Fever and Dehydration in a Child: Why Fluids Matter and What to Watch For
A fever increases how much fluid your child loses through sweat and faster breathing, while feeling unwell often makes them drink less. Offer small sips often, watch for fewer wet nappies or a dry mouth, and seek help if your child stops drinking altogether.
Why hydration matters during a fever
A higher body temperature means your child loses more fluid through sweat and faster breathing, even without vomiting or diarrhoea. At the same time, a sore throat, tummy upset, or just feeling unwell often means they eat and drink less than usual.
That combination, more fluid going out and less coming in, is why dehydration can build up faster during a fever than people expect. Babies and younger toddlers have smaller fluid reserves, so they can tip from fine to dehydrated more quickly than an older child or adult.
Early signs your child needs more fluids
These are a cue to offer drinks more often, not a reason to worry on their own:
- A mouth and lips that feel drier than usual
- Fewer wet nappies or diapers than normal, or urine that looks darker and stronger
- Asking for drinks more often, or seeming thirstier than usual
- A little less energetic than usual, but still alert, responsive, and able to settle
Signs of more serious dehydration
Call your GP or polyclinic the same day if your child is drinking much less than usual, or has clearly fewer wet nappies over several hours.
Go to your nearest 24-hour A&E if your child has a fever and any of these:
- !No wet nappy or urine passed for 12 hours or more
- !A dry mouth and tongue, with no tears when crying
- !Sunken eyes, or a sunken soft spot on the head (fontanelle) in a baby
- !Very drowsy, floppy, or hard to wake
- !Cold hands and feet with mottled or pale skin
What to offer, and how much
Frequent small amounts are easier to keep down and absorb better than trying to get your child to finish a full cup in one go, especially if they feel nauseous.
Continue breast milk or formula as usual, offered more often. Plain water is not recommended as a substitute for milk feeds at this age.
Water, breast milk, formula, or diluted feeds are all fine. There is no need to switch to anything special for a fever alone.
Useful if your child is also vomiting or has diarrhoea, because it replaces salts as well as fluid. Mix it exactly as the packet instructs, not stronger.
Undiluted fruit juice, soft drinks, and sports drinks. The sugar content can make diarrhoea worse and does not replace electrolytes the way ORS does.
A fever on its own, without vomiting or diarrhoea, does not need ORS. Plain fluids your child will actually drink are the priority.
If your child is also vomiting or has diarrhoea
Wait 15 to 30 minutes after a vomiting episode, then offer a teaspoon or two of fluid every 5 to 10 minutes. Increase the amount gradually as your child tolerates it. ORS is particularly useful here, since it is designed to be absorbed even when the gut is unsettled.
If your child cannot keep any fluid down for several hours, or the diarrhoea is frequent and watery, see a doctor sooner rather than waiting it out at home.
→ Fever with vomiting in a child: what to do and when to get help
→ Toddler fever 39°C: when to monitor vs seek help
→ Fever in a baby under 3 months: what to do
→ What happens when you take a feverish child to A&E
Frequently asked questions
How can I tell if my child is dehydrated during a fever?+
Early signs include a drier mouth than usual, fewer wet nappies or darker urine, and being more thirsty than normal. Your child may seem a little less energetic but should still be alert and responsive. These early signs are a cue to offer fluids more often, not a reason to panic.
How much fluid should my child drink when they have a fever?+
There is no fixed target. Offer small sips often rather than a large amount at once, and let your child drink to thirst. A useful check is nappy output: a baby or toddler who is still producing pale urine at roughly the usual frequency is taking in enough fluid.
Can I give my child plain water during a fever?+
For babies under 6 months, stick to breast milk or formula as usual, offered more often. For older babies and toddlers, water is fine alongside their usual milk and food. Plain water on its own is not a substitute for oral rehydration solution if your child is also vomiting or has diarrhoea.
Does the fever itself cause dehydration, or is it the illness?+
Both. A higher temperature increases fluid loss through sweat and faster breathing, while feeling unwell often means your child eats and drinks less than usual. The combination is why dehydration can develop faster during a fever than people expect, especially in babies and young toddlers.
When should I give my child oral rehydration solution (ORS)?+
ORS is most useful if your child has a fever alongside vomiting or diarrhoea, because it replaces both fluid and the salts lost. Mix it exactly as the packet instructs. For a fever on its own, without vomiting or diarrhoea, plain fluids such as water, milk, or diluted feeds are usually enough.
When does dehydration during a fever need medical attention?+
Call your GP or polyclinic the same day if your child is drinking much less than usual or has noticeably fewer wet nappies. Go to your nearest 24-hour A&E if there is no wet nappy or urine for 12 hours or more, no tears when crying with sunken eyes, or your child becomes very drowsy or hard to wake.