Converting Between Celsius, Fahrenheit and Kelvin
Temperature scale conversion looks trivial until a gas-law numerical asks for the answer in Kelvin and you have only been given a temperature in Celsius, or a physics problem quotes Fahrenheit. Every formula here is fixed and exact — the only real risk is doing the arithmetic in the wrong order.
The three conversion formulas
°C = (°F − 32) × 5/9
K = °C + 273.15
°C = K − 273.15
Kelvin has no "degree" symbol and no negative values in physical use — 0 K (absolute zero) is the lowest temperature theoretically possible. Every gas-law formula (PV = nRT and its relatives) requires temperature in Kelvin; using Celsius directly in those formulas is one of the most common numerical errors in Class 11 chemistry.
Worked example 1 — normal human body temperature
Convert 37°C to Fahrenheit and Kelvin.
°F = (37 × 9/5) + 32 = 66.6 + 32 = 98.6°F
K = 37 + 273.15 = 310.15 K
Worked example 2 — converting back, as a check
Convert 98.6°F back to Celsius, to confirm Example 1 was done correctly.
°C = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 333/9 = 37.0°C
This matches the original value — a useful habit whenever you convert a temperature and want to be sure no arithmetic slip crept in.
Worked example 3 — absolute zero
Convert 0 K to Celsius and Fahrenheit.
°C = 0 − 273.15 = −273.15°C
°F = (−273.15 × 9/5) + 32 = −491.67 + 32 = −459.67°F
Worked example 4 — the boiling point of water
Convert 100°C to Fahrenheit and Kelvin.
°F = (100 × 9/5) + 32 = 180 + 32 = 212°F
K = 100 + 273.15 = 373.15 K
Common mistakes that cost marks
- Adding 32 before multiplying by 9/5 when converting Celsius to Fahrenheit — the order matters: multiply first, then add 32.
- Subtracting 32 after multiplying by 5/9 when converting Fahrenheit to Celsius — subtract 32 first, then multiply.
- Using 273 instead of 273.15 for the Celsius-to-Kelvin offset — acceptable for a quick estimate, but not exact; use 273.15 unless a question tells you to round.
- Plugging a Celsius (or Fahrenheit) temperature directly into PV = nRT without converting to Kelvin first — this single mistake silently produces a wrong answer to an otherwise correctly set-up gas-law problem.
Where this is tested
| Exam / topic | Typical use |
|---|---|
| CBSE Class 11 Chemistry — States of Matter | Converting given temperatures to Kelvin before applying gas laws |
| CBSE/ICSE Physics — Thermometry and Calorimetry | Scale conversion and temperature-difference calculations |
| JEE/NEET numericals | Gas-law and thermodynamics problems that quote temperature in a non-Kelvin scale |
| GATE/IIT-JAM Physical Chemistry | Thermodynamic and kinetic calculations, always in Kelvin |
Convert instantly, in either direction. The calculator suite's Temperature converter handles Celsius, Fahrenheit and Kelvin conversions so you can check your working before using the result in a gas-law calculation.
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