Celsius to Fahrenheit & Kelvin Converter
This converter instantly translates any temperature value between the three most widely used scales: Celsius (°C), Fahrenheit (°F), and Kelvin (K). The core formulas are: °F = (°C × 9/5) + 32, °C = (°F − 32) × 5/9, and K = °C + 273.15. Used in science, cooking, weather reporting, medicine, and engineering, accurate temperature conversion is essential whenever data crosses metric and imperial systems — for example, converting a 100°C boiling point to 212°F or expressing absolute zero (0 K) as −273.15°C.
To convert Celsius to Fahrenheit: **°F = °C × 1.8 + 32**. Key values: 0°C = 32°F · 20°C = 68°F · 37°C = 98.6°F · 100°C = 212°F. For Kelvin: **K = °C + 273.15**. The only point where °C and °F are equal is −40° (both scales read −40).
When to use this calculator
- Converting an oven recipe from 375°F (US cookbook) to 190°C for a European convection oven
- Reporting a patient's fever of 38.9°C to a US nurse who reads in Fahrenheit (102°F)
- Setting a scientific experiment's water bath at 310.15 K (37°C / 98.6°F), the standard human body temperature used in NIH lab protocols
- Interpreting a NOAA weather forecast of 95°F as 35°C during a summer heat advisory
- Checking whether liquid nitrogen at −195.79°C (−320.42°F / 77.36 K) is safe for a cryogenic storage procedure
- Converting NASA mission thermal specs expressed in Kelvin back to Celsius for ground-support equipment rated in metric units
Example: Body temperature 37°C to Fahrenheit
- Start: 37°C (normal body temperature)
- Formula: °F = 37 × 9/5 + 32 = 66.6 + 32
- Result: 98.6°F
How it works
3 min readFormulas (NIST Standard)
| Conversion | Formula |
|---|---|
| °C → °F | °F = (°C × 9/5) + 32 |
| °F → °C | °C = (°F − 32) × 5/9 |
| °C → K | K = °C + 273.15 |
| K → °C | °C = K − 273.15 |
| °F → K | K = (°F − 32) × 5/9 + 273.15 |
| K → °F | °F = (K − 273.15) × 9/5 + 32 |
Mental shortcut: multiply Celsius by 2, then add 30 (±5% error — good enough for weather chat). For precision, always use the exact formula.
> Note: Kelvin uses no degree symbol (°). It is an absolute scale — 0 K is absolute zero (−273.15°C / −459.67°F), the lowest theoretically possible temperature.
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Reference Table: 20 Key Temperatures
| Description | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Liquid nitrogen (boiling) | −195.79 | −320.42 | 77.36 |
| Dry ice (CO₂ sublimation) | −78.5 | −109.3 | 194.65 |
| Home freezer | −18 | −0.4 | 255.15 |
| Water freezes (sea level) | 0 | 32 | 273.15 |
| Refrigerator | 4 | 39.2 | 277.15 |
| Average room temperature | 20 | 68 | 293.15 |
| Human body (normal) | 37 | 98.6 | 310.15 |
| Fever threshold (CDC) | 38 | 100.4 | 311.15 |
| High fever | 40 | 104 | 313.15 |
| Milk pasteurization (FDA) | 72 | 161.6 | 345.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
| Moderate oven (baking) | 177 | 350 | 450.15 |
| Hot oven (pizza) | 230 | 446 | 503.15 |
| Paper ignition point | 233 | 451 | 506.15 |
| Aluminum melts | 660 | 1220 | 933.15 |
| Iron melts | 1538 | 2800 | 1811.15 |
| Surface of the Sun | 5,505 | 9,941 | 5,778 |
| Lightning bolt | 27,727 | 49,941 | 28,000 |
| Big Bang (initial) | ~10²⁷ | — | ~10²⁷ |
Sources: NIST, NOAA, CDC, USDA/FDA
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Worked Examples
Example 1 — Cooking conversion
A US recipe calls for 375°F:
°C = (375 − 32) × 5/9 = 343 × 5/9 = 190.6°C ≈ 190°CMost European ovens have markings at 180°C and 200°C; 190°C is the correct midpoint.
Example 2 — Medical fever
A patient reads 39.4°C. The ER chart uses Fahrenheit:
°F = (39.4 × 9/5) + 32 = 70.92 + 32 = 102.9°FThe CDC classifies ≥100.4°F (38°C) as fever. At 102.9°F this patient is febrile.
Example 3 — Scientific lab (Kelvin)
A chemistry protocol requires 25°C (room temperature). For gas law calculation:
K = 25 + 273.15 = 298.15 KThis value goes into PV = nRT where R = 8.314 J/(mol·K).
The −40 Crossover
−40°C = −40°F (the ONLY temperature identical on both scales). Proof:
Setting °F = °C: C = (C × 9/5) + 32 → −4C/5 = 32 → C = −40.
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Common Mistakes
1. Forgetting to subtract 32 before multiplying (°F → °C): The step °C = °F × 5/9 (without −32) is wrong. Example: 0°F without −32 gives 0°C, when it should be −17.78°C.
2. Using 273 instead of 273.15 for Kelvin: The offset is exactly 273.15. For cryogenic or thermodynamic calculations this 0.15° systematic error matters.
3. Writing "°K": Incorrect. Since the 1967 SI revision, the correct notation is K (no degree symbol). It is an absolute unit, not a "degree" of a relative scale.
4. Assuming boiling/freezing points are universal: The standard values (100°C / 212°F boiling) apply only at sea-level pressure (101.325 kPa). In Denver (~1,609 m), water boils at ~95°C / 203°F.
5. Confusing intervals with absolute values: A 1°C difference equals a 1.8°F difference, but 1°C absolute ≠ 33.8°F. The interval factor (1.8×) and the conversion formula (×1.8 + 32) are not the same.
6. Celsius vs. Centigrade: "Centigrade" is an older synonym for Celsius. They are identical for practical purposes, but since 1948 the official SI name is Celsius (°C).
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Frequently asked questions
What is the exact formula to convert Celsius to Fahrenheit?
The exact formula is °F = (°C × 9/5) + 32, equivalently °F = (°C × 1.8) + 32. For example, 25°C → (25 × 1.8) + 32 = 45 + 32 = 77°F. The factor 9/5 accounts for the different size of each degree (a Fahrenheit degree is 5/9 the size of a Celsius degree), and the +32 corrects for Fahrenheit's higher zero point (32°F = 0°C).
How many Fahrenheit is 100 Celsius?
100°C = 212°F. This is the boiling point of water at sea level (101.325 kPa). Formula: 100 × 9/5 + 32 = 180 + 32 = 212°F. This is one of the most important reference points for both cooking and science.
At what temperature are Celsius and Fahrenheit the same?
Celsius and Fahrenheit are equal at exactly −40° — that is, −40°C = −40°F. You can verify algebraically by setting °F = °C in the formula: C = (C × 9/5) + 32 → −4C/5 = 32 → C = −40. This crossover point is practical in extremely cold-weather and cryogenic contexts (and recorded in Arctic/Antarctic stations).
Why does Kelvin have no degree symbol?
Since the 13th General Conference on Weights and Measures (1967–68), the Kelvin was redefined as a base SI unit, not a 'degree' of a scale. NIST and the BIPM specify that the unit is written K (not °K). This reflects that Kelvin is an absolute thermodynamic scale starting at 0 K (absolute zero), rather than an arbitrary reference like the freezing point of water.
What is absolute zero, and can it be reached?
Absolute zero is 0 K = −273.15°C = −459.67°F, the temperature at which particles have minimum thermal motion. According to the third law of thermodynamics, it is impossible to reach absolute zero in a finite number of steps. Scientists at MIT have cooled atoms to within 500 picokelvin (5 × 10⁻¹⁰ K) of absolute zero, but true 0 K remains unreachable.
Does water always boil at 100°C / 212°F?
Only at standard sea-level pressure (101.325 kPa / 1 atm). Boiling point decreases with altitude: in Denver (~1,609 m), water boils at ~95°C (203°F); on Mount Everest (~8,849 m), it boils at roughly 70°C (158°F). This is why high-altitude baking and cooking instructions differ — pasta takes longer and candy thermometer readings must be adjusted.
What temperature does the CDC define as a fever?
The CDC defines fever as a body temperature of ≥100.4°F (38.0°C / 311.15 K) measured orally. Normal adult body temperature is approximately 98.6°F (37°C / 310.15 K), though it naturally varies by ±0.5°F throughout the day. A temperature of 103°F (39.4°C) or higher in adults is considered high fever and typically warrants medical evaluation.
What is 'standard room temperature' in science?
NIST and ISO define standard room temperature as 20°C (68°F / 293.15 K) for most measurement and material specifications. The IUPAC standard for gas measurements is 0°C (STP) or 25°C (SATP). The ASHRAE comfort range for occupied spaces is typically 68–76°F (20–24.4°C).
How do I convert Fahrenheit directly to Kelvin without going through Celsius?
Use the direct formula: K = (°F − 32) × 5/9 + 273.15. Example: 98.6°F → (98.6 − 32) × 5/9 + 273.15 = 66.6 × 0.5556 + 273.15 = 37.0 + 273.15 = 310.15 K (exactly human body temperature). No intermediate Celsius step needed.
Why do the US and a few other countries still use Fahrenheit?
The United States, its territories, and a handful of other countries (including the Cayman Islands and Liberia) retained Fahrenheit after most of the world adopted Celsius in the 1960s–70s. The US Metric Conversion Act of 1975 made metrication voluntary, not mandatory, so Fahrenheit persisted in everyday life. Scientific and medical fields in the US universally use Celsius and Kelvin per SI standards (NIST HB 44).
What is the quick mental trick for converting Celsius to Fahrenheit?
A popular mental shortcut: double the Celsius and add 30. Example: 25°C → 25×2 + 30 = 80°F (exact answer: 77°F, ~4% error). More accurate trick: multiply by 2, subtract 10%, add 32. Example: 25°C → 50 − 5 + 32 = 77°F (exact). For precision, always use °F = °C × 1.8 + 32.