To ward off dehydration, many people have heard that they should drink drink eight, 8-ounce glasses of water every day. However, the 8×8 fluid guideline is more of a guesstimate than exact science. Below are how dietitians determine how much water (and other fluids) is enough to optimize hydration status.
Numerous factors can impact fluid needs, including age (since children and the elderly are at increased risk for dehydration), physical activity, caloric intake, body composition and health status. Environmental factors, such as ambient temperature, humidity, altitude and indoor heat, can also affect hydration status. Therefore, when assessing fluid requirements, it’s important to take all of these elements into consideration.
How much water do you need? Use these three formulas to help you find out.
- 1 Mililiter per Calorie
If you know how many calories you need daily, you can get a good idea of how much fluid you need. Each calorie burned requires 1 ml of water. If you burn 2,000 calories, you need 2L of fluid per day. Remember, 20 percent of your daily fluids come from food, so if you need 2L of fluid, you have to drink 1.6L to stay hydrated. If you’re exercising strenuously or are in a hot environment, you would probably need even more fluids.
Body Weight-Based Formula
- 30-35 mL per Kilogram Body Weight (Adults >30 Years Old)
Fluid needs are often calculated on a per body weight basis, with an adult baseline of 30-35mL per kilogram body weight or 13.5-16 ml per pound.
- Nine 8-ounce Servings Per Day for Women
- Thirteen 8-ounce Servings Per Day for Men
Based on national food and beverage intake surveys, the National Academy of Medicine (NAM) set Adequate Intake (AI) values for total water from all beverages based on median intakes of a national sample of adults.
The AI for men is roughly thirteen 8-ounce servings (3 liters) of total fluids per day. The AI for women is approximately nine 8-ounce servings (2.2 liters) of fluids per day. (5). All beverages, including coffee, tea, fruit juices, dairy and moisture present in food, which contributes about 20% of total fluid intake, count toward daily fluid requirements.
For more detailed fluid recommendations from NAM based on age and sex. The NAM recommendations do not take into consideration medical conditions, activity levels, environmental or other factors that impact hydration status.
10 Signs and Symptoms of Dehydration
- Infrequent urination and low urine output
- Deep yellow or amber color urine
- Dry mouth, skin and nasal passages
- Lack of tears or dry eyes
- Increased heart rate
- Lack of skin elasticity
1. Popkin BM, D’Anci KE, Rosenberg IH. Water, Hydration and Health. Nutrition Reviews. 2010;68(8):439-458.
2. American College of Sports Medicine; American Dietetic Association; Dietitians of Canada. Joint Position Statement: nutrition and athletic performance. American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada. Med Sci Sports Exerc. 2000 Dec;32(12):2130-45.
- Sucher K, Nelms M, Roth et al. Nutrition Therapy and Pathophysiology, Third Edition. Boston, MA: Cengage Learning; 2016.
- Chidester JC, Spangler AA. Fluid intake in the institutionalized elderly. J Am Diet Assoc. 1997 Jan;97(1):23-8; 29-30.
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, D.C.: National Academies Press; 2004.