Temperature and fever

Convert between Fahrenheit and Celsius, plus what the number means for an adult, a child or a baby.

Your details

38.4°C is

101.1°F

A high temperature

Celsius38.4°C
Fahrenheit101.1°F
NHS high temperature line38°C (100.4°F)
Taken in the armpitfine; follow the instructions that came with your thermometer
Days with a high temperatureadd them if it has gone on for a while

The NHS counts 38C or above as a high temperature. It is usually a sign of your body fighting an infection and often gets better on its own; rest and plenty of fluids help.

Ask for an urgent GP appointment or call 111 if you have been treating a high temperature at home and it is not getting better or is getting worse. Call 999 in an emergency.

The working

38.4°C for an adult

Celsius x 9 ÷ 5 + 32 = Fahrenheit, so 38.4 x 1.8 + 32 = 101.1°F. The NHS counts 38°C or more as a high temperature, so this is one. For a baby under 3 months, the same reading would mean an urgent GP appointment or a call to 111.

The lines

When the number matters most

Who	Line	What to do
Baby under 3 months	38°C or more	urgent GP appointment or 111
Baby 3 to 6 months	39°C or more	urgent GP appointment or 111
Children and adults	38°C or more	a high temperature; watch how they are
Anyone	below 35°C	hypothermia, call 999

Taking it

A fair reading

Use a digital thermometer; for a child, tuck it into the top of the armpit and hold the arm against their side
After a bath or being wrapped in a blanket, wait a few minutes and check again
Follow the instructions that came with your thermometer
Feeling hot, cold or shivery can mean a high temperature even if the number is under 38

How and how long

Where it was taken, and for how many days

Armpit: NICE recommends a digital thermometer in the armpit for babies under 4 weeks, and the armpit or an ear thermometer for children up to 5.
By mouth: fine for older children and adults, but NICE advises against it for children under 5.
Forehead strips: NICE calls them unreliable. If that is what you used, check again with a digital thermometer.
Days: in a child, a fever lasting 5 days or more needs a doctor to check them (NICE NG143), so the page says so. For adults, the NHS advice is to get an urgent GP appointment or call 111 if a high temperature is not getting better.

Read with care

What this cannot tell you

Tell you the causeIt is often a common infection, but a number cannot say.
Judge how ill someone isHow they look and act matters more.
Replace 111If you are worried, call.

Questions

Common questions

What is a high temperature in an adult?

The NHS says a high temperature is usually 38°C or above.

What is 38C in Fahrenheit?

100.4°F.

When is a baby's temperature an emergency?

Under 3 months with 38°C or more, or 3 to 6 months with 39°C or more, needs an urgent GP appointment or 111. Call 999 if they are hard to wake, have a rash that does not fade under a glass, or struggle to breathe.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Talk your results through

If a result worries you, book a chat with your GP or practice nurse, or ask a pharmacist. Call NHS 111 if you feel unwell and are not sure what to do, and 999 in an emergency.