Why is Kelvin used in gas-law calculations?

Gas laws describe properties proportional to absolute temperature. Kelvin starts at absolute zero, the lowest physically possible temperature, so K values are always non-negative and proportional relationships work correctly.

What is absolute zero?

0 K = −273.15 °C = −459.67 °F. It is the theoretical temperature at which molecular thermal motion ceases. It cannot be reached, only approached.

Why does Fahrenheit have an offset of 459.67?

Because the Fahrenheit zero is set arbitrarily at a brine freezing point, not at absolute zero. Rankine is the absolute version of Fahrenheit (°R = °F + 459.67).

What is Rankine used for?

Engineering practice in the United States, especially thermodynamics for power plants and refrigeration, often uses Rankine alongside Fahrenheit.

Can Kelvin values be negative?

No. Kelvin measures absolute temperature, and 0 K is the lowest temperature physically possible. Any calculation that produces a negative Kelvin value points to an error in the inputs, which is why this calculator flags it instead of returning a result.

Temperature Unit Converter

Instant two-way conversion between Kelvin, Celsius, Fahrenheit, and Rankine.

Temperature Converter

Type in any field, and the others update instantly.

Kelvin (K)

Celsius (°C)

Fahrenheit (°F)

Rankine (°R)

°C = K − 273.15
°F = (K − 273.15) × 9/5 + 32
°R = K × 9/5

Why Kelvin Is the Gas-Law Standard

Every fundamental gas law, Charles', Gay-Lussac's, Combined, and Ideal, requires temperatures in Kelvin because they describe quantities proportional to absolute temperature. Celsius and Fahrenheit place their zero points at conventional physical references (water's freezing point, brine), not at the true thermal zero. Plugging Celsius into V₁/T₁ = V₂/T₂ would, for example, divide by zero at the freezing point of water, which is clearly nonsense.

Absolute Zero and the Kelvin Scale

The Kelvin scale was constructed so that 0 K corresponds to absolute zero, the temperature at which the entropy of a perfect crystalline solid reaches its theoretical minimum and ideal gas pressure (and volume, at constant P) extrapolates to zero. No object has ever been cooled to exactly 0 K; the closest laboratory experiments reach within a fraction of a billionth of a degree, close enough to confirm the prediction but never all the way to it. Each Kelvin is exactly the same size as a Celsius degree, so conversion is simple addition: K = °C + 273.15.

Conversion Formulas

Kelvin to Celsius: °C = K − 273.15
Kelvin to Fahrenheit: °F = (K − 273.15) × 9/5 + 32
Kelvin to Rankine: °R = K × 9/5
Celsius to Kelvin: K = °C + 273.15
Fahrenheit to Kelvin: K = (°F − 32) × 5/9 + 273.15

How to Convert Between Units

Pick the unit you already have and apply the matching formula above to reach Kelvin first, then convert onward if you need a third unit. For example, to turn 98.6 °F into Celsius: convert to Kelvin with K = (98.6 − 32) × 5/9 + 273.15 = 310.15 K, then subtract 273.15 to get 37 °C, human body temperature. Doing the conversion in two short steps, through Kelvin, avoids the more error-prone direct Fahrenheit-to-Celsius formula and keeps every gas-law calculation on this site consistent with a single reference scale.

Reference Temperatures

Reference	K	°C	°F	°R
Absolute zero	0	−273.15	−459.67	0
Water freezing point	273.15	0	32	491.67
Room temperature	293.15	20	68	527.67
Human body temperature	310.15	37	98.6	558.27
Water boiling point	373.15	100	212	671.67