Kusatsu Onsen — photo: おにく · CC0 · Wikimedia Commons
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How hot is onsen water? Source and bath temperatures in Japan, town by town
"How hot is an onsen?" has two answers. The bath you sit in is usually kept around 40 to 42 °C (104 to 108 °F). The water that comes out of the ground can be anything from cold to boiling: in our records it runs from 6.3 °C to 105 °C. This guide explains the gap between the two, gives the source temperature for the towns people ask about most, and says plainly where we do not have a number.
The short answer
- In the bath: most baths are kept at about 40 to 42 °C. Some are hotter, 43 to 45 °C, especially public baths fed straight from a hot source. A few towns are known for very hot baths on purpose.
- At the source: of the 1,167 onsen towns in our records with a measured temperature, half come out of the ground at 41 °C or less and half above. 573 are 42 °C or hotter; 237 are below 25 °C.
- So most baths are adjusted: hot water is cooled before it reaches you, and cool water is heated. That is normal, and the notice in the changing room tells you which was done.
Two temperatures: at the source and in the tub
Japanese has separate words for them. 泉温 (sen'on) is the temperature at the source, measured where the water leaves the ground, and it is the number on the official analysis and in our records. 湯温 (yuon) is the temperature of the water in the bath. Some baths post it on a small board by the tub, and it can change through the day.
The two are rarely the same. An 84 °C source like Gero cannot be bathed in as it is; a 30 °C source is colder than your body. Only water that surfaces at roughly bathing temperature can go into the tub unchanged, and that is the smallest group in our records.
What our records say
We hold 1,538 onsen towns, and 1,167 have a source temperature on record. Taking the hottest source in each town, they fall into the four bands used on Japanese analysis sheets:
- Below 25 °C, 冷鉱泉 (reikōsen), cold mineral spring: 237 towns. Under Japanese law these still count as onsen if they carry enough minerals.
- 25 to 34 °C, 低温泉 (tei-onsen): 176 towns.
- 34 to 42 °C, 温泉 (onsen) in the narrow sense: 181 towns.
- 42 °C or hotter, 高温泉 (kō-onsen): 573 towns. Of those, 218 reach 60 °C, 78 reach 80 °C and 51 reach 90 °C.
Only 121 towns have a hottest source between 38 and 42 °C, close enough to a normal bath to use with little or no change. Everywhere else, someone has to cool the water down or warm it up. Where those hot and cold towns are on the map is in where Japan's hot springs are.
The hottest and the coldest
The hottest source in our records is Obama Onsen in Nagasaki, on the coast below Unzen, at 105 °C. Next come four towns at 100 °C, including Kannawa in Beppu, where steam rises from the street vents, Tsuetate in Kumamoto and Atagawa on the Izu coast. Unazuki in Toyama is 98.3 °C, and Tamagawa in Akita, Yumura in Hyōgo and the top source at Arima are 98 °C.
At the other end, 60 towns come out below 15 °C. The coldest on record is Yanagisawa in Tochigi at 6.3 °C. Better-known cold springs include Shimobe in Yamanashi at 21.7 °C and the black water of Tokyo's neighbourhood baths, around 18 °C; see onsen in Tokyo.
Famous towns, by the numbers
Source temperatures from our records, hottest source first where there is a range. Many towns have several sources, so a range means different baths in the same town draw different water.
- Kannawa, Beppu: 100 °C
- Arima: 40 to 98 °C
- Kurokawa (Kumamoto): 80 to 98 °C
- Kusatsu: 44.9 to 94.5 °C, with about 32,300 litres a minute
- Yunomine: 92 °C
- Hirayu: 90 °C
- Noboribetsu: 45 to 90 °C
- Nozawa: 42 to 90 °C
- Gero: 84 °C
- Kinosaki: 37 to 83 °C
- Shima: 43 to 82 °C
- Ibusuki: 82 °C
- Ginzan: 44 to 65 °C
- Shuzenji: 65 °C
- Ubako, Hakone: 44 to 63 °C
- Dōgo: 42 to 51 °C
- Naruko: 44 °C
- Tamatsukuri: 42 °C
For some towns people search for, we have no single source temperature: Hakone as a whole, central Beppu, Ikaho, Nyūtō, Unzen, Ureshino, Shirahone, Zaō, Kinugawa and Atami. Most of these are large areas with dozens of separate sources, and no one figure would be honest. Each inn or public bath posts the analysis for its own water, and the temperature is on it.
Of the towns with a range on record, 80 have sources at least 20 °C apart and 26 at least 40 °C apart. In those towns, the next bathhouse down the street may feel quite different.
How hot water is cooled, and cold water warmed
A spring that comes out at 90 °C has to lose about half its heat before anyone can sit in it. The usual ways:
- Adding water. Mixing in tap or well water, 加水 (kasui), is the simplest way. It also thins the minerals, which is why some baths avoid it.
- Letting it cool on the way. Water runs through long channels, pipes or a pool before it reaches the bath. The wooden channels of the yubatake in the middle of Kusatsu are the best-known example.
- Stirring. Kusatsu's yumomi, turning the water with long wooden boards, cools it without adding any. The town shows it to visitors several times a day at Netsu-no-yu, by the yubatake.
- Heat exchangers. Many inns pass the water through pipes that carry the heat away, so the minerals are not thinned.
Cool springs go the other way and are heated, 加温 (kaon). Heated baths are often also filtered and recirculated, 循環 (junkan), to save fuel. None of this is a trick. Whether it was done is on a notice in the changing room, marked 有 (ari), yes, or 無 (nashi), no. How to read that notice is in onsen water types.
The very hot baths: Kusatsu's jikan-yu
Most towns cool their water to a comfortable level. Kusatsu also keeps an old custom called 時間湯 (jikan-yu), "timed bath": the water is stirred, bathers pour it over themselves, and then everyone gets in together at about 48 °C (118 °F) for three minutes, timed by an attendant who calls out the count. It is offered at only a few bathhouses in the town, under the attendant's rules, and it is not what an ordinary Kusatsu bath is like. If you are curious, the yumomi show is the easy way to see the custom without taking part.
Elsewhere, a public bath fed straight from a hot source can be 44 or 45 °C. Local regulars often like it that way.
If you do not like hot water
- Choose a town whose source is near bathing temperature. Examples from our records: Tamatsukuri and Kakeyu at 42 °C, Tawarayama at 41 °C, Yunogō at 40 °C, Nagato Yumoto at 39 °C and Kirizumi at 38.9 °C.
- Try every tub. Places with more than one bath rarely keep them at the same temperature. Test with a hand or a foot first, then lower yourself in slowly.
- Look for a board by the tub. Where the temperature is posted, you know what you are getting into.
- Keep soaks short. Get out, sit for a while, and go back in if you like. Our guide to when to soak, how long, and how many times a day has more on this.
Things not to do
Do not run the cold tap into a shared bath without asking. In many local baths the regulars want it hot, and there may be a rule about it.
Do not read the source temperature as the bath temperature. A 90 °C town does not have 90 °C baths.
Do not stay in if you feel dizzy or too hot. Get out, sit down and drink some water.
New to this? Start with how an onsen stay actually works and onsen etiquette. For the water itself, see onsen water types, the best onsen water in Japan and sulfur onsen.