Fahrenheit ↔ Celsius Converter

Reading a temperature correctly, in either scale

Most American thermometers read Fahrenheit while discharge instructions, imported devices, and clinical literature use Celsius. That mismatch causes real confusion at exactly the wrong moment, when someone is unwell at 2am and the instruction sheet says to call if the temperature passes 38 degrees. The conversion is arithmetic: Fahrenheit equals Celsius times nine fifths plus 32, so 38 degrees Celsius is 100.4 degrees Fahrenheit, the standard fever threshold.

For a rough estimate without a calculator, double the Celsius figure and add 30. That turns 37 into 104, which is four degrees high, so it works for judging a ballpark but never for deciding whether to seek care. Around body temperature the useful anchors are worth memorizing: 36.1C is 97F, 37C is 98.6F, 38C is 100.4F, 39C is 102.2F, and 40C is 104F.

Fever in older adults behaves differently

The familiar 100.4F rule was written for the general population, and it under-detects infection in seniors. Baseline body temperature falls with age, so an 85 year old whose normal reading is 97.0F may be genuinely febrile at 99.5F, well below the classic cutoff. Geriatric guidance therefore treats a persistent reading of 99F or higher, or a rise of two degrees Fahrenheit above that person's own baseline, as a possible infection.

This is why establishing a baseline matters. Take the temperature at the same time of day for several healthy days and write it down. Behavioral change often arrives before the number does: new confusion, unusual drowsiness, loss of appetite, or a sudden change in continence can signal infection in an older adult whose thermometer still looks reassuring.

Getting an accurate reading

Site changes the number. Oral readings run about 0.5 to 1 degree Fahrenheit below rectal, armpit readings run roughly a degree lower still, and forehead scanners drift with room temperature and sweat. Wait 15 minutes after a hot or cold drink before an oral reading, use the same site every time, and note the site alongside the number. For a fuller walkthrough of what a reading means, use the body temperature checker, and pair it with the water intake calculator since fever increases fluid needs noticeably.