Onsen & Sauna Tracker

Your bathing journal, written by your Apple Watch.

Step into the water. Furolog starts recording: temperature, heart rate, and every transition between soak, sauna, and cold plunge. No buttons to press.

Download on the App Store See how it works ↓

iOS 26 + watchOS 26

47 Prefectures

13 Spring types

12 Sauna types

01 · Record

Records the moment you step in.

Apple Watch's underwater sensor detects submersion and begins logging instantly. Temperature every five seconds. Heart rate throughout. Soak, cold plunge, sauna, rest, classified as you move.

Auto water detection Submerge. Recording starts within seconds. No taps required.
Five-second sampling Continuous temperature capture while you bathe.
Sauna auto-detect Heart-rate rise triggers sauna segments automatically.

02 · Analyze

Every degree. Every beat.

Sessions become charts become patterns. Thermal contrast, heart-rate recovery, sauna climb profiles. The feeling in your body, made visible.

Temperature trends How your preferences shift by season, location, and time of day.
Recovery tracking Karvonen-based HRR with sauna-specific post-session analysis.
Five recovery patterns Your body's response classified across five distinct profiles.

03 · On the watch

Hands-free, water-side.

Everything you need on the wrist. Live duration, heart-rate zones, session summaries. The phone is in the locker. Your watch does the work.

Features

Built around the bath.

Automatic Tracking

Water detection, temperature sampling, activity classification. Nothing to press.

Deep Analytics

Trends, heatmaps, thermal contrast, HR recovery. Furolog Pro unlocks the full picture.

Discover & Explore

Nearby onsen, sauna, and sento on an interactive map. 47 prefectures to collect.

Personalize

Seven themes, nine alternate icons, custom share cards. Make it yours.

Technology

Proprietary algorithms, built for water.

Activity Detection

Submersion sensors, accelerometer, and heart rate combine to auto-classify soaks, cold plunges, saunas, and rest. 10-second debounced exit detection. Motion-aware state changes.

Real-Time Health

Karvonen-based Heart Rate Reserve. Alerts at 75% HRR, recovery confirmed at 50%. Five post-sauna recovery patterns. Per-session HR zone analysis.

SwiftUISwiftDataCloudKitHealthKitCoreMotionMapKitWatchConnectivityiOS 26+watchOS 26+

FAQ

Questions

How do I record a session?

Open Furolog on your Apple Watch and tap Start Session. The watch automatically detects water and begins recording temperature. Tap the sauna button for sauna segments. Swipe to controls and tap End Session when done.

How does temperature detection work?

Furolog uses Apple Watch's built-in water temperature sensor (Ultra and Series 10 or later). Temperature is recorded every five seconds when submerged.

What are activity segments?

Sessions are divided into Soak (≥ 25°C), Cold Plunge (< 25°C), Sauna (manual toggle), and Rest. Shown in the segment flow bar on every session.

What does Furolog Pro include?

Advanced statistics, activity heatmaps, temperature trends, heart-rate recovery analysis, custom share backgrounds, premium banners, and alternate app icons.

Can I use it without Pro?

Yes. All core features are free: recording, session details, basic stats, prefecture tracker, the explore map, sharing, and all Watch functionality.

Is my data private?

Home and personal locations never leave your device. iCloud sync is between your own devices only. No data is sent to third-party servers.

Start your bathing journal.

Free to download. Pro unlocks the deeper analytics once you're ready for them.

Download on the App Store

Available for iOS 26 and watchOS 26.