Keeping them safe · 4 min read
How to tell if they're dehydrated
The vomiting or the diarrhea is unpleasant but survivable. Dehydration is the thing underneath it that actually turns a bug into an emergency, and it is the reason anyone asks you about wet nappies.
It is also one of the few things you can genuinely assess at home, if you know what you are looking at.
Last updated August 19, 2026
Mild to moderate — what to check
Urinating less often than usual. For infants, AAP gives the number: FEWER THAN SIX WET NAPPIES A DAY. That is the single most useful thing to start counting, and it is why it is the first question you will be asked.
A parched, dry mouth. Look inside rather than at the lips — lips dry out from mouth-breathing and a fever without meaning much.
Fewer tears when crying. In a child old enough to make tears, a proper cry with nothing coming out is informative.
A sunken soft spot on the head, in a baby or toddler young enough to still have one. It should be flat and level; a visible dip is a sign.
None of these on its own is an emergency, and together they are the point at which to ring rather than keep watching.
Severe — this is now
Very fussy, or excessively sleepy. Either end of that range counts, and 'finally settled after a difficult day' is exactly how the sleepy version gets misread.
Sunken eyes.
Cool or discolored hands and feet.
Wrinkled skin.
Urinating only once or twice a day.
This list is not for monitoring. Any of it means urgent assessment now — these are signs of a circulation that is being affected, not simply a child who needs a drink.
Age changes the maths
Babies dehydrate quickly, and the same stretch without fluids means something quite different at six weeks than at three years.
AAP's specific instruction: if your baby is younger than 3 MONTHS and has a fever as well as diarrhea, call your pediatrician at once. Not in the morning.
Older than 3 months, with mild diarrhea and a slight fever for more than a day: check whether they are passing a normal amount of urine, and then call.
Note the structure of that advice — it asks you to gather the urine information FIRST and then call, which is exactly what makes the call useful.
What to have ready when you ring
How old they are. When it started. How many wet nappies in the last 24 hours, and when the last one was. Whether they are keeping any fluid down and how much. Whether they are alert and interested, or unusually sleepy. And whether there is a fever.
Those six things are what the conversation is going to be about, and having them counted rather than guessed changes what happens next.
If you are watching and waiting at home, keep counting the nappies rather than the hours — a child who has gone quiet and settled but has not passed urine is the situation this page exists for.
Common questions
- How many wet nappies should a baby have?
- AAP's threshold for concern in infants is fewer than six wet nappies a day. Below that, alongside a bug, is a reason to call rather than continue watching.
- They're sleeping a lot. Is that just being unwell?
- It can be — and excessively sleepy is on AAP's SEVERE dehydration list. A child who is unusually hard to rouse during an illness with vomiting or diarrhea needs urgent assessment, not a quiet afternoon.
- What does a sunken soft spot look like?
- The fontanelle should sit flat and level with the skull. A visible dip or hollow is a sign of dehydration in a baby or toddler young enough to still have one.
- My baby is 8 weeks with a fever and diarrhea.
- Call now. AAP is explicit that under 3 months, a fever together with diarrhea is a call-at-once situation — and under 3 months, fever alone is its own emergency.
If it's this, not that
Still not sure
This page is the general answer. Yours is a specific child.
The advisor can be. It has read everything here, and it knows how old your children actually are — which is the whole difference wherever guidance changes with age. Ask it anything.
- She's got a temperature of 100. Is that something to worry about?
- He's obsessed with space and galaxies — what toys are worth getting him?
Free. It comes with the note.
It answers from the same cited pages you are reading, and when the question is where to go, from the real clinician, hospital and childcare records behind this site — not a link to go search them yourself. It says so when we have not written about something, and it is not a doctor; for anything urgent it gives you the threshold and tells you to call.
Where this comes from
Every claim on this page traces to one of these. See every source we use →
Written from the sources named above, and it is not a substitute for your own doctor. If something feels wrong to you, that instinct is a good enough reason to call — you know yourself and this family better than any page does.