Early signs
Mild dehydration often shows up before you feel very thirsty:
- Dark yellow urine — the simplest, most reliable check (aim for pale straw).
- Thirst and a dry mouth or lips
- Headache
- Tiredness and low energy
- Trouble concentrating or feeling foggy
- Less frequent urination
More serious signs
Significant dehydration needs prompt attention:
- Dizziness or light-headedness
- Rapid heartbeat
- Very dark urine or barely urinating
- Sunken eyes, dry skin that's slow to bounce back
- Confusion (seek medical help)
What causes it
Simply not drinking enough is the most common cause, but heat, exercise and sweating, illness (vomiting, diarrhoea, fever), alcohol, and high altitude all increase fluid loss. Older adults and children are more vulnerable.
How to rehydrate
For mild dehydration, sip water steadily — don't gulp huge amounts at once. After heavy sweating or illness, you also lose electrolytes (sodium, potassium), so an oral rehydration drink or a salty snack with your water helps your body hold onto the fluid. Foods like fruit and soup count too.
How much you need
A practical target is around 35 ml per kg of body weight plus extra for exercise and heat — for many adults that's 2–3 litres. Our water intake calculator gives your personal number, and the easiest daily check is simply keeping your urine pale.
When to get help
Seek medical care for severe symptoms — confusion, fainting, a racing heart, or no urination — or for dehydration in a baby, young child, or older adult that doesn't improve with fluids.
Why replacing water alone is sometimes not enough
Sweat is not just water — it contains sodium, and to a lesser extent potassium, chloride and magnesium. After heavy or prolonged sweating, or after illness involving vomiting or diarrhoea, you have lost both fluid and electrolytes.
Drinking large volumes of plain water in that state can be counterproductive. It dilutes the sodium remaining in your blood, and because your body regulates sodium concentration rather than total volume, it responds by excreting the excess water — so you urinate frequently while still feeling unwell and still being under-replaced.
The fix is to include sodium alongside the fluid. An oral rehydration solution is the purpose-built option and is what to use after significant illness. For ordinary heavy sweating, a salty snack with your water, a normal meal, or milk all work perfectly well. Milk in particular performs surprisingly strongly in rehydration studies, because it provides fluid, sodium, potassium and protein together.
For everyday mild dehydration — the kind you get from being busy and forgetting to drink — plain water is entirely sufficient and this whole consideration does not apply.
Who is at higher risk
Dehydration does not distribute evenly, and some groups need more attention than a general urine-colour check provides.
Older adults are at the highest everyday risk. The thirst mechanism becomes less sensitive with age, kidney concentrating ability declines, and total body water is lower to begin with. Some older people also deliberately limit fluids to reduce bathroom trips, particularly at night. In this group, confusion or a sudden change in alertness can be the first noticeable sign of dehydration rather than a late one.
Infants and young children have a higher surface-area-to-weight ratio, lose fluid faster, and cannot reliably communicate thirst. Warning signs include fewer wet nappies, no tears when crying, a sunken soft spot, unusual drowsiness, and dry mouth. Dehydration in a baby escalates quickly and warrants prompt medical attention.
People who are unwell — fever raises fluid loss, and vomiting or diarrhoea can lose litres quickly. This is where dehydration becomes genuinely dangerous rather than uncomfortable.
People on diuretics, and those with diabetes, kidney conditions, or who are pregnant or breastfeeding, all have altered fluid requirements. Some conditions require fluid restriction, in which case general guidance does not apply and your clinician's does.
Building the habit of noticing
Most people discover they are dehydrated after the headache arrives. The useful skill is catching it earlier, and that is mostly a matter of having one check you actually perform.
Urine colour works because it requires no equipment and no memory — you are already in the bathroom. Deciding once that you will glance and act on it turns an occasional realisation into a reliable signal.
The second habit worth building is anticipating rather than reacting. Hot weather, a long flight, a hard training session, or a night of drinking all predictably increase your needs. Drinking a little more before and during those, rather than catching up afterwards, avoids the deficit entirely.