Celsius vs Kelvin: same step, different zero

TL;DR. Celsius and Kelvin use the same step size — 1 K = 1°C — and differ only in where zero is. Kelvin starts at absolute zero (−273.15°C), so convert with K = °C + 273.15. Use Kelvin for any equation involving temperature ratios; Celsius for everyday reporting.

Celsius and Kelvin measure the same physical quantity at the same step size. The only difference: where the scale starts. Celsius zero is water’s freezing point (a convenient anchor for everyday use). Kelvin zero is absolute zero — the theoretical minimum temperature where all molecular motion ceases.

The conversion

K = °C + 273.15
°C = K − 273.15

That’s it. No multiplication, no fractions, no offsets beyond the constant 273.15. A 10°C temperature change equals a 10 K temperature change — the two scales agree on the meaning of one degree.

Quick reference:

• 0 K = −273.15°C — absolute zero
• 273.15 K = 0°C — water freezes
• 293.15 K = 20°C — typical room temperature
• 310.15 K = 37°C — human body temperature
• 373.15 K = 100°C — water boils at standard pressure
• 5778 K — surface of the Sun
• 13.7 billion K — moments after the Big Bang (estimated)

Why two scales exist

Celsius — anchored to water

Anders Celsius (1742) proposed a scale based on water’s phase changes at standard atmospheric pressure. Originally he had water boiling at 0° and freezing at 100° — yes, that way around — but the scale was inverted after his death. Adopted by SI in 1948 as the practical scale for everyday temperature.

Kelvin — anchored to physics

William Thomson, Lord Kelvin (1848) realised that absolute zero was the natural “true zero” for temperature — the point where molecular motion ceases and ratios of temperatures become physically meaningful. The SI base unit since 1968; the kelvin (lowercase “k,” symbol K — note: not “°K”) is defined via the Boltzmann constant since the 2019 SI redefinition.

Where Celsius is the right choice

• Everyday temperature reporting. Weather, cooking, thermostats — anywhere the numbers are familiar and the precision required is degrees, not fractions. Negative numbers are uncomfortable for end-users but correct.
• Temperature changes.A 10°C increase is unambiguous. Saying “10 K increase” is equivalent but reads as overly formal in casual contexts.
• Most engineering work. Mechanical, chemical, civil engineering all use Celsius unless the calculation requires absolute temperature ratios.

Where Kelvin is mandatory

• Gas laws. PV = nRT requires absolute temperature. Using Celsius would produce nonsensical results for gas at -10°C vs +10°C (the ratio is 263/283 ≈ 0.93 in Kelvin; 10/-10 = -1 in Celsius).
• Thermodynamics. Entropy, heat capacity, thermal efficiency calculations all assume absolute temperature.
• Light colour temperature. Warm white lamps ~2700 K, daylight ~5500 K, overcast sky ~7500 K. The Kelvin numbers describe the temperature of a black body that would emit the same spectrum.
• Astrophysics, plasma physics, low-temperature physics. Anywhere temperatures span many orders of magnitude, Kelvin is the only sensible unit.

The notation rule that catches people

Celsius uses the degree symbol: °C. Kelvin does not: K, not °K. The convention was codified by the SI in 1968 when kelvin was made a base unit — it’s a unit of temperature directly, not a temperature scale, so the degree mark is redundant.

You’ll see “°K” in older textbooks and in some popular-science writing. It’s not wrong, but modern scientific publication style is just K.

The pragmatic rule

For anything you’d describe in words involving the word “cold” or “hot” — use Celsius. For anything you’d describe with an equation — use Kelvin. The conversion is one addition; pick whichever fits the audience and the math.

Numeric facts

• Offset: exactly 273.15 (defined by SI). Not 273 — the 0.15 matters for thermodynamic calculations.
• Step size: 1 K = 1°C identically. There is no scaling factor anywhere in the conversion.
• Boltzmann constant (basis of post-2019 kelvin definition): 1.380 649 × 10⁻²³ J/K exactly.
• Coldest temperature ever achieved in a lab: ~38 picokelvin (MIT, 2021) — that’s 0.000 000 000 038 K above absolute zero.
• Cosmic microwave background: 2.725 K (−270.425°C) — the residual radiation from the Big Bang fills all of space at this temperature.
• Liquid helium boils at 4.222 K = −268.928°C; liquid nitrogen at 77.355 K = −195.795°C.
• ITS-90 (International Temperature Scale of 1990) defines 17 fixed reference points for calibrating thermometers; the triple point of water is exactly 273.16 K by definition.
• Notation rule (SI Brochure 9th ed.): the symbol is K, the unit name is kelvin (lowercase). “°K” has been wrong since 1968.

Decision matrix

Sources

• BIPM SI Brochure, 9th edition (2019) — defines the kelvin via the Boltzmann constant — bipm.org.
• NIST — The International Temperature Scale of 1990 (ITS-90) — nist.gov.