Water Tracker

What water actually does in the body

An adult body is roughly 50–65% water by weight — muscle holds about 75% water, fat tissue about 10–30% — totaling about 35–40 liters for a 70 kg man and slightly less for women, whose average composition carries more fat. That water divides into intracellular (about two-thirds, the working medium of every cell) and extracellular (about one-third: blood plasma, the fluid between cells, and the connective tissue matrix). You lose roughly 2–3 liters per day through urine, breath, and skin evaporation before lifting a finger; exercise in heat multiplies skin losses several-fold.

Hydration state touches the things you actually care about. Cognitive work: even mild dehydration (1–2% of body mass) measurably degrades concentration, working memory, and mood in controlled studies, and registers as fatigue and headache long before you connect it to water. Physical work: blood volume shrinks when you are dry, so the heart pumps a smaller stroke volume faster to deliver the same oxygen — the same run feels harder at a higher heart rate. Kidney economy: well-hydrated kidneys concentrate urine cheaply; chronically low intake over years is associated with a higher risk of kidney stones and recurrent urinary tract infections.

How much water: the rule, the guidelines, and the tracker's math

The tracker's rule is 35 ml per kilogram of body weight: a 70 kg person lands at 2,450 ml per day. That single multiplier sits deliberately between the two major official frameworks. The European Food Safety Authority sets adequate total water intake at 2.5 L/day for men and 2.0 L/day for women — where "total water" includes water inside food (roughly 20% of intake on a typical diet). The US Institute of Medicine sets 3.7 L/day for men and 2.7 L/day for women as total water, with about 3.0 L and 2.2 L respectively coming from drinks. The weight-scaled rule personalizes the same science: it rises for heavier bodies and sinks for lighter ones, instead of prescribing one number to every adult.

The tracker clamps the rule between 2,000 ml and 4,500 ml. The floor exists because even small, sedentary bodies need a workable daily minimum; the ceiling exists because the rule is a drinking target, not a dare — pushing far beyond it has its own risks covered in the overhydration section below. Between the clamps the number scales linearly: 55 kg → 1,925 clamped to 2,000 ml; 80 kg → 2,800 ml; 120 kg → 4,200 ml.

What moves your personal number

Heat and sweat: add roughly 400–800 ml per hour of sweaty exercise — more in genuine heat or with heavy sweating. A practical field rule from sports medicine: weigh yourself before and after a hard session; each kilogram lost is about a liter of sweat, and the classic replacement guidance is to drink about 1.5 liters per kilogram lost over the hours that follow. Altitude above roughly 2,500 m increases respiratory and urinary water losses. Fever, vomiting, and diarrhea all add losses that need deliberate replacement, and air-conditioned dry air evaporates skin water silently all day.

Caffeine is weaker than folklore claims: up to about 400 mg per day (roughly four cups of coffee) does not dehydrate habitual drinkers — the water in the cup vastly outweighs the mild diuretic effect, and tolerance develops within days. Alcohol is different: it suppresses the antidiuretic hormone and genuinely increases urine output, so a drinking evening deserves deliberate water alongside. Pregnancy and breastfeeding raise needs substantially (breastfeeding adds roughly 600–700 ml/day), and those guidelines belong to the treating clinician rather than any generic calculator.

Recognizing real dehydration

The honest tell is urine, not a feeling: pale straw color says you are fine; dark amber says drink now. Volume matters too — urinating roughly every 3–4 hours with a reasonable stream is the normal rhythm; twice a day means you are running dry. By the time you feel clearly thirsty, fluid deficit has usually already passed 1% of body mass; headaches, unusual fatigue, difficulty concentrating, and constipation all arrive in the mild-dehydration band before anything dramatic. Dry lips and mouth are early local signs, not reliable overall measures.

The two situations where mild dehydration turns urgent: heavy exercise in heat, where sweat losses can reach 1–2 liters per hour and thirst lags far behind the deficit; and illness in infants and the elderly, whose thirst signals and kidney concentration ability are weaker. Dizziness on standing, confusion, cessation of sweating in heat, or urine output falling to near zero are stop-and-seek-care signs, not hydration-app territory.

The opposite error: overhydration and hyponatremia

Drinking far beyond need dilutes blood sodium — hyponatremia — and it is a real, occasionally fatal emergency seen in marathon and ultramarathon runners who over-drink plain water for hours, and in people persuaded that heroic gallon-per-day targets are health. Symptoms (nausea, headache, confusion, swelling) echo dehydration's, which leads sufferers to drink more — the wrong turn. The defense is built into this tracker's design: the 35 ml/kg rule with its 4.5-liter ceiling is nowhere near the danger zone for healthy kidneys, and during very long sweaty events, replace a meaningful share of losses with electrolyte-containing drinks rather than plain water alone.

Healthy kidneys can excrete about 0.8–1.0 liter per hour, so steady sipping across the day is effectively risk-free; the danger pattern is rapid, forced intake of several liters in a short window. Medical conditions that retain water — heart failure, cirrhosis, SIADH — follow fluid limits prescribed by physicians, not weight-scaled rules; if any of those apply to you, this tool's number is a conversation piece, not a prescription.

The water you eat

On a typical mixed diet, roughly 20% of total water arrives inside food — more on a produce-heavy one. Logging drinks alone therefore undercounts your true intake, which is fine for a drinking target (the tracker's number is water to drink, matching how EFSA and IOM separate their figures) but worth understanding. The table shows why a cucumber-and-watermelon summer day is almost a hydration strategy by itself.

A 300 g portion of watermelon alone is about 275 ml of water — more than a small glass — plus electrolytes. Soup, stew, and porridge traditions across every cuisine exist partly because boiling water into food is a culturally durable way to hydrate; the salty-sweaty cuisines of hot regions pair them with pickles and teas for the electrolyte half of the equation.

How this tracker works — features and privacy

The workflow takes three seconds a day: enter your weight once and the goal computes at 35 ml/kg within the safe clamps; then tap the cup button through the day (a standard 250 ml glass per tap) or log a custom amount — a bottle, a kettle, a restaurant glass. The progress ring fills toward your goal, the remaining number tells you exactly what is left, and a seven-day bar history reveals your real weekly rhythm — the pattern matters more than any single day, and a Thursday at 60% is a nudge, not a failure. Reset any day, undo a mis-tap, and adjust the goal manually whenever judgment says the rule is wrong for you.

Everything stays on your device: the log and settings live in your browser's local storage, no account is required, and nothing about your water habits leaves the page unless you later choose to save results with an account. That also means the log is bound to this browser — clearing site data resets history, and switching devices starts fresh; the habit, not the archive, is the asset.

A closing word on the pursuit of perfect numbers: hydration is a rhythm, not a religion. The tracker's goal is a well-reasoned default from your body weight and the official frameworks; urine color and how you feel remain the ground truth. Aim to finish most days between 80% and 120% of the goal, adjust upward on hot, training, or caffeinated-and-alcoholic days, and let the seven-day picture — not the daily verdict — drive small changes.

Electrolytes — water's silent partner

Water never travels alone: sodium, potassium, magnesium, and chloride ride with it and hold the electrical gradients that nerves, muscles, and kidneys run on. Hydration trouble is usually a pair problem — too little water, or too much water without enough electrolytes (the hyponatremia mechanism above). Ordinary days and ordinary meals handle the pairing automatically: salt in food covers sodium, and produce covers potassium. The situations that deliberately break the pairing are the ones that need deliberate repair: heavy sweating (salt leaves in sweat), long endurance events, illness with vomiting or diarrhea, very low-carbohydrate diets (which flush sodium faster in the first weeks), and scorching climates.

The practical repair menu, from kitchen to pharmacy shelf: for training sessions, water with a pinch of salt plus a splash of citrus makes a functioning sports drink at home; for long events, commercial electrolyte mixes matter mainly for their sodium content (check the label for sodium, not for the marketing words around it); and for illness, the world's most evidence-backed rehydration recipe is the humble oral rehydration solution — water, salt, and a carbohydrate source in the ratios medicine has used for decades to save lives. On ordinary days, none of this is necessary: eat food, drink water, and let kidneys do the arithmetic they were designed for.

Frequently asked questions about this tool