Enter a temperature reading and how it was taken. Fever thresholds shift depending on the method, so the same number can mean something different on the ear, the mouth or under the arm.
Reading interpretation
This tool offers a general interpretation based on commonly used fever thresholds. It is not a diagnosis and does not replace the judgement of a doctor, nurse or pharmacist. If in doubt, or if a fever is high, persistent or affects a baby, seek medical advice.
Why the same number can mean different things
Body temperature readings shift depending on where they are taken. A rectal or tympanic (ear) reading tends to run about 0.5°C higher than an oral (mouth) reading, while an axillary (armpit) reading tends to run about 0.5°C lower. This tool adjusts the classification for the method you select, using oral thresholds as the reference point, in line with the general approach used in NHS patient guidance.
How the classification is built
- Normal - below 37.5°C oral-equivalent.
- Low-grade fever - 37.5 to 37.9°C oral-equivalent.
- Fever - 38.0 to 38.9°C oral-equivalent. The NHS generally describes a temperature of 38°C or higher as a high temperature.
- High fever - 39.0 to 39.9°C oral-equivalent.
- Hyperpyrexia - 40.0°C oral-equivalent or above, a medical emergency.
When to seek medical advice
A fever in a baby under 3 months old always warrants urgent medical attention in UK guidance, even if it seems mild, since infection can progress quickly at that age; NHS advice is to call 999 or go to A&E for a baby under 3 months with a temperature of 38°C or higher. For older children and adults, contact a GP, pharmacist or NHS 111 if a fever lasts more than a couple of days, climbs into the high fever or hyperpyrexia range, or comes with symptoms such as a stiff neck, rash, breathing difficulty or confusion.
What this tool cannot tell you
A single temperature reading does not identify the cause of a fever, and a normal temperature does not rule out illness. This tool does not account for individual baseline temperature, medication such as paracetamol or ibuprofen, or the accuracy of the thermometer used.
This tool is not a medical device and does not provide a diagnosis. If you have any concern about a fever, particularly in a baby or young child, contact a GP, pharmacist, NHS 111 or emergency services. All calculations happen in your browser; nothing you enter is saved or sent anywhere.