Why hydration varies so much between people
The "eight glasses a day" rule is a simplification — it doesn't account for body size, climate, or activity. A 100kg man training outdoors in Brisbane in summer needs significantly more water than a 55kg woman working in an air-conditioned office in Melbourne in winter. This calculator adjusts for the main variables that drive individual hydration needs.
What increases your water needs
- Higher body weight — more tissue requires more water to maintain
- Exercise — sweat losses during moderate exercise range from 0.5–2L per hour
- Heat and humidity — sweat rate increases significantly in hot conditions
- Pregnancy — blood volume increases by up to 50%, raising fluid needs
- Breastfeeding — breast milk is ~87% water; producing it significantly increases daily needs
- Illness — fever, vomiting, and diarrhoea all increase fluid losses