Body weight is one of the strongest predictors of baseline water requirements — far more useful than a single number applied to everyone regardless of size. A widely used clinical starting point is approximately 35ml per kilogram of body weight per day for a sedentary adult, before accounting for activity, climate, sleep, or stress.
Below is a reference table using this baseline. These are starting points for a sedentary day in moderate conditions — not personalised targets, and not adjusted for the daily variables (sleep, stress, activity, weather) that meaningfully shift requirements day to day.
| Body weight | Baseline daily water (35ml/kg) |
|---|---|
| 50 kg (7st 12lb) | 1.75 L |
| 55 kg (8st 9lb) | 1.93 L |
| 60 kg (9st 6lb) | 2.10 L |
| 65 kg (10st 3lb) | 2.28 L |
| 70 kg (11st 0lb) | 2.45 L |
| 75 kg (11st 11lb) | 2.63 L |
| 80 kg (12st 8lb) | 2.80 L |
| 85 kg (13st 5lb) | 2.98 L |
| 90 kg (14st 2lb) | 3.15 L |
| 95 kg (14st 13lb) | 3.33 L |
| 100 kg (15st 10lb) | 3.50 L |
Why this is a starting point, not a target
This table answers "how much would a sedentary person of this weight need on an average day in moderate conditions?" It does not answer "how much do I need today" — because today's number depends on factors that shift daily and are not captured by weight alone:
- Sleep quality — poor sleep increases overnight fluid loss and disrupts the hormones that regulate fluid retention
- Stress — elevated cortisol affects aldosterone, increasing how much water your kidneys excrete
- Activity level — exercise can add anywhere from 300ml to over 2L depending on duration, intensity, and conditions
- Temperature and humidity — a hot day can add 500ml to over 1L compared to a cool one
A 70kg person on a cool, low-stress rest day after good sleep might genuinely need close to the 2.45L baseline. The same person on a hot day, after poor sleep, with a training session planned, could need 3.5-4L — a difference of over a litre, despite their weight not changing at all.
Why body composition also matters
Two people of identical weight can have meaningfully different requirements based on body composition. Muscle tissue holds significantly more water than fat tissue, so a more muscular person at a given weight will generally have a somewhat higher baseline requirement than a less muscular person at the same weight.
From baseline to actual target
The weight-based baseline is a reasonable floor for a quiet day. perpHect uses your body profile as one of five inputs — combined with last night's sleep, today's stress and energy, your planned activity, and live weather data — to calculate your actual target each morning. This means the same person gets a different number on different days, reflecting what their body genuinely needs that day rather than a static figure based on weight alone.
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Your weight is the starting point — not the answer
perpHect combines your body profile with sleep, stress, activity, and weather to calculate your actual target each morning.
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