The 8 glass-a-day myth: How much water do you really need? The ‘eight glasses of water a day’ rule doesn’t fit everyone, experts say. Hydration needs vary by lifestyle, health, and diet, with urine color being a simple indicator for water intake. But, do they ever get into the fine print on how much water you actually need? Is the general advisory of ‘eight glasses a day’ really a one-size-fits-all solution?