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Fever Guide — When Is a Temperature a Fever?

A temperature of 100.4°F (38.0°C) or higher is a fever, at any age. In babies, a rectal reading is the most reliable. Fever is one of the commonest reasons families seek care, and the rules are simpler than the worry suggests: one threshold for everyone, one urgent exception for the youngest babies, and a short list of warning signs that matter more than the number itself.

The under-3-months rule: Any fever in a baby under 3 months old — 100.4°F (38.0°C) or higher, measured rectally — needs medical care the same day. Do not simply watch and wait at home, and ask a clinician before giving fever medicine at this age.

Fever thresholds by age

AgeWhat counts, and what to do
Under 3 months100.4°F (38.0°C) or higher, measured rectally, is a fever and needs medical care the same day. A temperature below 97.7°F (36.5°C) in a newborn also needs prompt advice.
3–6 months100.4°F (38.0°C) or higher is a fever. Contact your child's clinician the same day if it reaches 102°F (38.9°C), if your baby seems unusually sleepy, irritable or unwell, or whenever you are worried.
6 months – 2 yearsFever is common at this age and is usually part of fighting an infection. Seek advice if a fever over 102°F (38.9°C) lasts more than a day, if any fever rises above 104°F (40.0°C), or if your child seems dehydrated, very sleepy or develops a rash.
2–12 yearsHow the child looks and behaves matters more than the exact number. Seek care for a fever lasting more than 3 days, a fever above 104°F (40.0°C), trouble breathing, a stiff neck, unusual drowsiness, or a rash that does not fade when pressed.
Teens and adultsSeek care for a fever above 103°F (39.4°C), a fever lasting more than 3 days, or any fever with severe headache, stiff neck, confusion, chest pain, trouble breathing or a rash that does not fade when pressed.
Older adultsFever can be blunted with age. A rise of about 2°F (1.1°C) above the person's usual temperature counts as a fever for them — and new confusion, unusual weakness or a fall during an illness warrants prompt medical advice even if the thermometer reading looks modest.

How to measure, by age

The right method depends on age — and in young babies the method matters, because the fever threshold is defined on a rectal reading.

AgeBest method
Under 3 monthsRectal — the most accurate method at this age, and the reading fever guidance for young babies is based on.
3 months – 4 yearsRectal is still the most accurate. An armpit (axillary) reading is a reasonable first check, but it reads lower and less precisely, so confirm an important reading rectally.
4 years and upOral (mouth) works well once a child can keep the thermometer under the tongue with lips closed. Ear and forehead thermometers are convenient alternatives — follow the device instructions carefully, as technique affects the result.
Adults and older adultsOral is standard. Wait 15–30 minutes after eating, drinking or smoking, and keep the thermometer under the tongue with the mouth closed.

When to seek care promptly

Whatever the thermometer says, seek medical care promptly — the same day, or urgently if the person looks seriously unwell — when any of these are present:

  • A baby under 3 months old has a temperature of 100.4°F (38.0°C) or higher — same-day care, every time.
  • Trouble breathing, noisy breathing, or lips or face that look blue or grey.
  • Hard to wake, unresponsive, or unusually floppy or drowsy.
  • A stiff neck, a severe headache, or light hurting the eyes.
  • A rash that does not fade when a glass is pressed against it.
  • Signs of dehydration: very little urine, no tears when crying, a dry mouth, or unusual sleepiness.
  • A seizure (fit) — especially a first seizure, or one lasting more than a few minutes.
  • A fever above 104°F (40.0°C) at any age, or a fever lasting more than 3 days.
  • The person has a weakened immune system, is on immune-suppressing medicines, or you are simply worried — trust that instinct and seek advice.

For the normal temperature range at each age, see the vital signs by age chart or the page for each band, such as newborn and infant vital signs.

Frequently asked questions

Is a fever dangerous in itself?

Fever is a sign, not an illness — it is the body’s normal response to infection, and the height of the fever alone does not tell you how serious the illness is. What matters most is the age of the person (under 3 months is the urgent exception), how they look and behave, and whether any of the warning signs on this page are present.

Should I try to bring every fever down?

The goal of fever medicine is comfort, not a normal number. A child who is drinking, playing and comfortable can usually be watched without medicine even with a fever, while a miserable child may be given fever medicine for comfort — follow the product’s age guidance or your clinician’s advice, and ask a clinician first for any baby under 3 months.

Why do babies under 3 months have a different rule?

Young babies have immature immune systems and cannot show the usual signs of a serious infection reliably, so a fever at this age is always checked in person, the same day. The threshold is 100.4°F (38.0°C) or higher measured rectally — the most accurate method at this age.

Can a fever be too low instead?

Yes, at the extremes of age. A newborn whose temperature falls below 97.7°F (36.5°C) and does not come up with warmth needs prompt advice, and in older adults a serious infection sometimes shows as a lower-than-usual temperature or a rise from a low baseline rather than a textbook fever.

Please note: This page is patient education, not medical advice. Normal ranges are guides for a person who is awake, resting and measured correctly — a single reading outside the range is not a diagnosis. Abnormal readings in someone who is unwell, breathing with difficulty, unusually drowsy or simply worrying you warrant medical care — seek it promptly rather than re-checking at home.