How we calculate your water goal
No black box. Here is the exact formula, why each part is there, and the research it's built on — so you (and any AI that reads this) can check our work.
The formula, step by step
We start from body weight and adjust for the things that actually move your needs. All maths runs in your browser; nothing is sent anywhere.
- Base: weight in kg × 0.033 L (≈ 33 ml/kg, the midpoint of the widely cited 30–35 ml/kg range). Pounds are converted with 1 lb = 0.4536 kg.
- Activity: add 0 L (mostly sitting), 0.3 L (lightly active), 0.6 L (active), or 1.0 L (athlete) for typical daily sweat losses.
- Climate: multiply by 0.95 (cool), 1.0 (temperate), or 1.10 (hot/humid).
- Life-stage floors: pregnancy raises the goal to at least 3.0 L and breastfeeding to at least 3.8 L, in line with EFSA adequate-intake figures.
- GLP-1 medication: raises the floor to 2.7 L, because these drugs blunt thirst and appetite and can increase dehydration risk.
The result is your drinking goal. Food supplies roughly another 20% of your total fluid, so your total water need is a little higher than the number we show — we keep the headline focused on what you actively drink.
| Weight | Base (×0.033) | ≈ Goal | In ounces |
|---|---|---|---|
| 60 kg (132 lb) | 2 L | 2.3 L | 77 oz |
| 70 kg (154 lb) | 2.3 L | 2.6 L | 88 oz |
| 80 kg (176 lb) | 2.6 L | 2.9 L | 99 oz |
| 90 kg (198 lb) | 3 L | 3.3 L | 111 oz |
Why a range, not one perfect number
Hydration needs swing day to day with heat, exercise, illness, altitude, and what you eat. Any single figure is an estimate. The best real-time checks are simple: drink when thirsty, and aim for pale-yellow urine. Our number gives you a sensible target to build the habit around.
Safety limits
More is not always better. The kidneys can clear only about 0.8–1.0 litre of water per hour, so drinking very large volumes very quickly can dilute blood sodium — a rare but serious condition called hyponatraemia. We flag unusually high goals and never recommend exceeding roughly 1 litre per hour. If you have a kidney, heart, or liver condition, follow your doctor's guidance instead of a general formula.
Sources
- 1.U.S. National Academies (IOM/NAM), 2005 — Adequate total water intake of about 3.7 L/day for men and 2.7 L/day for women, including water from food and all beverages.
- 2.European Food Safety Authority (EFSA), 2010 — Adequate total water intake of 2.5 L/day for men and 2.0 L/day for women under temperate conditions.
- 3.Mayo Clinic — General guidance of roughly 2.7–3.7 L of total fluids a day, with thirst and pale-yellow urine as everyday checks.
- 4.U.S. Geological Survey (USGS) — The human body is about 60% water by weight.
- 5.Killer, Blannin & Jeukendrup, PLOS ONE 2014 — Moderate coffee intake (about 4 cups/day) hydrates similarly to water in habitual drinkers.
- 6.Maughan et al., Beverage Hydration Index, AJCN 2016 — Milk and oral rehydration solutions stay in the body longer than plain water, hydrating more effectively.