Convert Temperature: Celsius, Fahrenheit & Kelvin
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.
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 Calculation
- 100°C → °F
- 212°F
How it works
3 min readHow It's Calculated
The three temperature scales are linearly related (except for the Kelvin-Celsius offset). All six conversion paths reduce to two constants: 32 (Fahrenheit offset) and 273.15 (absolute zero offset in Celsius).
# Celsius → Fahrenheit
°F = (°C × 9/5) + 32
# Fahrenheit → Celsius
°C = (°F − 32) × 5/9
# Celsius → Kelvin
K = °C + 273.15
# Kelvin → Celsius
°C = K − 273.15
# Fahrenheit → Kelvin
K = (°F − 32) × 5/9 + 273.15
# Kelvin → Fahrenheit
°F = (K − 273.15) × 9/5 + 32> Note: Kelvin uses no degree symbol (°). It is an absolute scale — 0 K is absolute zero, the lowest theoretically possible temperature, equal to −273.15°C / −459.67°F (NIST, 2018 SI redefinition).
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Reference Table
| Description | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Liquid nitrogen (boiling) | −195.79 | −320.42 | 77.36 |
| Dry ice (sublimation) | −78.5 | −109.3 | 194.65 |
| Water freezes (sea level) | 0 | 32 | 273.15 |
| Average room temperature | 20–22 | 68–72 | 293–295 |
| Human body (normal) | 37 | 98.6 | 310.15 |
| Fever threshold (CDC) | 38 | 100.4 | 311.15 |
| Pasteurization (milk, FDA) | 72 | 161 | 345.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
| Baking (moderate oven) | 175–190 | 347–375 | 448–463 |
| Aluminum melts | 660.3 | 1220.5 | 933.45 |
| Iron melts | 1538 | 2800 | 1811 |
| Surface of the Sun (approx.) | 5,505 | 9,941 | 5,778 |
Sources: NIST, NOAA, CDC, USDA/FDA
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Typical Examples
Example 1 — Cooking conversion
A US recipe calls for 375°F. To set a European oven in Celsius:
°C = (375 − 32) × 5/9
°C = 343 × 5/9
°C = 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 on a digital thermometer. The ER chart uses Fahrenheit:
°F = (39.4 × 9/5) + 32
°F = 70.92 + 32
°F = 102.9°FThe CDC classifies a temperature ≥100.4°F (38°C) as a fever. At 102.9°F this patient is clearly febrile.
Example 3 — Scientific lab (Kelvin)
A chemistry protocol requires a reaction at −40°C (also the unique crossover point where °C = °F):
K = −40 + 273.15 = 233.15 K
°F = (−40 × 9/5) + 32 = −72 + 32 = −40°F ✓ (crossover confirmed)---
Common Mistakes
1. Forgetting to subtract 32 before multiplying (°F → °C): A common error is computing °C = °F × 5/9 without the −32 step. This yields −17.8°C for 0°F instead of the correct −17.78°C — and completely wrong results for most other values.
2. Adding 273 instead of 273.15 for Kelvin: The offset is exactly 273.15, not 273. For everyday use the difference seems minor, but in cryogenic or thermodynamic calculations it introduces systematic error. NIST defines 0°C = 273.15 K precisely.
3. Writing "°K" instead of "K": The Kelvin unit does not use a degree symbol. Since the 1968 revision of the International System of Units (SI), the correct notation is simply K, not °K. Many textbooks still print the old form.
4. Assuming boiling/freezing points are universal: The standard values (100°C / 212°F boiling; 0°C / 32°F freezing) apply only at sea-level pressure (101.325 kPa). At Denver's altitude (~1,609 m / 5,280 ft), water boils at ~95°C / 203°F — relevant for cooking and lab work.
5. Confusing Celsius and Centigrade: "Centigrade" is an older synonym for Celsius. They are identical for practical purposes, but since 1948 the official SI name is Celsius (°C), named after Anders Celsius.
6. Using the wrong direction of the 9/5 fraction: When converting °C→°F you multiply by 9/5 (≈1.8); for °F→°C you multiply by 5/9 (≈0.556). Inverting these is the single most common arithmetic error.
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Related Calculators
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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. 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).
At what temperature are Celsius and Fahrenheit equal?
Celsius and Fahrenheit are equal at exactly −40° — that is, −40°C = −40°F. You can verify this algebraically by setting °F = °C in the formula: C = (C × 9/5) + 32 → C − 9C/5 = 32 → −4C/5 = 32 → C = −40. This crossover point is useful in extremely cold-weather and cryogenic contexts.
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 Bureau International des Poids et Mesures (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.
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.
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 to cook and candy thermometer readings must be adjusted.
What temperature is used for '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 US OSHA and ASHRAE comfort range for occupied spaces is typically 68–76°F (20–24.4°C), consistent with the NIST reference.
How do I convert Fahrenheit directly to Kelvin without going through Celsius first?
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. This is exactly human body temperature, confirming the formula. No intermediate Celsius step is 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).