Three temperature scales are in common use worldwide — and they each define their zero point differently. Understanding the logic behind each one makes conversions intuitive rather than formula-dependent.

Celsius (°C) – The Metric Standard

Developed by Anders Celsius in 1742. Defined originally by two fixed points: 0°C = water freezes, 100°C = water boils at standard atmospheric pressure. Used by approximately 95% of the world. SI unit for temperature in science and everyday life outside the US.

Fahrenheit (°F) – The US Standard

Developed by Daniel Fahrenheit in 1724. Defined by a more complex set of reference points: 32°F = water freezes, 212°F = water boils. Human body temperature is 98.6°F. Used primarily in the United States, a few Caribbean islands, and some Pacific territories. Weather in the US is always reported in Fahrenheit.

The key conversion formulas:
C to F: (C x 9/5) + 32
F to C: (F – 32) x 5/9

Kelvin (K) – The Scientific Standard

No degree symbol — just K. The absolute temperature scale, defined by absolute zero (0K = -273.15°C = -459.67°F) — the theoretical point at which all molecular motion stops. The size of one Kelvin equals one degree Celsius. Water freezes at 273.15K and boils at 373.15K. Used universally in physics, chemistry, and engineering. Conversion: K = C + 273.15.

Rankine (°R) – The American Absolute Scale

The Fahrenheit-based absolute scale. Used in some engineering applications in the US. Absolute zero = 0°R. Water freezes at 491.67°R. Conversion: R = F + 459.67.

Key Reference Points

• Absolute zero: 0K / -273.15°C / -459.67°F
• Water freezes: 0°C / 32°F / 273.15K
• Room temperature: ~21°C / ~70°F / ~294K
• Human body: 37°C / 98.6°F / 310.15K
• Water boils: 100°C / 212°F / 373.15K
• Sun surface: ~5,505°C / ~9,941°F / ~5,778K