Temperature Converter

Free Online Temperature Converter

This temperature converter handles every temperature scale used in physics, engineering, and everyday life — Celsius, Fahrenheit, Kelvin, Rankine, and Réaumur. Enter any value, pick a "From" and "To" unit, and the calculator instantly converts it, shows a step-by-step solution using the exact conversion formula, and plots both temperatures on a labeled scale diagram running from absolute zero up to the core of the Sun so you can see exactly where your temperature sits in the physical universe.

Unlike length or mass, temperature scales don't just differ by a multiplication factor — each has its own zero point, which is why converting between them always needs both a scale factor and an offset. This calculator routes every conversion through Kelvin, the SI base unit of temperature, so the math stays consistent and accurate no matter which two units you're converting between.

What Is Temperature? (In Simple Terms)

Temperature is a measure of how hot or cold something is, and more precisely, it reflects the average kinetic energy of the particles (atoms and molecules) making up a substance — the faster those particles jiggle and move, the higher the temperature. Temperature is one of the seven SI base quantities, and its official SI base unit is the Kelvin (K), a scale built directly on the concept of absolute zero: the coldest possible temperature, where particle motion is at its theoretical minimum.

A temperature converter is a tool that changes a temperature reading from one scale into the equivalent reading on a different scale, without changing the actual physical hotness or coldness being described. Because different countries, industries, and scientific fields historically settled on different reference points (freezing water, boiling water, absolute zero, or body temperature), the world ended up with several temperature scales that are all still in active use today.

Temperature Conversion Formulas

Every conversion in this calculator works the same way: convert the input to Kelvin first (the SI base unit), then convert from Kelvin into the target unit. Here are the core formulas used internally:

• Celsius to Kelvin: K = °C + 273.15 Kelvin to Celsius: °C = K − 273.15
• Fahrenheit to Kelvin: K = (°F − 32) × 5/9 + 273.15 Kelvin to Fahrenheit: °F = (K − 273.15) × 9/5 + 32
• Celsius to Fahrenheit (direct shortcut): °F = °C × 9/5 + 32 Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• Rankine to Kelvin: K = °R × 5/9 Kelvin to Rankine: °R = K × 9/5
• Réaumur to Kelvin: K = °Ré × 5/4 + 273.15 Kelvin to Réaumur: °Ré = (K − 273.15) × 4/5

Temperature Units and Their Reference Points

These are the five temperature scales supported by this calculator, along with what each one is built around:

• Kelvin (K) — the SI base unit; 0 K is absolute zero, the coldest possible temperature, and each Kelvin degree is the same size as a Celsius degree.
• Celsius (°C) — the standard metric scale used almost everywhere outside the US; 0°C is defined as water's freezing point and 100°C as water's boiling point at standard atmospheric pressure.
• Fahrenheit (°F) — used mainly in the United States for weather and everyday temperatures; water freezes at 32°F and boils at 212°F.
• Rankine (°R) — an absolute scale (like Kelvin, it starts at absolute zero) but uses Fahrenheit-sized degrees; historically used in US thermodynamics and engineering calculations.
• Réaumur (°Ré) — a historical European scale where water freezes at 0°Ré and boils at 80°Ré; largely obsolete today but still occasionally used in some European food, dairy, and candy-making recipes.

Worked Example: Human Body Temperature, Fahrenheit to Celsius

Normal human body temperature is often quoted as 98.6°F. Here's how to convert that to Celsius, step by step:

• Step 1 — Subtract 32: 98.6 − 32 = 66.6
• Step 2 — Multiply by 5/9: 66.6 × 5/9 = 37
• So 98.6°F equals exactly 37°C — enter these exact numbers above (or click the 'F → C' shortcut) to see this identical result on the diagram and step-by-step solution.

Worked Example: Water Boiling Point, Celsius to Kelvin

Water boils at 100°C at standard atmospheric pressure. Converting that to Kelvin:

• Step — Add 273.15: 100 + 273.15 = 373.15 K
• So the boiling point of water is 373.15 K — this is exactly why Kelvin is the preferred scale in thermodynamics: it has no negative numbers for any real physical temperature, since 0 K is the true physical floor.

Why Physics Needs Multiple Temperature Scales

Temperature scales exist because different fields needed a scale anchored to reference points that made sense for their work. Celsius was built around water, the most common substance in daily life and chemistry. Fahrenheit was built around a mix of brine freezing point and (roughly) human body temperature, and stuck in the US for historical reasons. Kelvin was built around absolute zero itself, making it the natural choice for physics and thermodynamics, since gas laws, statistical mechanics, and radiation formulas all work cleanly with an absolute (never-negative) scale. Rankine does the same job as Kelvin but for engineers already working in Fahrenheit-based units. The scale diagram above this section shows how all of these compare on a single, absolute Kelvin-based timeline from the coldest physically possible temperature up to the surface and core of a star.

How to Use This Temperature Converter

Enter the numeric value you want to convert, then choose your "From" and "To" units — the converted result and both diagrams update instantly. Use the swap button to reverse the conversion direction, or tap one of the popular temperature conversion shortcuts (like Celsius → Fahrenheit, Fahrenheit → Kelvin, or Celsius → Réaumur) to jump straight to a common comparison.

The conversion flow diagram shows the exact two-step arithmetic — your value converted to Kelvin, then converted out to the target unit — with the precise formula used labeled directly on each arrow. The scale diagram plots your chosen temperatures on a logarithmic Kelvin number line running from absolute zero up to the core of the Sun, with familiar reference points (liquid nitrogen's boiling point, water's freezing and boiling points, room temperature, human body temperature, and the Sun's surface and core) marked along the way, so you can immediately see how hot or cold your value really is on a universal scale. Scroll down for the complete step-by-step solution, written out exactly as you'd solve it on paper.

Real-Life Applications of Temperature Converter

Temperature conversion isn't just a classroom exercise — it shows up constantly in science, industry, and daily life across the world:

• Weather forecasting and travel — countries report temperatures in either Celsius or Fahrenheit, so travelers, pilots, and international news outlets convert constantly to understand real-world conditions.
• Cooking and baking — oven temperatures and recipe instructions are given in Celsius in most of the world but Fahrenheit in the US, and some European or heritage recipes still use Réaumur for sugar and dairy work.
• Medicine and healthcare — body temperature and fever thresholds are recorded in Celsius or Fahrenheit depending on the country, and accurate conversion matters for correctly identifying a fever.
• Thermodynamics and physics research — gas laws (like the ideal gas law), heat transfer equations, and blackbody radiation formulas require absolute temperature in Kelvin, since using Celsius or Fahrenheit directly would produce incorrect results.
• Cryogenics — the science of extremely low temperatures (like liquid nitrogen at 77 K or liquid helium near 4 K) is described almost exclusively in Kelvin, since it's built around the concept of absolute zero.
• Materials science and manufacturing — melting points, annealing temperatures, and furnace settings for metals (like tungsten melting at 3695 K) are specified precisely, often requiring conversion between Celsius and Kelvin for lab equipment.
• Astrophysics and astronomy — the surface and core temperatures of stars (the Sun's surface is about 5,778 K, its core about 15.7 million K) are always reported in Kelvin, the natural scale for extreme physical temperatures.
• HVAC, refrigeration, and engineering — Rankine is still used in some US thermodynamic cycle calculations, particularly in older engineering textbooks and legacy industrial equipment.
• Climate science — global temperature change is tracked in Celsius or Kelvin in scientific literature, even though public communication in the US often uses Fahrenheit.
• Chemistry and laboratory work — reaction temperatures, boiling and melting points of compounds, and calibration of lab thermometers require quick, accurate conversion between Celsius, Fahrenheit, and Kelvin.

Absolute Zero and the Kelvin Scale: A Quick Physics Refresher

Absolute zero (0 K, equal to −273.15°C or −459.67°F) is the coldest temperature theoretically possible — the point at which particles have the minimum possible kinetic energy allowed by quantum mechanics. It can be approached extremely closely in laboratories but never fully reached, a consequence of the third law of thermodynamics. Because Kelvin starts exactly at this physical floor, it's the only major temperature scale with no negative values for any real, physically possible temperature, which is why it's the standard scale used throughout physics, chemistry, and engineering formulas involving temperature.

Tips for Converting Temperature Units Correctly

A few habits prevent the most common mistakes when converting between temperature scales:

• Never treat temperature conversion like a simple multiplication — remember the offset. Doubling a Celsius temperature does NOT double the Fahrenheit or Kelvin equivalent, because each scale's zero point sits in a different place.
• Always route through Kelvin for physics and engineering calculations (gas laws, radiation, thermodynamic cycles) rather than converting Celsius directly to Fahrenheit — this avoids compounding rounding errors.
• Remember the two easiest fixed points: water freezes at 0°C / 32°F / 273.15 K, and water boils at 100°C / 212°F / 373.15 K, at standard atmospheric pressure — use these to sanity-check any conversion.
• Watch for values below absolute zero (0 K or −273.15°C or −459.67°F) — these are physically impossible, and this calculator will flag them so you can catch data entry errors.
• Round consistently: temperature is rarely reported past 1–2 decimal places outside of scientific research, so match your rounding to the context (weather report vs. lab measurement).

Frequently Asked Questions

What is a temperature converter and how does it work? A temperature converter changes a temperature reading from one scale into an equivalent reading on another scale by converting through a common base — Kelvin, in the SI system — using each scale's specific offset-and-ratio formula.

What is the SI base unit of temperature? The Kelvin (K), an absolute scale where 0 K represents absolute zero, the theoretical coldest possible temperature, with the same degree size as Celsius.

How do you convert Celsius to Fahrenheit? Multiply the Celsius value by 9/5 (or 1.8) and add 32: °F = °C × 9/5 + 32. To go the other way, subtract 32 first, then multiply by 5/9.

Why is 0°C not the same as 0 K? Because each scale sets its zero point differently — Celsius sets 0°C at water's freezing point, while Kelvin sets 0 K at absolute zero, which is −273.15°C.

What is absolute zero? Absolute zero (0 K, −273.15°C, −459.67°F) is the coldest temperature theoretically possible, the point where particle motion reaches its quantum-mechanical minimum. It can be approached but never fully reached.

What is the Rankine scale used for? Rankine is an absolute temperature scale (like Kelvin) but uses Fahrenheit-sized degrees, historically used in US engineering and thermodynamic cycle calculations.

Is the Réaumur scale still used today? Rarely — it's mostly historical, though it occasionally appears in older European recipes, particularly for sugar work and dairy processing.