Baby Bathtub Thermometer shows whether bath water is too cold, just right, or too hot by reading the water temperature sensor in your Apple Watch.
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Only watches with a water temperature sensor: Apple Watch Ultra (every generation) and Apple Watch Series 10 or later, running watchOS 11 or later. On other models the app says the sensor isn’t available. The iPhone app runs on iOS 17 or later and is a guide plus settings; it cannot measure on its own.
The watch hasn’t detected water yet. Make sure the back of the watch is under the surface and hold it still for a few seconds. Readings only appear while the watch is submerged; an old number is never left on screen once the watch leaves the water.
The sensor takes a few seconds to match the water temperature. While it settles the app shows the sensor’s own accuracy figure (for example ±1.2°). Wait until that figure drops before acting on the colour, and test the water with your wrist or elbow as well.
The water temperature sensor is part of Apple’s Core Motion framework, so watchOS requires that permission to read it. If you declined, turn it back on under Settings › Privacy & Security › Motion & Fitness on the watch.
On the watch, tap the gear on the start screen to switch between °C and °F. The “just right” range is set in the iPhone app, in the Just-right range section of the guide. Pick one of the presets, each quoted from the organization named next to it with a link to its page, or set your own range after a safety reminder. The watch shows the range but cannot change it, so a splash or a sleeve can’t move it. Changes sync between the two devices.
The presets and their sources:
Those organizations have no connection with the app, and their advice can change; check the linked page before relying on it. Advice about water heaters (for example the American Academy of Pediatrics’ 120 °F / 49 °C limit) is about the hot tap, not the bath: it is a scald limit far above any bath temperature.
Baby Bathtub Thermometer is an aid for adults, not a medical or safety device. Green means the reading is inside the range you set, not that the water is safe. Before your baby goes in, always check the water with the inside of your wrist or elbow. Never leave a baby alone in or near water, even for a moment. Readings lag while the sensor settles and can be off by a degree or more, so stir the water and wait for the number to steady. The first time you open the app it asks you to read and accept this notice; the full Terms of Use are in the app and here.