A fluid loss of just 1–2 % of body weight already impairs concentration, mood, and physical performance. In heat, exercise, or illness it can escalate to serious dehydration within hours.
This guide shows how to quantify your risk — by comparing intake against the daily requirement (35 ml/kg, EFSA reference) and a clinical symptom score adapted from WHO and CDC.
How much fluid do you actually need?
The European Food Safety Authority (EFSA) recommends 2.0 L (women) to 2.5 L (men) of fluids per day from beverages — about 35 ml per kilogram of body weight. For a 70 kg adult that is ~2.45 L.
Factors that raise the requirement:
- Heat: +0.5–1.0 L above 30 °C — sweat rates can hit 1–2 L per hour
- Exercise: 400–800 ml per hour of training, depending on intensity
- Fever: +10 % per 1 °C above 37 °C
- Pregnancy / breastfeeding: +300 ml or +700 ml respectively
Dehydration severity grades
| Severity | Deficit | Typical signs |
|---|---|---|
| Mild | 1–3 % | Thirst, darker urine, mild fatigue |
| Moderate | 3–6 % | Dry mouth, headache, dizziness, tachycardia |
| Severe | > 6 % | Confusion, no urination, low blood pressure, loss of consciousness |
A loss of 10 % is life-threatening. Infants and the elderly deteriorate much faster than healthy adults.
The eight clinical symptoms in the score
Our calculator weighs eight signs derived from WHO and CDC criteria:
| Symptom | Points | Meaning |
|---|---|---|
| Thirst | 1 | Earliest, most sensitive sign |
| Dark urine | 1 | Concentrated urine, scale 5–8 |
| Dry mouth | 1 | Reduced saliva production |
| Fatigue | 1 | Decline in physical and cognitive performance |
| Headache | 1 | Reduced plasma volume |
| Lightheaded on standing | 2 | Orthostatic hypotension |
| Rapid heartbeat | 2 | Tachycardia as compensation |
| Confusion | 3 | Red-flag sign — seek medical care |
Confusion is always treated as a severe red flag — regardless of intake history. Call emergency services if it appears.
Calculate your dehydration risk now
Fluid intake, body weight, 8 clinical symptoms — instant severity grade and replacement guidance. Anonymous and free.
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Treatment depends on severity:
- Mild (1–3 %): 250–500 ml of water or diluted juice over 30–60 minutes.
- Moderate (3–6 %): 500–1000 ml over 1–2 hours, ideally with electrolytes (sodium, potassium, glucose). Oral rehydration salts (ORS) are first-line.
- Severe (> 6 %): call emergency services — oral rehydration alone is usually insufficient and intravenous fluids are required.
Caution: drinking very large amounts of plain water in a short time can cause hyponatremia — particularly after long endurance events. Drink steadily and replace electrolytes.
Related calculators
Estimate your individual daily target precisely with the water intake calculator. If you drink a lot of coffee, the caffeine calculator puts caffeine's mild diuretic effect into context. For people with frequent dizziness or electrolyte issues, the sodium correction calculator adds another layer.
Bottom line
Dehydration is more than just "feeling thirsty". Performance starts to drop once losses reach 1–2 % of body weight. Use our dehydration risk calculator to spot early signs in seconds — or to recognise an emergency. When in doubt, seek help sooner rather than later.