Water Intake Calculator

Why a personal number beats eight glasses

The old advice of eight glasses a day is easy to remember but has no clear scientific origin, and it ignores the obvious: a 55 kg office worker and a 100 kg lifter training in a hot gym do not need the same amount.

Even mild dehydration, around 2% of body weight lost as sweat, has been linked to reduced endurance, higher perceived effort and poorer concentration. For people who train, getting this right is a cheap performance gain.

How to use the calculator

Enter your body weight, the number of minutes you exercise on a typical day and whether you live or train in a temperate or hot climate. The result is shown in litres, 250 ml glasses and US cups and fluid ounces.

Run it twice if your training varies: once for rest days and once for training days. The difference tells you how much extra to drink around your sessions.

How the number is calculated

Base = 35 ml × body weight (kg)

Exercise = + 350 ml per 30 minutes of exercise

Climate = + 500 ml if hot

Example: a 70 kg person who trains for 60 minutes in a temperate climate needs about 2,450 + 700 = 3,150 ml, or roughly 3.2 litres a day. In a hot climate that becomes about 3.7 litres.

The 35 ml per kg base is a widely used clinical rule of thumb for healthy adults. The exercise allowance covers a moderate sweat rate. Heavy sweaters, long sessions and very humid gyms can need more.

How your target compares with official guidance

The European Food Safety Authority set adequate intakes of 2.0 litres a day for women and 2.5 litres for men. Those figures are total water, including the roughly 20% that typically comes from food such as fruit, vegetables, soups and dairy.

Because this calculator adds allowances for training and heat, most active people will see a number above those references. That is expected. The table gives a quick sense of typical results.

Check yourself, not just the number

The simplest hydration check is urine colour. Pale straw yellow usually means you are well hydrated. Dark yellow or amber suggests you need more. Completely clear urine all day can mean you are drinking more than you need. Note that B-vitamin supplements can turn urine bright yellow regardless.

For training, weigh yourself before and after a session. Each kilogram lost is roughly a litre of sweat, and replacing about 1.25-1.5 times that over the next few hours restores fluid balance.

Electrolytes, sweat and hot gyms

Sweat is not just water. It carries sodium, along with smaller amounts of potassium, magnesium and calcium. Sodium losses vary widely between people, and some heavy sweaters lose well over a gram per litre of sweat, which is why some people finish a session with white salt marks on their clothes.

For most gym sessions under an hour, normal meals replace these losses and plain water is enough. Electrolytes become worth adding when:

• you train longer than 60-90 minutes, or do two sessions in a day
• you train in heat or high humidity
• you are a heavy or salty sweater
• you are cramping or feeling unusually flat despite drinking enough
• you follow a low-carbohydrate or keto diet, which increases sodium loss

A simple approach is a pinch of salt in your bottle or an electrolyte drink that provides roughly 300-700 mg of sodium per litre. People with high blood pressure or on a sodium-restricted diet should check with a doctor first.

Heat also raises your needs in ways a single number cannot fully capture. Acclimatising to a hot environment takes around one to two weeks, during which your sweat rate rises. Start sessions hydrated, drink to thirst during them, and weigh yourself before and after to learn your own sweat rate.

Can you drink too much?

Yes. Drinking very large volumes in a short time, particularly during long endurance events, can dilute blood sodium and cause hyponatremia. Early signs include nausea, headache, confusion and swelling. It is rare in normal gym training, but it is the reason not to force down far more than your thirst and this target suggest.

People with kidney, heart or liver conditions, and anyone told to restrict fluids, should follow their doctor's advice instead of any calculator. Needs also rise during pregnancy and breastfeeding.

Making it easy

• Keep a marked bottle with you and know how many refills hit your target.
• Drink a glass with every meal and one when you wake up.
• Count tea, coffee and milk. Moderate caffeine does not dehydrate habitual drinkers.
• Eat water-rich foods. A diet built from the macro calculator with plenty of fruit and vegetables covers a good share.

Creatine draws water into muscle, so if you take it, aim for the top end of your range. The creatine guide explains why.