How much water to drink a day: thirst kicks in late.

How much water should you drink a day? At work, hardly anyone asks. Then 3pm comes: three empty mugs on the desk, and this morning's bottle still half full. A thick head, and a craving for something sweet.

You blame lunch, or a bad night. Often it's simpler than that: you haven't drunk enough. When I ask the people I work with, many say the same: under a litre. And none of them feels thirsty.

Not drinking enough water: the signs you miss

Drinking too little rarely feels like thirst. It feels like a bad day.

• Tired and foggy. Mid-afternoon you reread the same line three times.
• Fake hunger. Mostly for something sweet: mild dehydration is easily mistaken for hunger.
• Dark urine, few trips to the loo. Pale is fine. Dark means you're running low.
• Dry mouth and puffiness. Ankles and fingers more swollen by evening, sometimes because you drink too little.

Why you miss it

You lose two and a half litres a day, even at a desk. A litre and a half in urine, half a litre in sweat, the rest through your gut and your breath. Breathing alone, about 300ml.

Thirst fades with age. The signal weakens. Your body needs water and you feel nothing.

It doesn't take much. Your brain is three-quarters water. Lose about one per cent of your body's water, without topping it up that day, and head and body both start working less well.

The less you drink, the more you hold on to. When water runs short, your body raises a hormone that turns down the tap at your kidneys, so you pee less and retain fluid. The puffiness you blame on water often comes from a lack of it.

How much water should you drink a day?

About 30ml per kilo. It's the simplest way to work out how much water to drink for your weight. At 70kg (11 stone) that's just over two litres; at 80kg, nearly two and a half.

The minimum and the extra. Drink at least a litre and a half. The rest comes from food, about 700ml from fruit, vegetables and soups. In the heat, or when you sweat or train, go up to 50ml per kilo.

Most of it by mid-afternoon. A large glass as soon as you're up, then front-load the morning and early afternoon. After 5pm, just the odd sip. Drink a lot in the evening and your body wakes you to get rid of it.

Tie it to what you already do. A glass when you get to the office, one before each meeting, one before you sit down to eat. Eight large glasses make two litres, with nothing to remember.

A glass before the sweet stuff. When the 3pm craving hits, drink and wait a few minutes. A lot of sugar cravings are thirst.

If you sweat a lot, salts too. In real heat or a long session, water alone falls short: you need sodium too. On a normal office day we already eat more than enough salt.

Start tomorrow with the bottle on your desk. It's the simplest habit I know, and the first one people forget.