Dehydration Risk Assessment

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

The rates this page uses, for a 70 kg adult in mild weather:

Each includes about 30 mL an hour lost through breath and skin. They scale with body weight, and heat raises them: by 10% above 25°C, 30% above 30°C and 60% above 35°C, with 20% off below 10°C. The exercise figures sit inside the 0.5 to 2 litres an hour the American College of Sports Medicine reports for sweat rates, which vary widely between people. Weighing yourself before and after an hour of your usual exercise gives your own figure: every kilogram lost is about a litre of sweat.

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. Catching up takes about 1.7 litres, not 1.1, for the reason below. 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

Catching up:

The American College of Sports Medicine advises drinking about 1.5 litres for every kilogram lost, not just the kilogram. Some of what you drink passes straight through as urine, so replacing the exact deficit leaves you short. Spread it over a few hours, with some salt from food or a sports drink.

Tips:

• For everyday life, thirst is a good enough guide. In long exercise in the heat it lags behind, so drink on a schedule instead.
• In hot weather, drink 200–300 mL (7–10 oz) every 15–20 minutes during exercise, adjusted to your own sweat rate.
• 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.
• Drinking far beyond what you lose is not safer. Over long events it dilutes the blood’s sodium, which is how marathon runners end up in hospital with hyponatraemia.