Understanding Temperature Scales

Three temperature scales see everyday use. Celsius (°C) is anchored to water: 0 °C is the freezing point and 100 °C is the boiling point at sea level — an intuitive system for most of daily life. Fahrenheit (°F) was calibrated in the early 1700s using a brine solution for 0 °F and a rough estimate of human body temperature (~96 °F in Fahrenheit's original scale). Kelvin is the scientific scale, starting at absolute zero (0 K = −273.15 °C), the point at which molecular motion theoretically stops entirely.

The −40° Coincidence

There is exactly one temperature where Celsius and Fahrenheit give the same number: −40°. It's the only intersection point of the two scales, a consequence of their different offsets and scaling factors. For quick mental conversion from °C to °F, a useful approximation is to double the Celsius value and add 30 (e.g., 20 °C → 70 °F). For precision, the actual formula is: multiply by 9/5, then add 32.

Real-World Reference Points

Human body temperature: 37 °C / 98.6 °F. A comfortable room: 20–22 °C / 68–72 °F. Water boils at 100 °C / 212 °F at sea level (lower at altitude, which is why high-altitude cooking instructions differ). The surface of the Sun reaches approximately 5,500 °C. At the opposite extreme, absolute zero is −273.15 °C / −459.67 °F / 0 K — a limit that can be approached but never physically reached.

Why the US Uses Fahrenheit

The US adopted Fahrenheit in the colonial era, when it was the dominant scale in the English-speaking world. An attempt to switch to Celsius in the 1970s — part of a broader push toward metrication — stalled due to public resistance and was largely abandoned by 1982. Most other countries made the switch to Celsius during the 20th century as part of national metrication programs. The result is that the US stands nearly alone in using Fahrenheit for everyday weather and cooking.