20 °C or 20°C? How to write degrees Celsius
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In a nutshell
Write 20 °C, with a space. The degree sign and the letter C together form one unit symbol, °C, and like every unit symbol it is separated from the number by a space, just as in 25 kg or 3 m.
The exception is angles: 30°, with no space. And the kelvin needs no degree sign at all: 293.15 K.
One symbol, one space
The rule is the same for every unit: a space is left between the numerical value and the unit symbol. For temperature this means t = 30.2 °C, not 30.2°C and not 30.2° C. The degree sign belongs to the unit, not to the number, so it is never split from the C.
The same holds when the value describes something in English: a 20 °C room, not a 20-°C room. In the same way a 10 kΩ resistor takes no hyphen.
Angles are the exception
The only symbols written without a space are the degree, minute and second of plane angle: a bearing of 30°, or 30°22′8″. That is where the habit of writing 20°C comes from. A temperature is not an angle, so the space stays.
Degrees Celsius and kelvins
The Celsius temperature t is defined from the thermodynamic temperature T as t = T − T0, where T0 = 273.15 K exactly. So 20 °C is 293.15 K, and 0 °C is 273.15 K.
The two units are the same size. A rise of 10 °C is a rise of 10 K, which is why temperature differences can be given in either unit.
The kelvin itself is written with its symbol K and no degree sign: 293.15 K. How the kelvin is defined today is explained on the Base units page.
The same goes for per cent
The NIST guide to the SI applies the same logic to the per cent sign: a space is left between the number and the symbol, so 0.25 %, not 0.25%. Style guides of some publishers differ here, but the space is the rule in the NIST guide.
Sources
- BIPM, The International System of Units (SI Brochure), 9th edition, checked 28 October 2026
- NIST, Guide for the Use of the International System of Units (SP 811), Chapter 7: Rules and style conventions for expressing values of quantities, checked 28 October 2026
- NIST, Guide for the Use of the International System of Units (SP 811), Chapter 4: Two classes of SI units and the SI prefixes, checked 28 October 2026