What Is a Temperature Converter?
A temperature converter is a calculation tool that instantly translates a temperature reading from one scale to another — for example, converting Celsius to Fahrenheit, Kelvin to Celsius, or Fahrenheit to Rankine. It is used daily by home cooks following international recipes, travellers checking weather forecasts, engineers working across metric and imperial specifications, medical professionals comparing fever references, and scientists performing thermodynamic calculations.
Temperature measurement is one of the most geographically divided unit systems in everyday life. The United States uses Fahrenheit for weather, cooking, and body temperature. Nearly every other country uses Celsius. Science and engineering worldwide use Kelvin as the SI base unit. Several historic scales — Rankine, Réaumur, Delisle, Newton, and Rømer — still appear in older documentation and specialised industrial contexts. This tool handles all of them with formula-accurate precision.
How to Use This Temperature Converter
Results appear in real time as you type — no submit button required.
- Enter the temperature value — Type the numeric temperature you want to convert. Negative values are fully supported (e.g., –40 for the -40 crossover point).
- Select the source scale — Choose the scale your original temperature is in, such as Celsius, Fahrenheit, or Kelvin.
- Select the target scale — Choose the scale you need the result in.
- Read the result — The converted value appears instantly with full-precision output.
Supported Temperature Scales
This converter supports all major and historical temperature scales:
- Celsius (°C) — Standard in most countries; water freezes at 0°C and boils at 100°C at sea level.
- Fahrenheit (°F) — Primary scale in the USA; water freezes at 32°F and boils at 212°F.
- Kelvin (K) — SI base unit; absolute zero = 0 K = −273.15°C. No degree symbol used.
- Rankine (°R) — Absolute scale using Fahrenheit degree increments; used in some US engineering contexts.
- Réaumur (°Ré) — Historical European scale; water freezes at 0°Ré, boils at 80°Ré.
- Delisle (°De), Newton (°N), Rømer (°Rø) — Historical scales still referenced in scientific literature.
Temperature Conversion Formulas
Unlike unit conversions that use a simple multiplication factor, temperature scale conversions require both multiplication and addition (or subtraction) because the scales have different zero points. These are the internationally standardised formulas:
°C to °F
°F = (°C × 9/5) + 32
Example: 100°C (boiling water) = (100 × 1.8) + 32 = 212°F.
°F to °C
°C = (°F − 32) × 5/9
Example: 98.6°F (normal body temperature) = (98.6 − 32) × 5/9 = 37°C.
°C to K
K = °C + 273.15
Example: 0°C (freezing point) = 273.15 K. Absolute zero = −273.15°C = 0 K.
K to °C
°C = K − 273.15
Example: 300 K = 26.85°C — a typical warm room temperature.
°F to K
K = (°F + 459.67) × 5/9
Example: 32°F = (32 + 459.67) × 5/9 = 273.15 K.
°C to °R
°R = (°C + 273.15) × 9/5. One Rankine degree equals one Fahrenheit degree in magnitude; Rankine zero = absolute zero. Used in US thermodynamic engineering to avoid dealing with negative absolute temperatures in some calculations.
Understanding Your Conversion Results
Temperature conversions are exact mathematical transformations — there are no approximation factors involved (unlike, say, the inch, which was originally defined by convention). The formulas above are exact definitions established by the International Bureau of Weights and Measures (BIPM). Displayed results are rounded for readability; internal arithmetic uses full double-precision to prevent rounding accumulation.
The −40 Crossover Point
Celsius and Fahrenheit are equal at exactly −40 degrees: −40°C = −40°F. This is the only point where both scales agree. It serves as a useful sanity check when verifying a converter — if −40°C does not return −40°F exactly, the formula is wrong.
Absolute Zero
Absolute zero — the theoretical lower bound of thermodynamic temperature where a system has minimum thermal energy — is defined as exactly 0 K = −273.15°C = −459.67°F. No physical object can be cooled below this temperature. Kelvin is used in all thermodynamic equations precisely because it starts here, making all values non-negative and mathematically consistent.
Body Temperature Reference Points
Normal human body temperature is approximately 37°C / 98.6°F / 310.15 K. A fever is generally defined as a core temperature above 38°C / 100.4°F. Hypothermia risk begins below 35°C / 95°F. These reference points make the Celsius↔Fahrenheit conversion one of the most clinically important temperature conversions worldwide.
Example Calculations
Example 1 — Cooking: US Recipe in a European Oven
Scenario: A US recipe calls for an oven temperature of 375°F. A UK oven displays Celsius. What temperature to set?
°C = (375 − 32) × 5/9 = 343 × 0.5556 = 190.6°C (set to 190°C or Gas Mark 5).
Example 2 — Weather: US Heatwave in Celsius
Scenario: A US city reports 104°F. What is this in Celsius for an international news article?
°C = (104 − 32) × 5/9 = 72 × 0.5556 = 40°C. A temperature that would trigger heat health warnings in most countries.
Example 3 — Physics: Ideal Gas Law Calculation
Scenario: A gas at 25°C must be expressed in Kelvin for use in the ideal gas law PV = nRT.
K = 25 + 273.15 = 298.15 K. Kelvin is the required unit in all thermodynamic formulas; inserting Celsius directly would produce a completely wrong result.
Example 4 — Medical: Fever in Fahrenheit to Celsius
Scenario: A patient has a temperature of 101.3°F. A doctor in a metric-system country needs the equivalent in°C.
°C = (101.3 − 32) × 5/9 = 69.3 × 0.5556 = 38.5°C — a mild to moderate fever.
Temperature Scales by Application Domain
Everyday Weather & Climate
Celsius is used in weather forecasts across Europe, Asia, Africa, Oceania, Latin America, and Canada. Fahrenheit is used in the USA and, informally, in some Caribbean nations. Most international weather services publish both. A quick mental approximation: double the Celsius value and add 30 to get a rough Fahrenheit equivalent (e.g., 20°C ≈ 68°F; the estimate gives 70°F, close enough for casual reference).
Cooking & Food Science
Recipe temperatures in the USA are always in Fahrenheit. UK, EU, Australian, and Indian cookbooks use Celsius. Gas mark (UK) is a third system: Gas Mark 1 = 140°C / 275°F; Gas Mark 9 = 240°C / 475°F. Food safety standards globally use Celsius: the danger zone for bacterial growth is 4°C to 60°C (40°F to 140°F).
Medical & Clinical
Body temperature is measured in Celsius in most countries and in Fahrenheit in the USA. Clinical thermometers and electronic patient record systems in the UK and EU default to Celsius. Most medical research literature uses Celsius or Kelvin; Fahrenheit appears mainly in US-origin clinical studies.
Engineering & Thermodynamics
Kelvin is mandatory in all thermodynamic equations (entropy, enthalpy, heat capacity, gas laws). Rankine serves the same role in the US engineering tradition — it is the absolute scale that pairs with Fahrenheit degree increments, used in some HVAC and aerospace engineering in North America. Industrial process specifications may use °C or °F depending on the country of the equipment manufacturer.
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