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Signs of dehydration
Losing one to two percent of body mass in fluid is enough to blunt concentration and mood. It rarely announces itself as thirst — more often as a flat afternoon you blame on something else.
Early signs, and the ones that matter
The common early markers are dark or infrequent urine, a dull headache, fatigue that does not match your night's sleep, dry mouth and lips, and a drop in concentration or mood. Urine colour is the most practical of these: pale straw is the target, and anything darker than apple juice is worth acting on.
Thirst arrives late and unreliably. It is blunted with age, during intense exercise, in cold weather and when you are absorbed in something — so “I drink when I'm thirsty” works better in theory than on an August afternoon or a long meeting.
Some signs are not a hydration problem to solve with a glass of water. Confusion, fainting, a racing heart at rest, no urine for many hours, or heat exhaustion progressing to hot dry skin and disorientation need medical attention, not an app. The same is true for dehydration alongside vomiting or diarrhoea, especially in children and older adults.
What HydroPulse does here
- Nudging before the fatigue and the headache, which is the whole point of estimating rather than reacting.
- Naming the driver — heat, exertion or baseline — so the signal is interpretable.
- Escalating urgency as the estimated deficit grows.
- A history you can look back at when an afternoon went badly.
- Nothing diagnostic: it estimates fluid balance and makes no claim about your clinical state.
How HydroPulse decides
- You enter your weight, age and sweat profile once. That sets your personal baseline loss — the water you lose just by being alive and breathing.
- The watch reads your heart rate continuously and converts it into a heart-rate-reserve fraction: how hard you are working relative to your own resting and maximum heart rate, not to an average person's.
- Local temperature and humidity are fetched anonymously and turned into a heat-index multiplier, because the same effort costs far more fluid at 33 °C and 70 % humidity than at 18 °C.
- Every drink you log is credited back to the ledger. When the running balance crosses your threshold, HydroPulse tells you — with a specific millilitre amount and the reason behind it.
What to expect
Used consistently, the effect people report is not dramatic — it is the absence of the four-o'clock slump. What it cannot do is tell you whether you are dehydrated: it estimates a balance from sensors and your own logging, and the signs above remain the thing to watch.
Your balance as a dial, with the amount to drink now. 
One tap to log, one to snooze — the phone stays in your pocket.
Wear OS 3.0+. One-time purchase — no subscription, no ads, no account. The app interface is in English.
Common questions
Can a smartwatch detect dehydration?
No consumer watch measures hydration directly. HydroPulse estimates fluid balance from heart rate, activity, weather and what you log — a well-founded estimate, not a measurement, and not a diagnosis.
Is urine colour reliable?
It is the most practical everyday check. Pale straw suggests you are on track; darker suggests you are behind. Some vitamins, medications and foods colour it independently.
When should I see a doctor?
Confusion, fainting, a racing heart at rest, no urination for many hours, or signs of heat illness need urgent care. So does dehydration with persistent vomiting or diarrhoea, particularly in children and older adults.
Does the app warn me if I am dehydrated?
It warns you when its estimated balance crosses your threshold. That is a prompt to drink, not a clinical assessment, and it should never delay seeking care.
Where the model comes from
The baseline draws on IOM and EFSA adequate-intake figures, the sweat-rate component on ACSM exercise-hydration guidance, and the environmental multiplier on the US National Weather Service heat-index formula. These are population references adapted to what a watch can actually sense — a well-founded estimate, not a laboratory measurement.
Not a medical device
HydroPulse produces an estimate, not a measurement. It is a wellness tool, not a medical device, and it does not diagnose, treat or monitor dehydration or any other condition. If you take diuretics, restrict fluid on medical advice, or have a kidney, heart or endocrine condition, follow your clinician's intake guidance rather than this app's. Seek urgent care for confusion, fainting, or heat illness.
Private by design
Your heart rate, activity and drink log never leave the watch. There is no account, no advertising SDK and no product analytics — nothing tracks what you do in the app. Two things do leave the device: a pair of coordinates to a public weather service, with no identifier attached, and a crash report if the app fails. Neither carries your health data.
More on hydration
- On Wear OSA water reminder built for the watch, not a phone app with a complication.
- Running & trainingSweat-rate-aware reminders for runs, rides and gym sessions.
- Hot weatherWhen heat and humidity raise your needs, the reminders move first.
- At workFor desk days where six hours vanish between two glasses.
- How much per dayWhere the 2-litre rule comes from, and why your number is not that.
- Heart rate & hydrationThe link between cardiac drift, effort and fluid loss.