Loading page
About this tool
Convert body temperature between Celsius, Fahrenheit and Kelvin and adjust for oral, rectal, ear, forehead or armpit readings.
The Fever Temperature Converter changes a thermometer reading between Celsius, Fahrenheit and Kelvin with the exact formulas °F = °C × 9/5 + 32 and K = °C + 273.15, then compares it against the fever threshold for the site it was taken from. Because those thresholds differ — 38.0 °C (100.4 °F) rectal or ear, 37.8 °C (100.0 °F) oral, 37.2 °C (99.0 °F) armpit — the same number can mean fever at one site and not another. It is written for parents and carers translating between a thermometer, a leaflet and a doctor who uses the other scale.
Open Fever Temperature Converter on AltFTool — it loads instantly in your browser.
Enter the Temperature reading and pick the Scale: Celsius (°C), Fahrenheit (°F) or Kelvin (K).
Choose Where it was measured — Oral (under the tongue), Rectal, Ear (tympanic), Forehead (temporal artery) or Armpit (axillary).
Read all three scales with the fever threshold for that site, your distance from it, the core-equivalent reading and a band from Hypothermia range to Hyperpyrexia.
Uses the published fever threshold for oral, rectal, ear, forehead and armpit readings instead of one number for all.
Celsius, Fahrenheit and Kelvin from a single entry, with the core-equivalent figure alongside.
Names the hypothermia and hyperpyrexia bands and when to seek urgent care rather than guessing.
A core or rectal temperature of 38.0 °C (100.4 °F) or higher is the standard definition of fever. The equivalent thresholds are about 37.8 °C (100.0 °F) taken orally and 37.2 °C (99.0 °F) under the arm, because those sites read lower than core.
Subtract 32 and multiply by 5/9. So 100.4 °F is 38.0 °C, 101.3 °F is 38.5 °C and 104 °F is 40 °C. Going the other way, multiply Celsius by 9/5 and add 32.
It is the least reliable of the common sites and typically reads about 0.5–0.8 °C below core, which is why its fever threshold is 37.2 °C rather than 38.0 °C. Use it as a screen and confirm anything borderline with an oral, ear or rectal reading.
Seek urgent care for any fever in a baby under three months, for a core temperature at or above 41 °C (105.8 °F), or at any temperature if there is a stiff neck, a rash that does not fade under pressure, difficulty breathing, a seizure, severe drowsiness or dehydration. This tool is informational and does not replace assessment by a clinician.
Add the Fever Temperature Converter widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
<iframe src="https://www.altftool.com/embed/widget/fever-temperature-converter"
title="Fever Temperature Converter — free AltFTool widget"
width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
loading="lazy" referrerpolicy="no-referrer-when-downgrade" allow="clipboard-write"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/fever-temperature-converter?utm_source=embed&utm_medium=widget" rel="nofollow">AltFTool — free online tools</a></p>