How do I calculate my daily water intake?

Multiply your body weight in kg by 30–35 ml (or weight in lb by 0.5 oz), then add 500–750 ml for every 30–60 minutes of exercise and 10–20% more in hot weather.

How much water should a 70 kg person drink?

Roughly 2.1–2.45 liters (71–83 oz) per day at rest, rising to about 2.7–3.2 liters with 45 minutes of exercise.

Is there an app that calculates water intake automatically?

Yes — Water Tracker by SupaHydrate calculates a personalized daily goal from your weight, activity level, and climate, and weights every drink by the peer-reviewed Beverage Hydration Index.

Home › Hydration Guides › Water Intake Calculator (by Weight)

Water Intake Calculator: Work Out Exactly How Much Water You Need

Quick answer: Multiply your body weight in kilograms by 30–35 ml (or your weight in pounds by 0.5 oz). Then add 500–750 ml (17–25 oz) for every 30–60 minutes of exercise, and 10–20% more in hot or humid weather. A 70 kg (154 lb) adult needs roughly 2.1–2.45 L (71–83 oz) per day at rest.

The daily water intake formula

Sports medicine and clinical nutrition commonly use body weight as the starting point for fluid needs. The formula is simple:

Step	Metric	Imperial
1. Baseline	Weight (kg) × 30–35 ml	Weight (lb) × 0.5 oz
2. Exercise	+ 500–750 ml per 30–60 min	+ 17–25 oz per 30–60 min
3. Hot / humid climate	+ 10–20%	+ 10–20%

Worked examples

65 kg office worker, temperate climate: 65 × 32 ml ≈ 2.1 L/day (71 oz).
85 kg runner, 45-minute run, summer: 85 × 33 ml ≈ 2.8 L, + 600 ml for the run, + 15% for heat ≈ 3.9 L/day (132 oz).
150 lb adult: 150 × 0.5 ≈ 75 oz/day (2.2 L) before exercise.

These figures align with the U.S. National Academies' adequate intakes (2.7 L/day total for women, 3.7 L/day for men, including ~20% from food). For the full background, read how much water should I drink a day.

Why a static calculator isn't enough

A one-time number has two blind spots:

Your needs change daily. A rest day in an air-conditioned office and a long hike in July are completely different hydration problems.
Not every drink counts the same. Research in the American Journal of Clinical Nutrition (Maughan et al., 2016) shows coffee and tea hydrate at roughly 85–95% the rate of water, while electrolyte drinks hydrate at ~110%. If you count a latte as pure water, your "calculator result" is off before lunch.

Let the app be your always-on water calculator

SupaHydrate runs this calculation for you — and keeps it current:

Personalized goal computed from your body weight, activity level, and climate during onboarding, in metric or imperial units.
Beverage Hydration Index weighting on every logged drink, so your progress reflects fluid your body actually retains.
Live progress on your lock screen, Dynamic Island, widgets, and Apple Watch.
Smart reminders that pace your intake across the day instead of leaving you 1.5 L short at 9 pm.

Skip the math — get your goal automaticallySupaHydrate computes and updates your daily water goal for you, free on iPhone & Apple Watch.

Quick reference: daily goals by weight

Weight	Baseline goal	With 45 min exercise
55 kg / 121 lb	1.7–1.9 L (57–65 oz)	2.3–2.6 L (78–88 oz)
70 kg / 154 lb	2.1–2.45 L (71–83 oz)	2.7–3.2 L (91–108 oz)
85 kg / 187 lb	2.55–3.0 L (86–101 oz)	3.2–3.7 L (108–125 oz)
100 kg / 220 lb	3.0–3.5 L (101–118 oz)	3.6–4.2 L (122–142 oz)

Sources: National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for Water (2005); Maughan et al., American Journal of Clinical Nutrition (2016); American College of Sports Medicine — Exercise and Fluid Replacement position stand.