Weather apps, oven manuals, fever thermometers, and science textbooks all disagree on one thing: how they write the temperature. If a forecast tells you it will be 28°C today, a recipe calls for a 180°C oven, and your doctor mentions a fever of 101°F, you need one quick tool to reconcile them all. This temperature converter from Smekys switches instantly between Celsius, Fahrenheit, and Kelvin using the exact fixed points defined by NIST: water freezes at 0°C (32°F, 273.15 K) and boils at 100°C (212°F, 373.15 K) at standard atmospheric pressure.
Type any number, pick the scale it comes from, pick the scale you want, and the result card shows the converted value plus the equivalent reading in the other two scales — so you never have to run a second conversion to double-check.
How to Use
- Enter the Temperature Value — the number you want converted. Decimals are fine, and negatives are allowed (many real temperatures are below zero).
- Choose the From Unit — the scale your number is written in: Celsius, Fahrenheit, or Kelvin.
- Choose the To Unit — the scale you want the answer in.
- Read the Converted Temperature on the result card. The same card also shows the equivalent reading in Celsius, Fahrenheit, and Kelvin side by side.
Two built-in limits are worth knowing. First, Kelvin has no negative numbers: the scale starts at absolute zero (0 K = -273.15°C = -459.67°F), the point where molecular thermal motion stops. If you type a value below absolute zero, the converter flags it instead of returning a nonsense number. Second, switching the From and To units gives the reverse conversion for free — convert 32°F to Celsius once, and you already know 0°C back-converts to 32°F.
The Conversion Formulas
Celsius is the pivot every conversion runs through, which keeps the math to two simple steps:
- Fahrenheit to Celsius: C = (F − 32) × 5/9
- Celsius to Fahrenheit: F = C × 9/5 + 32
- Kelvin to Celsius: C = K − 273.15
- Celsius to Kelvin: K = C + 273.15
The −32 (or +32) shift exists because the Fahrenheit scale sets its zero at an old freezing-mixture point rather than at water's freezing point, while the 9/5 factor exists because a Fahrenheit degree is smaller: 180 Fahrenheit degrees span the same freezing-to-boiling range as 100 Celsius degrees. That 9/5 ratio is why a 1°C change equals a 1.8°F change — useful for understanding why turning your thermostat down by 2°C can shave roughly 5–10% off a $150 monthly heating bill.
The famous crossover point is −40: −40°C and −40°F describe the exact same temperature. Drop either formula in with −40 and both sides agree, which makes it a handy sanity check whenever you doubt a calculator.
Three Scales, Three Habits
Celsius is the global standard and the default scale of science: water freezes at 0, boils at 100, normal body temperature sits near 37°C. Nearly every country on Earth reports weather in Celsius.
Fahrenheit survives mainly in the United States. Its advantage is human-scale granularity: 180 Fahrenheit degrees between freezing and boiling means typical outdoor temperatures land on a 0–100 scale that feels intuitive ("90 and humid" needs no further translation).
Kelvin is the absolute scale used in physics, chemistry, and engineering, where temperature must start from a true zero. Room temperature is about 293 K, a candle flame is roughly 1,400 K, and the surface of the Sun is about 5,778 K.
Examples
Example 1 — Weather on vacation. A friend abroad texts that it is 30°C at the beach. Converting 30°C to Fahrenheit gives 86°F — warm, but comfortable, especially at 40% relative humidity. At 70% humidity the same 30°C would feel closer to 90°F to your body even though the thermometer reads the same.
Example 2 — Body temperature. A thermometer reads 98.6°F. Converting gives 37.0°C (and about 310.15 K). Anything over 38°C (100.4°F) is generally treated as a fever, which is a useful cross-reference when you compare a Celsius fever strip against an old Fahrenheit ear thermometer.
Example 3 — A recipe from abroad. A European recipe says to bake at 180°C. Converting to Fahrenheit gives 356°F, which rounds neatly to the American baking mark of 350°F — close enough that you do not need to adjust the recipe.
Example 4 — Extreme heat. A physics problem states a plasma temperature of 1,000,000°C. Converting to Fahrenheit gives 1,800,032°F and about 1,000,273 K — the engine handles values above 1,000,000 without rounding errors, because the 9/5 and 273.15 factors apply identically at any magnitude.
FAQ
How do I convert Celsius to Fahrenheit?
Multiply the Celsius temperature by 9/5 (or 1.8) and then add 32. For example, 25°C × 9/5 = 45, and 45 + 32 = 77°F. Type 25, choose Celsius as the From unit and Fahrenheit as the To unit, and the result card shows 77°F along with the Kelvin equivalent.
How do I convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit temperature and multiply the remainder by 5/9. For example, 68°F − 32 = 36, and 36 × 5/9 = 20°C. Room temperature in Fahrenheit is a pleasant 68°F, which is exactly 20°C.
What does 0°C equal in Fahrenheit?
0°C equals 32°F — the freezing point of water — and 273.15 K. This is one of the two fixed points the conversion formulas are built around, the other being 100°C = 212°F, the boiling point at sea level.
Why do Celsius and Fahrenheit read the same at -40?
Because both formulas collapse at that point: solve C = C × 9/5 + 32, and you get C = -40. At −40°, the Fahrenheit and Celsius scales genuinely intersect, so −40°C and −40°F are the same cold. It is also the fastest way to verify a converter is working correctly.
What is the difference between Kelvin and Celsius?
They have the same degree size — a 1 K change equals a 1°C change — but different zero points. Kelvin starts at absolute zero, so to convert Kelvin to Celsius you simply subtract 273.15. Normal human body temperature, 37°C, is about 310.15 K. Kelvin has no "degrees" in formal usage; you say "293 kelvin," not "293 degrees kelvin."
Why won't the converter accept a negative Kelvin temperature?
Negative Kelvin is physically impossible in the ordinary sense of thermal temperature: 0 K is absolute zero, the theoretical lowest possible temperature. Entering -300°C in Celsius mode also triggers the same warning, because -300°C is 27.85 degrees below absolute zero. The converter rejects these values instead of producing a mathematically correct but physically meaningless answer.
Disclaimer
This temperature converter is provided for informational purposes only. While the conversion factors follow the NIST-defined fixed points (water freezes at 0°C / 32°F and boils at 100°C / 212°F at standard atmospheric pressure), results are approximations for everyday use and should not be relied upon for medical diagnosis, industrial process control, laboratory calibration, or any safety-critical application. For fever screening, follow your doctor's guidance; for cooking, aircraft, or laboratory work, use calibrated instruments. Always verify critical measurements with official reference sources.