Temperature is one of the few conversions where the two scales don't just use a different multiplier — they also start counting from a different zero point entirely. That's what trips people up more than any other conversion, whether it's checking an oven temperature from an international recipe, reading a weather forecast while traveling, or understanding a fever reading on a thermometer calibrated differently than you're used to. This guide walks through the exact formula, a chart of common reference points, and a couple of shortcuts for estimating in your head.

The Celsius-Fahrenheit formulas

°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9

The 32 in the formula exists because the Fahrenheit and Celsius scales don't share a zero point: 0°C (water freezing) is 32°F, not 0°F. The 9/5 (or 5/9 in reverse) accounts for the fact that a Fahrenheit degree is a smaller unit — there are 180 Fahrenheit degrees between freezing and boiling water, versus 100 Celsius degrees over the same range, so each individual Fahrenheit degree represents a smaller temperature change.

Kelvin, the third temperature scale used mainly in science, is simpler by comparison: it uses the same size degree as Celsius but starts at absolute zero, so converting Celsius to Kelvin is just addition (°C + 273.15), with no multiplication involved.

Reference chart for common temperatures

  • 0°C = 32°F (water freezes)
  • 20°C = 68°F (room temperature)
  • 37°C = 98.6°F (average body temperature)
  • 100°C = 212°F (water boils at sea level)
  • 180°C = 356°F (a common baking temperature)
  • −40°C = −40°F (the one point where both scales agree)

That last one isn't a coincidence worth puzzling over — it's simply the point where the two linear scales happen to cross mathematically, and it's a handy way to sanity-check any conversion tool you're using: if a converter tells you −40°C is anything other than −40°F, something's wrong with it.

A shortcut for quick mental estimates

For a rough Celsius-to-Fahrenheit estimate, double the Celsius number and add 30 (instead of the exact ×9/5+32). It's off by a degree or two at extreme temperatures but is close enough for checking whether to bring a jacket. For Fahrenheit to Celsius, subtract 30 and halve what's left.

This shortcut matters most for everyday weather checks and cooking, where being off by a degree or two rarely changes the outcome. For anything more precise — a medical reading, a scientific measurement, or a recipe with a narrow temperature window — use the exact formula or a calculator rather than the mental approximation.

Where this conversion matters most in everyday life

Oven temperatures are one of the most common places this comes up: a recipe calling for 180°C is 356°F, and 200°C is close to 392°F, though many home ovens and recipe writers round to the nearest common setting (350°F or 400°F) rather than the exact converted figure, so a small gap between a converted number and a printed oven dial setting is normal, not an error.

Weather is the other frequent case, especially when traveling: 30°C is a genuinely hot day (86°F), while 0°C is freezing (32°F) — useful mental anchors even without doing the exact math. For anyone tracking a fever, it's worth knowing that a commonly cited threshold for fever in adults is around 38°C (100.4°F), though what counts as concerning can vary by individual and by measurement method, so a thermometer reading is best interpreted with guidance from a healthcare provider rather than a fixed online number.

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Frequently asked questions

Why isn't 0°C equal to 0°F?

Because the two scales were defined independently, with different reference points. Celsius sets 0 at water's freezing point; Fahrenheit's zero was originally based on a brine mixture used by its 18th-century inventor, Daniel Fahrenheit.

What temperature is the same number on both scales?

−40 degrees. At that single point, −40°C and −40°F represent the same temperature, which is a useful fact for checking whether a converter is working correctly.

Is Kelvin harder to convert than Fahrenheit?

Actually easier for Celsius conversions — Kelvin uses the same degree size as Celsius, so converting between them is just adding or subtracting 273.15, with no multiplication step required.