Temperature Converter

Temperature Is Different from Every Other Unit

Converting length is multiplication. Converting weight is multiplication. Temperature conversion is fundamentally different — and this distinction trips up a surprising number of people. Each temperature scale has its own definition of "zero", and that zero does not represent the same physical state across scales. Celsius calls 0° the freezing point of water. Fahrenheit defines 0° as the temperature of a brine–ice mixture. Kelvin defines 0 as absolute zero — the coldest theoretically possible temperature, at which molecular motion completely stops. Rankine shares the same zero as Kelvin but uses Fahrenheit-sized degrees. Because of these shifted baselines, you cannot simply multiply to convert temperature — you must apply offset-and-scale formulas specific to each pair.

The 8 Temperature Scales Explained

• Celsius (°C): The international everyday standard. Water freezes at 0°C, boils at 100°C. Used by virtually every country outside the US for weather, cooking, and medicine.
• Fahrenheit (°F): Standard in the United States. Water freezes at 32°F, boils at 212°F. Human body temperature is approximately 98.6°F (37°C).
• Kelvin (K): The SI thermodynamic temperature unit. 0 K = absolute zero. Used in all scientific and engineering thermodynamics calculations. Note: no degree symbol — it is written as "273.15 K", not "273.15 °K".
• Rankine (°Ra): Absolute temperature scale using Fahrenheit-degree increments. Used in some US engineering contexts, particularly aerodynamics and thermodynamics in imperial units.
• Delisle (°De): An 18th-century Russian scale that runs backwards — higher values mean lower temperatures. Water boils at 0°De and freezes at 150°De.
• Newton (°N): Proposed by Isaac Newton in 1701. Water freezes at 0°N and boils at 33°N. Primarily of historical interest today.
• Réaumur (°Ré): Used in parts of 18th–19th century Europe for food production, particularly cheese and sugar. Water freezes at 0°Ré, boils at 80°Ré.
• Rømer (°Rø): The first standardised temperature scale, created by Danish astronomer Ole Rømer in 1701. Water freezes at 7.5°Rø and boils at 60°Rø.

The Conversion Formulas Behind the Tool

All conversions route through Kelvin as the internal base. Here are the to-Kelvin formulas for each scale:

• Celsius → Kelvin: K = °C + 273.15
• Fahrenheit → Kelvin: K = (°F − 32) × 5/9 + 273.15
• Rankine → Kelvin: K = °Ra × 5/9
• Delisle → Kelvin: K = 373.15 − (°De × 2/3)
• Newton → Kelvin: K = °N × 100/33 + 273.15
• Réaumur → Kelvin: K = °Ré × 5/4 + 273.15
• Rømer → Kelvin: K = (°Rø − 7.5) × 40/21 + 273.15

Converting from Kelvin to any target scale uses the inverse of each formula above. Conversions between any two non-Kelvin scales chain through Kelvin — for example, Fahrenheit → Rankine goes Fahrenheit → Kelvin → Rankine.

When You Actually Need a Temperature Converter

The most common use is Celsius ↔ Fahrenheit for everyday situations: checking if today's 95°F forecast in New York is as hot as yesterday's 35°C in Sydney, confirming that an oven recipe from a US cookbook (350°F) translates to the 175°C your European oven uses, or understanding a cold storage specification that lists 40°F when your thermometer shows Celsius. Beyond the everyday pair, Kelvin conversions come up in HVAC engineering, astrophysics, and chemistry lab work. Rankine appears in some American thermodynamics textbooks. Delisle, Newton, Réaumur, and Rømer are rare today but crop up in historical scientific literature, period cookbooks, and science history courses. If your project involves temperature alongside other physical quantities, the Speed Converter is useful for gas flow or wind speed contexts, and the Pressure Converter often accompanies temperature in thermodynamics problems.

Reference: Landmark Temperatures Across Scales

• Absolute zero: 0 K / −273.15°C / −459.67°F
• Water freezes: 273.15 K / 0°C / 32°F / 0°Ré / 7.5°Rø
• Average human body: 310 K / 37°C / 98.6°F
• Water boils (at sea level): 373.15 K / 100°C / 212°F / 80°Ré / 60°Rø
• Surface of the Sun: ~5,778 K / ~5,505°C / ~9,941°F