We can also say 'One degree centigrade'.

its one degree Celsius

"One degree Celsius." (Or "Centigrade," although I think that term is deprecated). As a computer programmer, this is an issue I had to cope with frequently when writing code to express numbers in words. For any unit, if the quantity is exactly one, you use the singular: "one unit." If it is anything else, including zero, you use the plural, "units." If it is expressed with a decimal point, you use the plural even if all of the digits after the decimal are zeroes. You use the singular for both positive and negative one. Thus: 39.37 inches: "Thirty nine point thirty seven inches." 1 g: "One gram." 2 cm: "Two centimeters." -1 ºC: "Minus one degree Celsius." 1.00 km: "One point zero zero kilometers." (Or, alternatively, "exactly one kilometers.") P.S. For programmers: if you can't be bothered to write the code to use the singular form, an easy way out is to use the phrase "number of;" for example, "number of words: 1". Because of the logical structure of the phrase, no matter how many words there are, "number of words" is always correct no matter what the number of words is.