Correct spelling

13th

Correct spelling, explanation: 13th is the correct form, because the full form is thirteenth, thus the suffix is -th (the same as at the end of the word). 13rd is incorrect, because suffix -rd occurs only in case of ordinal number third (so in every ordinal number which ends with third, for example 33rd, 653rd). Therefore 13th is the only correct way we write this number down.

Definition of 13th:
numeral, ordinal number between 12th and 14th
In Judaism 13th birthday is when a boy is regarded as ready to take part in public worship.
I think it’s like 13th time today I tell you to cover your mouth when you sneeze!


Incorrect spelling

13rd

Incorrect spelling, explanation: this form is mistaken since ordinal number thirteenth ends with -th, not -rd, so -th should be added to the number form: 13th. -rd is reserved for number third and any other number ending with third. This is why 13th is correct and not 13rd.

Incorrect spelling

13st

Incorrect spelling

Incorrect spelling

13nd

Incorrect spelling