Can you wear an Apple Watch in an onsen? Heat, minerals, and what Apple's guidance means for hot springs

Onsen are the reason Furolog exists, so this is the question we get most. The answer is yes, with care, and the care depends on two things Apple's guidance mentions only indirectly: how hot the water is, and what is dissolved in it.

Yes, with care. Apple rates Series watches to 50 m and Ultra to 100 m for swimming, and its list of things to avoid names steam rooms and saunas, not hot springs. The two things to manage are heat (keep soaks to about ten minutes in water around 41°C, which is also the safety guidance for you) and minerals (rinse in fresh water every time, and think twice about strongly acidic springs, since acids are on Apple's own avoid list). Only Ultra has a published water temperature range, 0 to 40°C, so a 42°C bath is outside Apple's numbers on every model.

Can you wear an Apple Watch in an onsen? What the water resistance rating covers

Apple Watch Series 2 and later are water resistant to 50 metres under ISO 22810:2010. Apple Watch Ultra is rated to 100 metres. Both are meant for swimming in a pool or the sea. Neither rating says anything about temperature, and Apple's guidance adds three caveats that matter in a hot spring:

• Water resistance "isn't a permanent condition and can diminish over time", and the watch cannot be rechecked or resealed.
• Contact with soap or soapy water, perfume, solvents, detergent, acids or acidic foods, lotions, sunscreen, oil, and similar substances should be avoided. If it happens, Apple says to rinse with fresh water and dry with a lint-free cloth.
• Separately, Apple's limited warranty excludes damage from liquid contact and from operating the product outside Apple's published guidelines.

Apple's list of things to avoid names steam rooms and saunas. It does not name hot springs or hot tubs. The closest official numbers are the general operating range of 0 to 35°C ambient, and the water temperature Apple specifies for underwater activities on Apple Watch Ultra: 0 to 40°C. Series 10 and Series 11 have the same water temperature sensor, but Apple publishes no water temperature range for them.

Many onsen baths are around or above the upper end of Ultra's stated range. A reading may still appear, but accuracy and device durability are not guaranteed outside Apple's specifications. Personal experience is not a substitute for the manufacturer's guidance.

Heat: the 41°C rule helps the watch too

Japan's Consumer Affairs Agency recommends water no hotter than 41°C and soaks of no more than about ten minutes to reduce the risk of bathing accidents. That advice is for people, not a durability guarantee for Apple Watch. Hotter water and longer exposure may place more stress on both the wearer and the device.

Minerals: this is where onsen differ from a pool

Onsen water is classified by what is dissolved in it, and some of those things are on Apple's list of substances to rinse off. Here is how the common spring types compare from the watch's point of view.

Two practical points come out of that table. First, rinse every time. Apple's own advice for any of the listed substances is fresh water and a lint-free cloth, and the habit costs ten seconds at the wash station. Second, acidic springs are the one case where we would say the guidance is against you and the chemistry agrees. The famous strong-acid baths are worth visiting, but maybe with the watch in the locker.

Etiquette: the part nobody writes about

Wearing a smartwatch in a shared bath is not against the rules at most onsen, but it can draw looks, and a few facilities ask that you remove watches and jewelry. Some worry about scratching the tub, some about cameras. Apple Watch has no camera, but staff and other bathers may not know that.

What has worked for us: keep the watch low and out of sight, never raise the wrist to look at it while other people are in the bath, and if a sign or a staff member says no watches, take it off without arguing. Furolog runs on the wrist and needs no attention during the soak, which is the point. The record is there when you get out.

What we have seen in a year

We have bathed with Apple Watch almost daily for about a year while building Furolog, mostly in simple, chloride, and sulfur springs between 40 and 44°C, with the occasional acidic bath rinsed off straight after. No seal failures, no display problems, no battery health difference from a watch that stayed dry, and no lasting smell on fluoroelastomer bands. Sulfur springs did dull the finish on a stainless case slightly over time.

That is our experience across a few watches, not a guarantee. If your onsen habit is a weekly hour in a 45°C acidic bath, you are outside anything we have tested.

The sensible way to bathe with one

• Rinse before and after. Before, because you should anyway. After, with fresh water, including the band and the sensor on the back.
• Keep soaks around ten minutes at 41°C. The heat shock rule and the watch's comfort zone are the same rule.
• Turn on Water Lock from Control Center before you get in. It stops the wet screen registering taps and ejects water from the speaker afterwards. It does not make the watch any more water resistant.
• Use a silicone or fluoroelastomer band. Leather absorbs the water and the smell. Metal bands trap minerals in the links.
• Skip strong acidic springs, or be quick. pH 2 water is the one thing on this list that Apple names directly.
• Dry the speaker grille and the back before charging.

What Furolog does in the bath

Furolog starts recording automatically when the watch detects submersion, and logs the water temperature every five seconds until you get out. The session shows the temperature curve, your heart rate, and how long you soaked. Tag the spring type and the prefecture afterwards and the session feeds your prefecture map and your per-spring-type statistics. A 41°C simple spring and a 43°C chloride bath end up looking quite different on the chart, which is half the fun.

Temperature logging needs a watch with the water temperature sensor: Series 10, Series 11, or any Ultra. The list is in which Apple Watch models have a water temperature sensor. For saunas, which are a different question, see can you wear an Apple Watch in a sauna.

Read next: Apple Watch in the bath and sento: what's safe · The 10 types of onsen water, explained

Sources

• Apple Support. "About Apple Watch water resistance."
• Apple Support. "Keep Apple Watch within acceptable operating temperatures."
• Apple. "Apple One (1) Year Limited Warranty."
• Apple Support. "How to use Water Lock and eject water from your Apple Watch."
• Apple Support. "Measure underwater temperature, duration, and depth on Apple Watch." Apple Watch User Guide.
• Consumer Affairs Agency of Japan. "冬季に多発する高齢者の入浴中の事故に御注意ください！" News release, 19 Nov 2020.
• Ministry of the Environment, Japan. "鉱泉分析法指針（平成26年改訂）" (Guidelines for analysis of mineral springs, 2014 revision).

Quick answer

• Rating: 50 m (Series) or 100 m (Ultra) for swimming. Says nothing about heat, and diminishes with age.
• Heat: typical onsen are 40 to 42°C, at or just above the 0 to 40°C water range Apple publishes for Ultra (Series 10 and 11 have no published range). Works in practice. Keep soaks short.
• Minerals: rinse every time. Sulfur tarnishes, salt corrodes, acid is the one Apple names. Skip pH 2 baths or rinse immediately.
• Etiquette: keep it low, never look at it in front of others, and comply if asked to remove it.