Dehydration Risk Assessment
Assess dehydration risk from body weight, temperature, activity level, and fluid intake.
Returns a rating from low to severe plus a rehydration target.
Dehydration occurs when your body loses more fluid than it takes in, impairing normal bodily functions. This assessment estimates your risk based on environmental conditions, physical activity, and current fluid intake.
Risk factors considered:
- Temperature: higher temperatures increase fluid loss through sweating
- Activity level: exercise and physical labor accelerate water loss
- Fluid intake: how much water and other fluids you have consumed
- Duration of exposure: longer periods in heat or during exercise compound risk
- Body weight: larger individuals need more fluids
Fluid loss estimates:
| Activity | Fluid Loss Rate |
|---|---|
| Sedentary (cool) | 1–1.5 L/day (0.26–0.4 gal) |
| Sedentary (hot) | 1.5–2.5 L/day (0.4–0.66 gal) |
| Moderate exercise | 0.5–1 L/hour (17–34 oz/hr) |
| Intense exercise (hot) | 1–2.5 L/hour (34–84 oz/hr) |
When to use this calculator:
- Planning outdoor activities in hot weather
- Monitoring hydration during exercise or sports
- Assessing whether your daily water intake is sufficient
- Caring for elderly or young individuals who are more vulnerable
Practical example: A 70 kg (154 lb) person exercising moderately for 2 hours in 35°C (95°F) heat loses roughly 1.6 litres. Drinking only 500 mL over that time leaves a deficit of about 1.1 litres, which is 1.6% of body weight and lands in the moderate band: enough to blunt performance and bring on a headache, not enough to be an emergency. Swap moderate for intense effort and the same two hours cost closer to 3 litres, which is where it turns serious.
The percentages matter more than the millilitres. Losing 2% of body weight measurably degrades endurance and concentration. Past 4% you are into cramping, nausea and a real drop in blood volume. Those two thresholds are what the risk bands below are built on, which is why the same 1-litre deficit is trivial for a 100 kg adult and significant for a 45 kg one.
Warning signs of dehydration:
- Mild: thirst, dark urine, dry mouth
- Moderate: headache, dizziness, reduced urine output
- Severe: rapid heartbeat, confusion, fainting: seek emergency help
Tips:
- Drink water before you feel thirsty: thirst means you are already mildly dehydrated.
- In hot weather, drink 200–300 mL (7–10 oz) every 15–20 minutes during exercise.
- Sports drinks help replace electrolytes lost during prolonged activity (over 60 minutes).
- Monitor urine color: pale yellow means well-hydrated, dark yellow means drink more.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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