as an onsen if it is at least 25 °C where it emerges, or if it contains enough of any one of a list of specified dissolved substances, whatever its temperature. That second door matters more than most visitors expect.
Of the 1,538 onsen towns in our records, 1,167 have a recorded source temperature. They spread out like this:
- 237 towns never reach 25 °C at any recorded source. Their water qualifies on its minerals alone and is heated before anyone can sit in it.
- 176 are between 25 and 34 °C: warm to the hand, still cooler than a bath.
- 181 are between 34 and 42 °C, around body temperature to a comfortable bath.
- 573 reach 42 °C or more at the source, and 51 of those reach 90 °C or more.
So about one onsen town in five is running a cold spring through a boiler. Kashio Onsen, high in the mountains of Nagano, is recorded at 14 °C and is a chloride spring: salt water, far from any sea. Tsunashima Onsen in Yokohama is recorded at 18 °C, a bicarbonate spring. Both are onsen in the full legal sense, and both are heated. A heated onsen is not a fake one. It is just a different kind of water from the ones that come out of the ground too hot to touch.
When a sento is also an onsen
This is where the neat three-way split breaks. A sento is defined by how it runs and what it charges, not by where its water comes from. If a neighbourhood bathhouse sits on a well that qualifies, it can be both: a sento at the standard price, filled with onsen water.
Tokyo's best-known case is the 黒湯 (kuroyu), "black water", of Ōta Ward in the south of the city. Ōta Ward's black-water springs are recorded at 18 °C and are chloride and bicarbonate springs. The water is the colour of strong tea, sometimes darker, dark enough that you cannot see your feet, and it feels smooth on the skin. Several neighbourhood sento in the area use it, so the tiled bathhouse with the chimney may in fact be an onsen, at sento prices.
Deep wells have put onsen water under the city centre too. Ōtemachi Onsen, among the office towers near Tokyo Station, is recorded at 36.5 °C, pH 7.3, a chloride spring. And Keio Takaosan Onsen at the foot of Mt Takao, a day-spa building of the super sento kind, is recorded at 26.2 °C and pH 9.9, strongly alkaline, which you feel as a slippery smoothness.
Reading the board on the wall
You do not have to guess. Any bath that uses onsen water must post a spring analysis, the 温泉分析書 (onsen bunsekisho), usually a framed sheet near the changing room. It lists the source temperature, the pH, the spring type, and what is dissolved in it. If a place calls itself an onsen and you cannot find this sheet, ask at the desk.
Near it, the bath must also say whether the water has been changed on its way to you. Look for four words:
- 加温 (kaon): heated. Expected for any source under about 40 °C.
- 加水 (kasui): water added, usually to cool water that is too hot or to stretch a small flow.
- 循環 (junkan): recirculated and filtered, rather than flowing through once.
- 消毒 (shōdoku): disinfectant added, usually chlorine, which you may smell.
Each will be marked あり (ari), yes, or なし (nashi), no. The phrase bathers look for is 源泉かけ流し (gensen kakenagashi): source water that flows into the tub and out over the edge, not reused. None of these makes a bath good or bad by itself, but they explain why two onsen with the same spring type can feel completely different.
If the sign says 人工温泉 (jinkō onsen), "artificial onsen", the water is ordinary water with minerals or bath salts added. It is not an onsen in the legal sense, whatever it looks like.
Which one to go to
Go to a sento for the ordinary city bath: cheap, short, local, open in the evening. It is the closest you will get to how people in Tokyo or Osaka actually bathe on a weekday.
Go to a super sento when you want a long afternoon somewhere comfortable, with food, a nap and several baths, and when you do not want to bring anything.
Go to an onsen town when the water itself is the point: a sulfur spring that smells of the mountain, an iron spring that turns rust-brown in the air, a bath fed straight from a source at 50 °C. You cannot bring that water to a city, and that is what the train ride is for.
Things not to do
Do not walk into a sento expecting towels and shampoo. Bring a small towel and cash; coins and ¥1,000 notes are safest.
Do not treat "onsen" on a sign as a promise of hot water from the ground. Read the board, and do not be disappointed by 加温: a fifth of Japan's onsen towns could not run a bath without it.
Do not assume one place's tattoo rule applies to the next. Neighbourhood sento are often more relaxed than large super sento, which are often the strictest of all, but neither is guaranteed.
And the rules in the water are the same everywhere: wash before you get in, keep the towel out of the tub, and keep your voice down.
New to all of this? Start with how an onsen stay actually works. For bathing without an overnight stay, see onsen day trips, and for tattoos, going to an onsen with tattoos.