Temperature Scales: Celsius, Fahrenheit, and Kelvin

Three countries use Fahrenheit for everyday temperatures: the United States, Liberia, and Myanmar. The rest of the world uses Celsius. And scientists everywhere use Kelvin. Here's why, and how to convert between them.

The Three Scales

Celsius (°C)

The metric system's temperature scale, based on water:

• 0°C: Water freezes
• 100°C: Water boils (at sea level)
• Also known as Centigrade (100 degrees between freezing and boiling)

Fahrenheit (°F)

The traditional US scale with a less intuitive origin:

• 32°F: Water freezes
• 212°F: Water boils
• Originally based on the coldest temperature Fahrenheit could achieve (0°F) and human body temperature (96°F, later refined to 98.6°F)

Kelvin (K)

The absolute scale for science:

• 0 K: Absolute zero (coldest possible temperature)
• 273.15 K: Water freezes
• 373.15 K: Water boils
• Note: No degree symbol—it's just "K", not "°K"

Conversion Formulas

Celsius ↔ Fahrenheit

// Celsius to Fahrenheit
const cToF = (c) => (c * 9/5) + 32;

// Fahrenheit to Celsius
const fToC = (f) => (f - 32) * 5/9;

The formula explained:

• Fahrenheit has 180 degrees between freezing (32) and boiling (212)
• Celsius has 100 degrees in the same range
• Ratio: 180/100 = 9/5
• Offset: 32°F = 0°C

Celsius ↔ Kelvin

// Celsius to Kelvin
const cToK = (c) => c + 273.15;

// Kelvin to Celsius
const kToC = (k) => k - 273.15;

Simple offset—same scale size, different zero point.

Fahrenheit ↔ Kelvin

// Fahrenheit to Kelvin
const fToK = (f) => (f - 32) * 5/9 + 273.15;

// Kelvin to Fahrenheit
const kToF = (k) => (k - 273.15) * 9/5 + 32;

Just combine the two conversions.

Quick Reference Table

Where They Meet

There's one temperature where Celsius and Fahrenheit show the same number:

x = (x × 9/5) + 32
x = 1.8x + 32
-0.8x = 32
x = -40

Answer: -40°C = -40°F

This is a useful fact to remember—at -40, both scales agree!

Mental Math Shortcuts

Celsius to Fahrenheit (approximate)

Double and add 30:

15°C → 15 × 2 + 30 = 60°F (actual: 59°F)
20°C → 20 × 2 + 30 = 70°F (actual: 68°F)
25°C → 25 × 2 + 30 = 80°F (actual: 77°F)
30°C → 30 × 2 + 30 = 90°F (actual: 86°F)

Works reasonably well for common weather temperatures.

Fahrenheit to Celsius (approximate)

Subtract 30, then halve:

68°F → (68 - 30) / 2 = 19°C (actual: 20°C)
86°F → (86 - 30) / 2 = 28°C (actual: 30°C)
50°F → (50 - 30) / 2 = 10°C (actual: 10°C)

Quick Landmarks

Memorize these anchor points:

• 0°C = 32°F (freezing)
• 10°C = 50°F (cool)
• 20°C = 68°F (room temp)
• 30°C = 86°F (warm)
• 40°C = 104°F (very hot)
• 100°C = 212°F (boiling)

Each 10°C ≈ 18°F

Why These Numbers?

The Origin of Fahrenheit

Daniel Gabriel Fahrenheit (1724) based his scale on:

• 0°F: A brine solution (water, ice, and salt)—the coldest he could reliably produce
• 96°F: Human body temperature (he was slightly off)
• 32°F and 212°F: Water's freezing and boiling points emerged from this scale

The seemingly random numbers make more sense knowing this history.

The Origin of Celsius

Anders Celsius (1742) designed a more logical scale:

• 0: Water freezes
• 100: Water boils
• Originally inverted (100 = freezing, 0 = boiling), later flipped

The Origin of Kelvin

Lord Kelvin (1848) created an absolute scale:

• 0 K: Absolute zero—where molecular motion stops
• Based on thermodynamic principles
• Each kelvin equals one Celsius degree
• Used in science because negative temperatures become unnecessary

When to Use Each Scale

Celsius

• ✅ Weather forecasts (most countries)
• ✅ Cooking (most recipes worldwide)
• ✅ Science (along with Kelvin)
• ✅ International communication

Fahrenheit

• ✅ Weather in the US
• ✅ Cooking (US recipes)
• ✅ Some argue it's better for weather because:
  • 0-100°F roughly spans human-comfortable outdoor temperatures
  • More granularity without decimals

Kelvin

• ✅ Scientific calculations
• ✅ Thermodynamics
• ✅ Astrophysics (color temperature of stars)
• ✅ When ratios matter (double kelvin = double thermal energy)
• ❌ Never for everyday use

Programming Examples

Complete Temperature Converter

const Temperature = {
    // Celsius conversions
    celsiusToFahrenheit: (c) => (c * 9/5) + 32,
    celsiusToKelvin: (c) => c + 273.15,
    
    // Fahrenheit conversions
    fahrenheitToCelsius: (f) => (f - 32) * 5/9,
    fahrenheitToKelvin: (f) => (f - 32) * 5/9 + 273.15,
    
    // Kelvin conversions
    kelvinToCelsius: (k) => k - 273.15,
    kelvinToFahrenheit: (k) => (k - 273.15) * 9/5 + 32,
    
    // Format with unit
    format: (value, unit, decimals = 1) => {
        return `${value.toFixed(decimals)}°${unit}`;
    }
};

// Usage
console.log(Temperature.celsiusToFahrenheit(20));  // 68
console.log(Temperature.format(68, 'F'));          // "68.0°F"

Python Version

class Temperature:
    @staticmethod
    def c_to_f(celsius):
        return (celsius * 9/5) + 32
    
    @staticmethod
    def f_to_c(fahrenheit):
        return (fahrenheit - 32) * 5/9
    
    @staticmethod
    def c_to_k(celsius):
        return celsius + 273.15
    
    @staticmethod
    def k_to_c(kelvin):
        return kelvin - 273.15
    
    @staticmethod
    def f_to_k(fahrenheit):
        return (fahrenheit - 32) * 5/9 + 273.15
    
    @staticmethod
    def k_to_f(kelvin):
        return (kelvin - 273.15) * 9/5 + 32

# Usage
print(Temperature.c_to_f(20))  # 68.0

C# Version

public static class Temperature
{
    public static double CelsiusToFahrenheit(double c) => (c * 9.0 / 5.0) + 32;
    public static double FahrenheitToCelsius(double f) => (f - 32) * 5.0 / 9.0;
    public static double CelsiusToKelvin(double c) => c + 273.15;
    public static double KelvinToCelsius(double k) => k - 273.15;
}

Interesting Temperature Facts

• Coldest temperature in the universe: About 1 K (in labs, even colder: nanokelvins)
• Hottest temperature ever achieved: ~5.5 trillion K (in particle accelerators)
• Surface of the Sun: ~5,778 K (5,505°C / 9,941°F)
• Human survival range: Roughly -40°C to 60°C with proper conditions
• Comfortable indoor range: 20-22°C (68-72°F)

Summary

Key Conversion: °F = (°C × 9/5) + 32

Need to convert temperatures quickly? Try our Temperature Converter!