Furolog
Breathing pacer
Box, long-exhale, or coherent breathing on the Watch while you soak. Optional, quiet, never automatic.
Onsen & Sauna Tracker for Apple Watch
Your baths & sauna, logged by your Apple Watch.
Furolog turns your Apple Watch into a journal of your bathing life. Water temperature, heart rate, and every round, recorded the moment you step in. See how your body responds, and collect every onsen you visit.
How many have you collected?
47
Prefectures to collect
13
Spring qualities
12
Sauna styles
How it works
Three steps. One of them is a bath.
-
Wear your watch
Open Furolog on Apple Watch before you undress. That's the only tap.
-
Step into the water
Submersion starts the recording. Move between baths, sauna, and rest. Furolog follows.
-
Read your session
Back on your phone: the temperature curve, your heart rate, every round on a timeline. Add a photo and a note.
01 · Record
Records the moment you step in.
The submersion sensor on Apple Watch starts the recording when you enter the water. Temperature is captured continuously, heart rate throughout, and soak, cold plunge, sauna, and rest are classified as you move. On watches without the sensor, tap Start and dial the temperature in with the Digital Crown.
- Auto water detection Submerge and the recording starts within seconds. Leave the water and it waits a moment, in case you're only resting your arms on the edge.
- Knows when you switch baths Move from the 42° bath to the 38° one without getting out, and Furolog logs two dips, not one.
- Sauna detection A sustained heart-rate rise after you sit down asks: In a sauna? One tap confirms. Leaving is detected the same way.
02 · Look after yourself
A tap on the wrist when it's time for a break.
Heat is the point, and too much of it is the risk. Furolog watches your heart rate against your own reserve, tells you when to step out, and when you've recovered enough to go back in.
- Heart-rate alerts A warning at 75% of your heart-rate reserve, a green Recovered once you're back under 50%. Both thresholds are yours to set.
- Left the bath? Walking detection and a quick confirmation keep the timeline honest without you touching the screen.
- Forgot to end it? Lost track in the changing room? Furolog trims the trailing rest. A session nobody is driving ends itself instead of running all night.
03 · Analyze
Find your perfect session.
Furolog learns which length, hour, weather, and bath type you rate highest, and shows you the pattern. Alongside it: heat and cold tolerance scores, heart-rate recovery after each sauna round, and how your comfort zone shifts with the seasons.
- Your Perfect Session Optimal duration, time of day, weather, and bath type, derived from the sessions you rated best.
- Heat and cold tolerance Scores that track how your body handles the sauna and the plunge, session after session.
- Personal bests Hottest, coldest, longest, biggest hot-to-cold contrast, busiest month.
04 · Collect
Forty-seven prefectures. How many have you soaked in?
Every session with a location lights up a prefecture on your map of Japan. Spring qualities and sauna styles are collected too, from simple and sulfur to löyly and ganbanyoku. And with anonymous data from other Furolog users, you can see how hot a bath runs before you go.
- Prefecture tracker An interactive heatmap of Japan, coloured by how many places you've visited in each prefecture. Works offline.
- 13 spring qualities, 12 sauna styles Tag each bath and watch your collection grow. Tanjun-sen to houshanou-sen, dry sauna to sand bath.
- Know the water before you go Community visit counts, ratings, water temperature per bath type, and the hours the regulars show up. Anonymous, facilities only, and yours to switch off.
05 · Journal
A journal, not just a log.
Add up to ten photos, a rating, and a note. Furolog fills in the rest: the place, the weather, the outdoor temperature, and every number from your wrist. Come back to the same onsen and it remembers the name.
- Photos, notes, and stars The view from the rotenburo belongs next to the numbers.
- Share cards for Instagram Stories Three layouts, eight gradient backgrounds, or your own photo. Straight to Stories, Messages, or Photos.
- Onsen trip or Tuesday-night bathtub Home sessions count too. Log a bath-salt soak at home and keep it off the public record.
06 · On the watch
Hands-free, water-side.
Everything you need on the wrist. Live temperature, heart rate, rounds, and a summary you can rate before you've dried off. The phone stays in the locker.
Every Watch feature is free. No Pro required.
And more
Built around the bath.
Apple Health
Save any session as a workout with estimated calories. Import your height, weight, and resting heart rate.
iCloud sync
Sessions, photos, and settings on every device. Between your devices only.
Seven themes, nine icons
Zen, Neon Sento, Wabi-Sabi, Showa, and more. The Saru Pals icon set comes with Pro.
°C or °F, English or Japanese
Every temperature and every screen in your units and your language, on both devices.
Never lose a session
Recording is saved every 30 seconds. A crash, a dead battery, or a force-quit costs at most half a minute.
Free and Pro
Everything that records is free.
Free
Forever- Recording on Apple Watch and iPhone
- Session details, editing, photos, and notes
- Basic statistics and streaks
- Prefecture tracker
- Explore map and place discovery
- Share cards with gradient backgrounds
- Every Watch feature
Furolog Pro
Monthly subscription- 25+ charts: activity, temperature, duration, and frequency trends
- Your Perfect Session
- Bath and sauna insights, tolerance scores
- Heart-rate recovery analysis
- Personal bests and records
- Monthly and daily stats sheets
- Photo share backgrounds, premium banners, Saru Pals icons
Try recording first. Pro is there when you want to see the patterns.
Technology
Proprietary algorithms, built for water.
Activity Detection
Submersion sensor, accelerometer, and heart rate combine to classify soaks, cold plunges, saunas, and rest. A dip splits when the temperature shifts more than 5° inside it. Exit detection is debounced and configurable from 5 to 30 seconds. Walking ends a dip; sitting still arms sauna detection.
Real-Time Health
Karvonen-based heart-rate reserve with alerts at 75% and recovery at 50%, both adjustable. Five post-sauna recovery patterns. Heat and cold tolerance scores. Per-session heart-rate zones, with a workout session keeping the sensor in high-frequency mode.
SwiftUISwiftDataCloudKitHealthKitCoreMotionMapKitWatchConnectivityiOS 26+watchOS 26+
From the blog
Guides on onsen, sauna, and Apple Watch.
Sauna and onsen tracker apps for Apple Watch, compared (2026)
Seven apps that log sauna, cold plunge, and bath sessions from an Apple Watch, side by side. What each records on the wrist, what it costs, and why Furolog, which is ours, is the one to pick if your visit includes water as well as heat.
Read →
Ganbanyoku: lying on hot stone, how to do it, and whether the Apple Watch comes along
岩盤浴 is a Japanese heat format that is not a sauna and not a bath: you lie on heated stone slabs in a 40°C room, dressed, for a long time. What it is, how a session goes, what it does to heart rate compared with a sauna, and what the watch makes of it.
Read →
Half-body vs full-body bathing: what changes in heart rate and temperature, and which one to use when
Half-body and full-body bathing differ in immersion depth, water pressure, and how quickly they feel warm. Compare practical timing, comfort, and heart-rate caveats.
Read →
FAQ
Questions
Do I need an Apple Watch?
No. iPhone can record on its own with a temperature slider and manual dip and sauna buttons. With a Watch you get automatic detection and heart rate. Ultra 2 and Series 10 or later add the water temperature sensor. Ultra 1 detects water and takes temperature from the Digital Crown. Every other Watch works in manual mode.
How do I record a session?
Open Furolog on your Watch and tap Start, or just step in: with auto-start on, submersion begins the session. Furolog detects dips and prompts you when it thinks you've entered a sauna or left the bath. End the session from the controls page, rate it on the wrist, and the details land on your phone.
How does temperature detection work?
Apple Watch Ultra 2 and Series 10 or later have a water temperature sensor. Furolog records it continuously while you're submerged. Sauna air temperature is entered with the Digital Crown and kept separate from water temperature in your stats.
What are activity segments?
Sessions are divided into Soak (25 °C and above), Cold Plunge (below 25 °C), Sauna, and Rest. Saunas are detected from your heart rate and confirmed with one tap, or started manually. Every session shows them as a colour-coded flow bar and a timeline.
What does Furolog Pro include?
The deeper statistics: 25+ charts and 35+ insight cards covering activity, temperature, duration, and frequency trends, Your Perfect Session, bath and sauna insights, heart-rate recovery, personal bests, and monthly and daily stats sheets. Plus photo share backgrounds, premium profile banners, and the Saru Pals app icons.
Can I use it without Pro?
Yes. Recording, session details and editing, photos and notes, basic stats, the prefecture tracker, the explore map, sharing, and every Watch feature are free.
Is my data private?
Your sessions sync through your own iCloud, between your own devices. Home and private sessions never leave your phone. Community data is anonymous, covers named facilities only, drops exact times and health data, and can be switched off in Settings.
Does it work in my bathtub at home?
Yes. Mark a session as Home, tag it Bathtub or Bath Salts, and it counts toward your stats and streaks while staying out of the public record.
Start your bathing journal.
Free to download. Every Watch feature included. Pro is there when you want to see the patterns.
Download on the App StoreiOS 26 and watchOS 26. Best on Apple Watch Ultra 2 and Series 10 or later. Works on every Apple Watch.