Celsius, Fahrenheit, and Kelvin
Enter a temperature and pick which scale it's in, and this tool shows the equivalent value on all three common scales at once. Celsius and Kelvin share the same size of degree (just offset by 273.15), while Fahrenheit uses a smaller degree, which is why its numbers look larger for everyday temperatures.
Why Celsius and Kelvin are directly related by a simple offset
Kelvin was designed specifically so its degree size matches Celsius exactly - the only difference is where zero sits, with 0K (absolute zero, the theoretical point where no further heat can be removed from a system) equal to -273.15°C. That's why converting between the two is just adding or subtracting 273.15, with no multiplication involved.
Why Fahrenheit needs a different formula entirely
Fahrenheit's degree is both a different size than Celsius and starts from a different zero point historically defined by Daniel Fahrenheit's own reference points - which is why converting to or from Fahrenheit requires both multiplying/dividing by a ratio (9/5) and adding or subtracting an offset (32), unlike the simple addition used for Celsius-to-Kelvin.
Where each scale is actually used today
Celsius is the standard for everyday use and science throughout most of the world; Fahrenheit remains the everyday standard primarily in the United States; Kelvin is used almost exclusively in scientific and engineering contexts where an absolute temperature scale (one that starts at true zero heat) matters, such as physics and thermodynamics calculations.