Water Intake by Body Weight: How to Calculate It

Q: How much water should I drink per kg of body weight?

The standard formula is 35 ml per kilogram of body weight. For a 70 kg person this is 2,450 ml (2.45 L) per day. Add 500–1,500 ml per hour of exercise, and further adjustments for hot climates, pregnancy, or illness.

Q: How many litres of water should a 70 kg person drink?

A 70 kg person needs approximately 2.45 L per day as a baseline. With moderate exercise, this increases to 3.0–3.5 L. With intense exercise in a hot climate, total daily fluid needs can reach 4–5 L including water from food.

Q: Does coffee or tea count toward daily water intake?

Yes — coffee and tea contribute to daily fluid intake. The mild diuretic effect of caffeine at moderate consumption (up to 400 mg/day) is offset by the fluid volume of the beverage. Research confirms caffeinated beverages contribute positively to daily hydration in regular consumers.

Generic hydration advice — "drink 8 glasses a day" — ignores one of the most important factors in determining how much water you actually need: body weight. Daily water intake by body weight is a more personalised and physiologically grounded approach. This guide explains the 35 ml/kg formula, how to use a water intake calculator by body weight, and how to adjust your target for exercise, hot climates, and specific life stages.

By the FitCalcHub Editorial Team — Updated July 2026 — 7 min read

Why Body Weight Is the Best Baseline for Water Intake

Total body water accounts for approximately 60% of body weight in men and 50–55% in women. A heavier person has more total body water, greater metabolic activity, and therefore a higher daily water requirement. Flat-volume recommendations (e.g., "2 litres per day") do not account for this variation — a 50 kg person and a 100 kg person have very different hydration needs.

Using daily water intake per kg body weight as the calculation basis produces a personalised target that scales with physiology. The most widely used formula — endorsed by clinical nutritionists — is:

Daily water (ml) = Body weight (kg) × 35 ml

For a 70 kg person: 70 × 35 = 2,450 ml (2.45 L) per day. This includes water from all sources — drinks and food. Roughly 20–30% of daily water intake typically comes from solid food; the remaining 70–80% from beverages.

Water Intake Reference Table by Body Weight

The following table shows baseline daily water intake targets using the 35 ml/kg formula, before activity or climate adjustments:

Body weight (kg)	Water intake (ml)	Water intake (L)	~Glasses (250 ml)
50 kg	1,750 ml	1.75 L	7 glasses
55 kg	1,925 ml	1.93 L	8 glasses
60 kg	2,100 ml	2.1 L	8–9 glasses
65 kg	2,275 ml	2.28 L	9 glasses
70 kg	2,450 ml	2.45 L	10 glasses
75 kg	2,625 ml	2.63 L	10–11 glasses
80 kg	2,800 ml	2.8 L	11 glasses
85 kg	2,975 ml	2.98 L	12 glasses
90 kg	3,150 ml	3.15 L	13 glasses
95 kg	3,325 ml	3.33 L	13–14 glasses
100 kg	3,500 ml	3.5 L	14 glasses

Use the Water Intake Calculator to calculate your personalised target instantly, including activity and climate adjustments.

EFSA Panel on Dietetic Products. "Scientific Opinion on Dietary Reference Values for water." EFSA Journal. 2010;8(3):1459. EFSA ↗

How to Adjust Water Intake for Exercise

The 35 ml/kg baseline assumes a sedentary-to-moderately active lifestyle. Exercise increases water loss through sweat, respiration, and increased metabolic activity. The American College of Sports Medicine recommends:

Add 500–1,000 ml per hour of moderate exercise (brisk walking, cycling, yoga).
Add 1,000–1,500 ml per hour of intense exercise (running, HIIT, heavy resistance training).
For high-sweat individuals or hot environments, weighing before and after exercise is the most accurate method — each 1 kg lost during a session represents approximately 1,000 ml of fluid to replace.

Special Adjustments: Climate, Pregnancy, and Age

Several factors increase daily water intake beyond the 35 ml/kg baseline:

Factor	Additional water	Notes
Hot climate (>30°C)	+500–1,000 ml/day	Sweat rate increases even at rest
Pregnancy	+300 ml/day above baseline	Supports amniotic fluid, blood volume, fetal development
Breastfeeding	+700 ml/day above baseline	Breast milk is approximately 87% water
High-altitude environment	+500 ml/day	Increased respiratory water loss
Illness (fever, vomiting)	+500–1,000 ml/day	Replaces elevated fluid losses

Practical Tips for Hitting Your Daily Water Intake by Body Weight

Start with water at meals: A 500 ml glass with each of three meals provides 1,500 ml — the majority of a sedentary baseline for most body weights.
Use urine colour as a real-time indicator: Pale yellow (similar to lemonade) indicates adequate hydration. Dark yellow or amber signals insufficient intake. See Signs of Dehydration for a full urine colour guide.
Pre-hydrate before training: Consume 400–600 ml of water in the 2 hours before exercise. This is the ACSM's pre-exercise recommendation and reduces dehydration risk during the session.
Count food water: Foods such as cucumber (~97% water), lettuce (~96%), tomato (~95%), strawberries (~91%), and orange (~87%) contribute meaningfully to daily water intake — typically 500–800 ml on a diet rich in fruits and vegetables.
Set phone reminders: Spaced across the day (e.g., every 2 hours), reminders to drink 250–300 ml ensure consistent intake without relying on thirst, which can be an unreliable signal especially in older adults.

Frequently Asked Questions

How much water should I drink per kg of body weight?

The standard formula for daily water intake by body weight is 35 ml per kilogram. For a 70 kg person this is 2,450 ml (2.45 L) per day; for a 90 kg person it is 3,150 ml (3.15 L). This baseline assumes moderate activity. Add 500–1,500 ml per hour of exercise depending on intensity, and further adjustments for hot climates, pregnancy, or illness.

Is the 35 ml/kg formula accurate?

The 35 ml/kg formula is a validated clinical starting point widely used in nutrition practice. It produces targets that align closely with the EFSA adequate intake recommendations (2.0 L/day for women, 2.5 L/day for men for sedentary individuals at normal temperatures). Like all formulas, it is an estimate — individual sweat rates, activity levels, and dietary water content from food vary considerably. Monitor urine colour as a real-time accuracy check.

How many litres of water should a 70 kg person drink?

A 70 kg person needs approximately 2.45 L (2,450 ml) of water per day as a baseline using the 35 ml/kg formula. With moderate exercise (1 hour), this increases to approximately 3.0–3.5 L. With intense exercise in a hot climate, total daily fluid needs can reach 4–5 L. This includes water from all drinks and food sources.

Does body weight include fat mass in the water calculation?

Yes, the 35 ml/kg formula uses total body weight, not lean mass. This is a simplification — fat tissue contains significantly less water than muscle (~10% vs ~75%). Individuals with very high body fat percentages may slightly overestimate their water needs with this formula, while heavily muscled individuals may slightly underestimate. For practical purposes, the formula is accurate enough for most people; very lean athletes or those with obesity can use 30–40 ml/kg depending on body composition.

How much water should I drink if I exercise?

Add 500–1,000 ml per hour of moderate exercise, or 1,000–1,500 ml per hour of intense training, to your body-weight-based baseline. The most precise method: weigh yourself before and after exercise. Each kilogram of body weight lost during a session represents approximately 1,000 ml of fluid to replace. Avoid losing more than 2% of body weight as sweat during exercise, as this level of dehydration measurably impairs strength and endurance performance.

Can drinking too much water be harmful?

Yes — excessive water intake (typically more than 1 litre per hour in non-extreme conditions) can cause hyponatremia, a dangerous dilution of blood sodium. This is rare in normal daily hydration but has been recorded in endurance athletes who drink plain water heavily without electrolyte replacement during events exceeding 3–4 hours. For most people, the risk of underhydration far exceeds overhydration; follow the body-weight formula plus urine colour guidance.

Does coffee or tea count toward daily water intake?

Yes — coffee and tea contribute to daily fluid intake despite containing caffeine. The mild diuretic effect of caffeine at moderate consumption (up to 400 mg/day, or approximately 3–4 cups of coffee) is offset by the fluid volume in the beverage. Research confirms that caffeinated beverages contribute positively to daily hydration status in regular consumers. Only at very high caffeine doses does net fluid loss occur.

How do I use the Water Intake Calculator?

Enter your body weight in kg (or lbs, converted to kg), select your activity level, and optionally specify climate and special conditions (pregnancy, breastfeeding). The calculator applies the 35 ml/kg base formula and adds activity and climate multipliers to output your total daily fluid recommendation in litres and equivalent glasses. The result represents all fluid intake — from water, beverages, and food combined.

This article is part of our complete Nutrition Guide.

Medical Disclaimer: This article is for educational purposes only. Water intake needs vary by health status, kidney function, medications, and medical conditions. Consult a physician if you have kidney disease, heart failure, or other conditions affecting fluid balance before significantly changing your daily water intake.

Scientific References

EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). "Scientific Opinion on Dietary Reference Values for water." EFSA Journal. 2010;8(3):1459. EFSA ↗
Sawka MN et al. "American College of Sports Medicine Position Stand: Exercise and Fluid Replacement." Med Sci Sports Exerc. 2007;39(2):377–390. PubMed ↗
Grandjean AC et al. "The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration." J Am Coll Nutr. 2000;19(5):591–600. PubMed ↗