Subjects and lessons to explore.

Law

General Chemistry I · Matter, Energy & Measurement

Temperature Conversions: Celsius, Fahrenheit, and Kelvin

5 min read
SourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Temperature measures the average kinetic energy of the particles in a sample — how vigorously its atoms and molecules are moving. Three scales are in everyday use: Fahrenheit (°F, common in the U.S.), Celsius (°C, used in science and most of the world), and Kelvin (K, the SI base unit, with no degree symbol). Celsius and Fahrenheit are "relative" scales with arbitrary zero points, but the Kelvin scale is absolute: 0 K ("absolute zero") is the temperature at which particle motion reaches its theoretical minimum. Because Kelvin starts at true zero, it is the only scale that behaves proportionally and can be plugged directly into physical laws such as the ideal gas law.

Why this matters

Most chemical quantities — gas volume, reaction rate, pressure — depend on absolute temperature. Using Celsius in an equation like PV = nRT gives absurd or negative answers because Celsius can go negative. Converting correctly, especially remembering the 273.15 shift, is a prerequisite for the gas laws, thermochemistry, and kinetic theory covered later. In daily life, the formulas also let you interpret a fever (37.0 °C = 98.6 °F), a weather forecast, or a lab instrument reading.

The college version

Key Ideas

  • Celsius (°C): defined by the freezing point (0 °C) and boiling point (100 °C) of water at 1 atm.
  • Fahrenheit (°F): freezing point 32 °F, boiling point 212 °F — a 180-degree span for the same 100-Celsius-degree change.
  • Kelvin (K): the SI unit; a change of 1 K equals a change of 1 °C (the units are the same size), but the scale is shifted so that 0 K = −273.15 °C.
  • Absolute zero = 0 K = −273.15 °C = −459.67 °F; the coldest possible temperature, where thermal motion is at its minimum (not fully stopped — quantum zero-point motion remains).
  • The size of one degree is identical on the Celsius and Kelvin scales; only the zero point differs.

Equations and Variables

K = °C + 273.15

°F = (9/5)°C + 32

°C = (5/9)(°F − 32)

Variables:

  • K = temperature in kelvin (no degree symbol, never "degrees Kelvin")
  • °C = temperature in degrees Celsius
  • °F = temperature in degrees Fahrenheit

Note the order of operations: for °F → °C, subtract 32 first, then multiply by 5/9. For °C → °F, multiply by 9/5 first, then add 32.

How It Works or Problem-Solving Method

  1. Choose the conversion based on the two scales involved (three formulas above cover all six directions).
  2. Respect order of operations exactly — this is the most common source of error.
  3. Round using significant figures: the conversion factors (9/5 and 5/9) and the 32/273.15 offsets are exact or define the scale, so the answer takes the significant figures of the measured starting temperature.
  4. Sanity-check direction: room temperature is about 20 °C ≈ 68 °F ≈ 293 K. A result of 20 K or 500 °C for "room temperature" signals an arithmetic error.

Worked Example

Problem 1: Convert 25.0 °C to kelvin.

K = °C + 273.15 = 25.0 + 273.15 = 298.2 K (report as 298.2 K, since the .0 in 25.0 carries to the tenths place).

Problem 2: Convert 98.6 °F to Celsius.

°C = (5/9)(°F − 32) = (5/9)(98.6 − 32) = (5/9)(66.6) = 37.0 °C.

Problem 3: Convert 300. K to °F (a two-step conversion).

First, °C = K − 273.15 = 300. − 273.15 = 26.85 °C (≈ 27 °C). Then °F = (9/5)(27) + 32 = 48.6 + 32 = 81 °F.

Problem 4 (conceptual): Why can't a gas be cooled to −300 °C?

Because −300 °C = (−300 + 273.15) K = −26.85 K, and temperatures below 0 K are physically impossible. The coldest possible temperature is 0 K = −273.15 °C.

Common Confusions

  • "K = °C + 273" (dropping the 0.15). Wrong for precise work — the exact shift is 273.15. The rounding is acceptable only for rough estimates.
  • "20 °C is 68 °F, so to convert I just add 48." Wrong — adding a fixed number works only near that one value because the scales have different-sized degrees; the true relationship is linear with slope 9/5, not a constant offset.
  • "0 K and 0 °C are both 'zero temperature.'" Wrong — 0 °C is the freezing point of water (273.15 K); 0 K is absolute zero (−273.15 °C). They differ by 273.15 degrees.
  • "Negative Kelvin is possible for very cold things." Wrong — 0 K is the floor; no temperature can go below it.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of three rulers that measure the same "hotness" but start at different places. Celsius puts 0 where water freezes; Fahrenheit puts 0 somewhere colder and stretches its marks a bit more (that's why 212 vs 100 at boiling); Kelvin puts 0 at the very coldest anything can ever be — like a race where the starting line is at the true beginning, so nobody can ever be "before the start" (no negative kelvins). Limit of the analogy: Kelvin isn't a different kind of hotness, just the same-sized steps with a shifted starting line — and "absolute zero" is a limit temperature that can be approached but never quite reached, not a place you can actually visit.

Key takeaways

  • K = °C + 273.15 (and °C = K − 273.15).
  • °F = (9/5)°C + 32; °C = (5/9)(°F − 32).
  • Absolute zero: 0 K = −273.15 °C = −459.67 °F.
  • 1 K = 1 °C in size; the scales differ only by a 273.15 shift.
  • Kelvin has no degree sign and no negative values.
  • Room temperature ≈ 293 K ≈ 20 °C ≈ 68 °F.
  • A 1 °C change equals a 1.8 °F change (9/5), but a 1 °F change equals 5/9 °C.
  • Three scales: °C, °F, and K (SI unit).
  • K = °C + 273.15; °F = (9/5)°C + 32; °C = (5/9)(°F − 32).
  • Absolute zero = 0 K = −273.15 °C = −459.67 °F.
  • Kelvin has no negative values and no degree symbol.
  • 1 K = 1 °C (same step size).
  • For °F → °C, subtract 32 first, then ×5/9; for °C → °F, ×9/5 first, then +32.
  • Gas laws require absolute (Kelvin) temperature.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • State the three common temperature scales and the physical meaning of absolute zero.
  • Convert temperatures between Celsius, Fahrenheit, and Kelvin using the exact relationships.
  • Recognize that the Kelvin scale has no negative values and begins at absolute zero (0 K).
  • Explain why only the Kelvin scale can be used directly in gas-law and other absolute-temperature calculations.

Sources & references

  1. OpenStax, *Chemistry 2e*, Ch. 1.4, "Measurements" (temperature scales).
  2. NIST, "SI Units — Temperature."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.