The textbook 98.6°F (37°C) is from an 1851 study using broken thermometers. Modern normal is lower.
If you have ever stared at a number on a form and wondered where it came from, you are not alone. The textbook 98.6°F (37°C) is from an 1851 study using broken thermometers. Modern normal is lower.
97.5°F / 36.4°C average. Range 97–99°F (36.1–37.2°C) depending on time of day and individual.
100.4°F (38°C) is the medical cutoff for fever in adults.
Lowest at 4–6am, peaks late afternoon. Ovulation raises baseline ~0.5°F.
Why is my baseline below 97? Hypothyroidism, aging, or just individual variation. Pattern matters more than one reading.
When is fever an emergency? Adults >103°F that persists, or any fever with stiff neck, confusion, or chest pain.
Bookmark the calculator linked above — you will need it more often than you think.