Do you want to know more about thermometers?
Well check this website, choose your scale and see how it's the formula.
Developed during the 16th and 17th centuries, a thermometer (from the Greek θερμός (thermos) meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g. the scale on a mercury thermometer).
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit (1686–1736). Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees. The temperature scale was replaced by the Celsius scale in most countries during the mid to late 20th century, but it remains the official scale of the United States, Cayman Islands and Belize.
This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 (approximately 0.00366) of the difference between absolute zero and the triple point of water. Thus, it sets the magnitude of one degree Celsius and that of one kelvin as exactly the same. Additionally, it establishes the difference between the two scales' null points as being precisely 273.15 degrees Celsius (−273.15 °C = 0 K and 0 °C = 273.15 K).
Rankine is a thermodynamic (absolute) temperature scale named after the Glasgow University engineer and physicistWilliam John Macquorn Rankine, who proposed it in 1859. (The Kelvin scale was first proposed in 1848.)
The symbol for degrees Rankine is R (or Ra if necessary to distinguish it from the Rømer and Réaumur scales). Zero on both the Kelvin and Rankine scales is absolute zero, but the Rankine degree is defined as equal to one degreeFahrenheit, rather than the one degree Celsius used by the Kelvin scale. A temperature of −459.67 °F is exactly equal to 0 R.
Some engineering fields in the U.S. measure thermodynamic temperature using the Rankine scale. However, throughout the entire scientific world thermodynamic temperature is measured in Kelvin. The US National Institute of Standards and Technology does not recommend using degrees Rankine in NIST publications.
Another temperature scales:
And here is the conversion formulae: