How To Convert Temperatures
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The four scales
To estimate C from F quickly: subtract 30, divide by 2. “86 °F” → 56 / 2 = 28 °C (actual: 30). Accurate enough for weather conversation. For F from C, double and add 30: 20 °C → 40 + 30 = 70 °F (actual: 68). These shortcuts lose accuracy at extremes but work for the range humans normally encounter.
Celsius to Fahrenheit and back
0 K = 0 °R = −273.15 °C = −459.67 °F. This is the temperature at which classical molecular motion stops. You cannot reach it (third law of thermodynamics); you can only asymptotically approach it. The coldest temperatures ever achieved in labs are a few billionths of a Kelvin above zero. Negative Kelvin or Rankine values are nonsensical in the normal sense.
Mental shortcuts
Normal body temperature is ~37 °C = 98.6 °F = 310.15 K. A fever is 38 °C (100.4 °F) and above. Hypothermia begins below 35 °C (95 °F). If your conversion produces a body temperature of 200 or 30, you slipped a digit. Also useful: room temperature is ~20–22 °C = 68–72 °F. An ice bath is 0 °C. Boiling water is 100 °C. Keep these in mind as guardrails.
Celsius and Kelvin
Oven recipes collide with scale differences. Key conversions:
Fahrenheit and Rankine
Most ovens round to 5 °C or 25 °F increments, so a recipe’s exact value matters less than the range. A recipe calling for 177 °C will do fine at 175 °C or 180 °C.
Absolute zero
Celsius: most of the world, weather, cooking outside the US, biology, meteorology. Fahrenheit: US consumer weather, US home cooking, US medical charts. Kelvin: physics, chemistry, engineering thermodynamics, astronomy. Rankine: some US aerospace and HVAC calculations.
Human body as a sanity check
For weather reports, one decimal is usually enough. For body temperature, tenths of a degree matter clinically. For scientific work, use the full formula and carry as many significant figures as your input. When reporting, round to match the precision of your measurement—saying 98.6001 °F when your thermometer reads to 0.1 is fake precision.