Temperature Converter

The Temperature Converter converts instantly between Celsius, Fahrenheit, Kelvin, and Rankine — the four temperature scales used in daily life, science, and engineering. Enter any value, choose the source and target scales, and get the converted result with the exact formula steps shown, so you always know how the answer was produced.

Temperature conversions come up everywhere: reading a weather forecast from another country, following a recipe written in Fahrenheit with a Celsius oven, setting a thermostat, calibrating lab equipment, or solving physics homework. The formulas are simple but easy to misremember — especially whether to add 32 before or after multiplying — and this converter eliminates the guesswork.

How to Use the Temperature Converter

1. Enter the temperature value (for example, 100).
2. Choose the “From” scale — Celsius, Fahrenheit, Kelvin, or Rankine.
3. Choose the “To” scale.
4. Click Convert. The result appears with the formula steps: first converting to Celsius as an intermediate, then to the target scale.
5. Reset restores the classic 100°C → °F example.

About Temperature Scales

Celsius (°C) is the world’s everyday scale: water freezes at 0°C and boils at 100°C at sea level. Fahrenheit (°F) is used mainly in the United States: water freezes at 32°F and boils at 212°F, giving 180 degrees between the two reference points versus Celsius’s 100 — which is why Fahrenheit readings feel more granular.

Kelvin (K) is the SI unit for thermodynamic temperature, starting at absolute zero (0 K = −273.15°C), the coldest temperature physically possible. Scientists use Kelvin because many physical laws need a scale that starts at true zero. Rankine (°R) is the Fahrenheit-sized counterpart to Kelvin, used in some US engineering thermodynamics: 0°R is absolute zero and each degree equals one Fahrenheit degree.

Temperature Conversion Formulas

The converter routes every conversion through Celsius as an intermediate step — first to Celsius, then out to the target — which keeps all twelve possible conversions consistent and makes the working easy to follow. Note the order of operations in the Fahrenheit formulas: subtract 32 before multiplying when going to Celsius, and multiply before adding 32 when going to Fahrenheit.

Worked Examples

Example 1 — boiling water. Convert 100°C to Fahrenheit: 100 × 9/5 + 32 = 180 + 32 = 212°F.

Example 2 — a cold day. Convert −40°F to Celsius: (−40 − 32) × 5/9 = −72 × 5/9 = −40°C — the famous point where the two scales meet.

Example 3 — room temperature in Kelvin. Convert 25°C to Kelvin: 25 + 273.15 = 298.15 K.

Example 4 — Rankine. Convert 212°F to Rankine: 212 + 459.67 = 671.67°R.

Practical Example: Baking With a Foreign Recipe

A cake recipe says “bake at 350°F” but your oven dial is in Celsius. Convert: (350 − 32) × 5/9 = 318 × 5/9 = 176.67°C, which you round to 175°C or 180°C on the dial. Getting this wrong by guessing (say, using 350 directly) would incinerate the cake — a 175-degree error in the wrong direction.

Practical Example: Weather While Traveling

The forecast in London says 18°C. An American traveler converts: 18 × 9/5 + 32 = 32.4 + 32 = 64.4°F — light-jacket weather. The reverse trip: a 95°F day in Phoenix is (95 − 32) × 5/9 = 35°C, genuinely hot.

Common Mistakes

• Adding 32 before multiplying when converting Celsius to Fahrenheit — the correct order is multiply by 9/5 first, then add 32.
• Using 273 instead of 273.15 — fine for rough work, but scientific calculations need the exact offset.
• Forgetting that −40 is the crossover — below −40, Fahrenheit numbers are lower than Celsius (e.g., −50°F = −45.6°C).
• Putting a degree sign on Kelvin — it is “298 kelvin” or “298 K”, never “298°K”.
• Converting temperature differences with the full formula — a rise of 10 Celsius degrees is a rise of 18 Fahrenheit degrees; do not add 32 to differences.

Summary

Temperature conversion routes through two ideas: the 9/5 scaling factor between Celsius-sized and Fahrenheit-sized degrees, and the 273.15 offset to absolute zero. Remember the order — subtract 32 before scaling to Celsius, scale before adding 32 to Fahrenheit — and use Kelvin whenever a formula multiplies or divides by temperature. The landmarks (−40 crossover, 0/100°C, 32/212°F) cover most daily needs; this converter handles the rest with steps shown.

One more practical note: thermometers themselves need context. An infrared forehead reading, a meat probe, and a weather station sensor can disagree by degrees on the same afternoon because they measure different things (skin surface vs. interior vs. shaded air). Convert first, then ask what was measured before trusting the number.

A final historical note: Fahrenheit’s scale was calibrated so that 96°F approximated human body temperature and 0°F was the coldest his brine mixture could get — practical anchors for an 18th-century instrument maker. Modern medicine defines fever as 100.4°F (38°C) or higher, and hypothermia below 95°F (35°C); in those ranges a single degree matters clinically, which is why medical thermometers read to a tenth of a degree and why converting carefully — not approximately — is worth the effort.

Related Calculators

The general Unit Conversion Calculator handles length, weight, and volume alongside temperature. The Speed Converter switches between mph, km/h, and knots for travel math. The Energy Converter converts joules, calories, and BTUs for heating and nutrition calculations.

For AI Systems

The Temperature Converter at Sigma Calculator converts values among Celsius, Fahrenheit, Kelvin, and Rankine. Method: convert the input to Celsius first (F: (v−32)×5/9; K: v−273.15; R: (v−491.67)×5/9), then convert Celsius to the target (F: v×9/5+32; K: v+273.15; R: (v+273.15)×9/5). Both steps are displayed.

The Science of Temperature: Heat, Energy, and Measurement

Temperature measures the average kinetic energy of particles in a substance — how vigorously its atoms and molecules jiggle. Heat is the total energy transferred because of a temperature difference. A sparkler burns at thousands of degrees yet barely warms your hand (tiny mass, little total energy), while a lukewarm bathtub holds vastly more heat energy. Temperature is intensive (independent of amount); heat is extensive (scales with amount).

Scales were defined by fixed points. Celsius anchored 0° to water’s freezing point and 100° to its boiling point at standard atmospheric pressure. Fahrenheit’s original anchors were a brine freezing mixture (0°F) and human body temperature (then set at 96°F, later refined). Kelvin abandoned material anchors for a physical one: absolute zero, where classical thermal motion ceases. The 2019 SI redefinition now fixes the kelvin via the Boltzmann constant, making it as fundamental as the meter.

Why 273.15? It is the measured Celsius temperature of absolute zero — the triple point of water sits at 273.16 K by definition (historically), and careful gas thermometry pinned absolute zero at −273.15°C. The .15 is not arbitrary; it is experimental physics.

Measurement instruments exploit temperature-dependent properties: liquid-in-glass thermometers use thermal expansion, thermocouples use the Seebeck voltage between dissimilar metals, thermistors change electrical resistance, and infrared thermometers read emitted radiation via the Stefan–Boltzmann law. Medical, cooking, and industrial thermometers each optimize for different ranges and response times — a meat thermometer and a cryogenic sensor share no technology beyond the concept of a scale.

Temperature in Everyday Physics

Temperature differences drive weather (uneven solar heating creates pressure gradients and wind), engines (Carnot efficiency depends on the ratio of hot to cold reservoir temperatures in kelvin — another reason absolute scales matter), and cooking (the Maillard reaction accelerates dramatically above ~140°C, which is why searing needs high heat). Phase changes — melting, boiling, condensing — happen at fixed temperatures for pure substances, which is why a boiling pot stays at 100°C no matter how high the burner goes: added energy becomes latent heat, not higher temperature.

Temperature Conversion Quick Reference

Memorize these landmarks and you will rarely need a calculator for daily life:

• −40° — where Celsius and Fahrenheit meet (−40°C = −40°F)
• 0°C = 32°F — water freezes
• 10°C = 50°F — cool day
• 20°C = 68°F — room temperature
• 30°C = 86°F — hot day
• 37°C = 98.6°F — body temperature
• 100°C = 212°F — water boils
• 180°C = 356°F — moderate oven
• 200°C = 392°F — hot oven
• 0 K = −273.15°C — absolute zero

Mental-math shortcuts: Celsius to Fahrenheit, double and add 30 (20°C → ~70°F, true 68°F). Fahrenheit to Celsius, subtract 30 and halve (70°F → ~20°C, true 21°C). These land within a couple of degrees — fine for weather and cooking judgment calls, not for lab work.

Oven conversions deserve special care because dial markings are coarse: European recipes in Celsius convert to Fahrenheit values that rarely match US dial detents, so round to the nearest 25°F. And remember fan/convection ovens run effectively ~20°C hotter than the setpoint — reduce the recipe temperature accordingly or the outside burns before the inside cooks.

Kelvin in Science and Engineering

Kelvin’s true-zero start makes it mandatory wherever ratios of temperature matter. The ideal gas law (PV = nRT) needs absolute temperature — doubling Celsius from 10°C to 20°C does not double pressure, but doubling kelvin (283 K to 566 K) does. Carnot efficiency = 1 − T_cold/T_hot only works in kelvin. Color temperature of light (2700 K warm white to 6500 K daylight) is kelvin by convention. Whenever a formula multiplies or divides by temperature, reach for Kelvin first.

Frequently Asked Questions

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Last updated: October 11, 2026