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Temperature Converter
Convert between Celsius, Fahrenheit, and Kelvin instantly
Reviewed by Dr. Rachel Martinez, Ph.D.
Thermodynamics Expert | Metrology Specialist
Last Updated: November 24, 2025
Understanding Temperature Scales
Temperature measures the average kinetic energy of particles in a substance. While this physical phenomenon is universal, humans have created three major scales to quantify it, each with its own history, logic, and modern applications.
The Three Temperature Scales
Celsius (°C) - The Metric Standard
Developed by Swedish astronomer Anders Celsius in 1742, this scale uses water's phase changes as reference points: 0°C for freezing and 100°C for boiling (at standard atmospheric pressure). The 100-degree span makes mental math easy. Today, Celsius is the global standard except in a few countries. It's the default for weather, cooking, and everyday measurement worldwide.
°C = (°F - 32) × 5/9
Fahrenheit (°F) - The American Holdout
Created by German physicist Daniel Gabriel Fahrenheit in 1724, this scale originally set 0°F as the freezing point of brine (salt water) and 96°F as human body temperature. The finer degree divisions (180° between water's freezing and boiling) provide more precision without decimals for everyday temperatures. While the US, Bahamas, Cayman Islands, and Palau still use Fahrenheit, it's increasingly seen as anachronistic.
Kelvin (K) - The Scientific Absolute
Proposed by Lord Kelvin in 1848, this scale starts at absolute zero (-273.15°C), the theoretical point where molecular motion ceases. Kelvin uses the same degree size as Celsius but eliminates negative numbers, crucial for thermodynamic calculations. It's the SI unit for temperature and mandatory in scientific research. Note: Kelvin doesn't use the degree symbol (°) - it's "300 K," not "300°K."
°C = K - 273.15
Practical Conversion Tips
Quick Mental Approximations:
- °C to °F: Double the Celsius, subtract 10%, add 32 (25°C → 50-5+32 = 77°F)
- °F to °C: Subtract 30, halve it (70°F → 40/2 = 20°C, actual: 21.1°C)
- °C to K: Add 273 (close enough for most purposes; exact is +273.15)
Key Reference Temperatures:
- Body temperature: 37°C = 98.6°F = 310K
- Room comfort: 20-22°C = 68-72°F = 293-295K
- Hot day: 30°C = 86°F = 303K
- Cold day: 0°C = 32°F = 273K
- Baking: 180°C = 350°F = 453K
Why Temperature Scales Matter
Cooking: Recipe disasters happen because of temperature confusion. "Bake at 180 degrees" means 180°C (356°F) in Europe but would be impossibly cold if interpreted as 180°F (82°C). Always confirm the scale. British recipes often give both °C and "Gas Mark" (a scale on old ovens).
Weather: Americans visiting Europe panic at "30-degree heat" until they realize that's 86°F, not a blizzard. Conversely, Europeans see "100 degrees" and assume apocalyptic conditions instead of a hot summer day.
Medicine: A fever of 38°C (100.4°F) is mild, but misread as 38°F (-3°C) would indicate death. Medical equipment specify units, but human error occurs. Field medics in international zones must master conversion instantly.
💡 Expert Tips from Dr. Martinez
The Intersection Point: At -40°, Celsius and Fahrenheit scales intersect: -40°C = -40°F. This is the only point where the temperatures are numerically identical. It's a useful sanity check - if you're converting temperatures near -40° and get wildly different numbers, you've made an error.
Precision Matters in Science: When converting for scientific work, use the full 273.15 for Kelvin, not rounded 273. That 0.15 difference seems trivial but compounds in thermodynamic calculations. For example, computing the efficiency of a heat engine at 100°C: using K=373 vs K=373.15 changes your answer in the third decimal place.
Altitude Affects Boiling Point: Those "100°C boiling" reference points assume sea-level pressure. At 2000m elevation (like Denver), water boils at 93°C (199°F), not 100°C (212°F). Pressure cookers compensate by raising pressure, bringing the boiling point back up.
⚠️ Common Mistakes to Avoid
- Forgetting to Subtract 32 First: The most frequent error is multiplying F by 5/9 directly. You MUST subtract 32 before multiplying. (100°F - 32) × 5/9 = 37.8°C, not 100 × 5/9 = 55.6°C. The operations aren't interchangeable.
- Rounding Too Early: When doing multi-step conversions (say, °F → °C → K), don't round intermediate steps. Calculate 77°F → 25.00°C → 298.15K. If you round to 25°C first, you lose precision in the final Kelvin value.
- Negative Temperature Confusion: -10°C ≠ -10°F. They're completely different. -10°C = 14°F (cold) while -10°F = -23.3°C (frigid). Never assume negative temperatures are equivalent across scales.
- Using "Degrees Kelvin": It's just "Kelvin" or "kelvins," never "degrees Kelvin." The unit is already named after a person, and "degree" is unnecessary. Write "300 K" not "300°K."
Historical Context
The multiplicity of temperature scales reflects scientific evolution. Early thermometers used arbitrary reference points - Fahrenheit's original scale used the coldest temperature he could create (ice-salt mixture) as zero. Celsius simplified this by using water, Earth's most common liquid. Kelvin revolutionized the concept by defining temperature relative to absolute zero, a fundamental physical limit, not a chemical accident like water's freezing point.
The resistance to Celsius adoption in America isn't mere stubbornness - it's infrastructure. Billions of dollars worth of equipment, regulations, and public familiarity are built around Fahrenheit. Switching would require replacing thermostats, road signs, medical devices, and retraining entire populations. Metrication efforts in the 1970s failed for these practical reasons.
Frequently Asked Questions
📚 Expert References & Further Reading
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI) (9th ed.). https://www.bipm.org/
- NIST - Temperature Measurement. https://www.nist.gov/
- Royal Society of Chemistry - Temperature Scales. https://www.rsc.org/
- American Meteorological Society - Temperature Glossary. https://www.ametsoc.org/