HealthScorer

Daily Water Intake Calculator

Personalised daily water target based on body weight, activity, climate, and life stage. EFSA 2010 + IOM 2005 baselines.

Last updated: Sources verified:

How much water do you actually need?

The answer depends on your size, what you do today, and the climate. The calculator personalises a target based on body weight (30 mL/kg) plus adjustments for exercise, hot environments, pregnancy, and breastfeeding.

This personalised approach lines up with EFSA 2010 (2.0 L for adult women, 2.5 L for men in temperate climates) and IOM 2005 (2.7 L for women, 3.7 L for men, including food water).

What the formula does

Base = body weight (kg) × 30 mL + activity = 500 mL per 60 minutes of moderate-to-vigorous exercise + hot climate = 500 mL + pregnancy = 300 mL + breastfeeding = 700 mL

The base of 30 mL/kg is a clinical rule of thumb that maps closely to the EFSA averages for typical body sizes (60–80 kg). Below 60 kg or above 90 kg it’s the more accurate way to set the target than a flat number.

The output is fluid from drinks. About 20–25% of total water intake comes from food (especially fruits, vegetables, soups, and yoghurt) — that’s accounted for separately and not displayed in the target.

Common scenarios

PersonBody weightActivityClimateRecommended drinks
Office worker, woman65 kg0 minTemperate~2.0 L
Office worker, man80 kg0 minTemperate~2.4 L
Recreational runner75 kg60 min runTemperate~2.8 L
Construction worker80 kgHigh activityHot~3.4 L
Pregnant, third trimester70 kgLight walkingTemperate~2.6 L

These are starting points. Use thirst plus urine colour (pale straw = well hydrated) as feedback.

Hydration markers — how to check yourself

Per the Armstrong 2016 systematic review:

  • Urine colour: pale straw is well-hydrated; dark amber suggests under-hydration.
  • Body weight changes during exercise: losing more than 2% of body weight during a session means you didn’t drink enough during it.
  • Thirst: a reliable signal in healthy adults. The “drink before you’re thirsty” rule applies mostly during prolonged heat or exercise, when thirst lags behind actual need.

You don’t need lab tests for everyday hydration awareness.

Pair with our other tools

Limits

  • Kidney disease, heart failure: fluid restriction is often medically prescribed. Follow your clinician, not this calculator.
  • Endurance athletes: target hydration during prolonged exercise to body-weight loss <2%. Pre-/post-session weight checks are more accurate than time-of-day fluid logs.
  • Children: this calculator is for adults. Paediatric needs are weight-based but use different ranges.

Privacy

All calculation happens in your browser. We never see, log, or store your inputs. Anonymous events (your locale) go to our privacy-first analytics.

Frequently asked questions

Is the '8 glasses a day' rule right?
It's a memorable number, not a clinical recommendation. EFSA 2010 recommends adequate intake of total water (food + drinks) of 2.0 L/day for adult women and 2.5 L/day for men in temperate climates. About 20–25% of that comes from food. So target fluid from drinks lands around 1.6 L/day for women and 2.0 L/day for men — close to but not exactly 8 glasses. Personalising by body weight (30 mL/kg) lands you in roughly the same range and adjusts for size.
Why does this calculator add water for exercise?
Exercise increases sweat losses. ACSM 2007 recommends roughly 500 mL extra per hour of moderate-to-vigorous exercise, more in heat. We add 500 mL per 60 minutes as a conservative average; if you're a heavy sweater or training above 60 minutes regularly, increase from there based on body weight checks (lose more than 2% of body weight during a session = under-hydrated).
Should pregnant or breastfeeding women drink more?
Yes. EFSA 2010 adds 300 mL/day to the baseline for pregnancy, and 700 mL/day for breastfeeding. The calculator factors this in if you check the relevant box. Plain water is generally the right choice; consult your obstetrician if you have any condition affecting fluid balance (pre-eclampsia, hyperemesis).
Does coffee or tea count?
Yes. The old idea that caffeine is dehydrating doesn't hold up. The Armstrong 2016 systematic review concluded that fluid balance after typical coffee or tea intake is similar to plain water for most adults. Coffee, tea, milk, juice, broth — all contribute to your fluid total. Soft drinks count too but bring sugar; alcohol is the exception (mildly diuretic at modest doses, more so at higher).
Does the data leave my device?
No. Calculation runs entirely in your browser. Anonymous events (locale only) are sent to our privacy-first analytics.

Sources

  1. Scientific opinion on dietary reference values for water — European Food Safety Authority (EFSA, 2010) (guideline, retrieved 2026-04-28)
  2. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate — Institute of Medicine (IOM, 2005) (guideline, retrieved 2026-04-28)
  3. Selected hydration markers and their use in the assessment of hydration status: a systematic review — Eur J Nutr (Armstrong et al., 2016) (peer reviewed, retrieved 2026-04-28)