Ordinal Suffix Rules: When to Use st, nd, rd and th
A clear guide to English ordinal suffixes, why 11th to 13th break the pattern, and how to write ordinals for large numbers, dates and lists.
The core rule and its one exception
English forms most ordinals by adding a two-letter suffix to a number. The final digit decides which suffix you use: 1 takes st (1st), 2 takes nd (2nd), 3 takes rd (3rd), and 4 through 9 plus 0 take th (4th, 7th, 10th). This covers the vast majority of numbers you will ever write. The single exception involves the numbers 11, 12 and 13, which take th even though their last digits are 1, 2 and 3. That is why we write 11th, 12th and 13th, not 11st, 12nd or 13rd.
Why the teens behave differently
The exception is not random. The suffixes st, nd and rd are shortened forms of first, second and third, but eleventh, twelfth and thirteenth do not end in those sounds, so th fits instead. The practical test is to look at the last two digits of any number rather than just the last one. If the last two digits are 11, 12 or 13, use th. Otherwise, use the last digit to pick the suffix. This single check handles every case, from 13th to 213th to 1013th, all of which end in th.
Ordinals for large numbers and hundreds
Big numbers follow the same last-two-digits test. In 101, the last two digits are 01, so it takes st and becomes 101st. In 112, the last two digits are 12, so it takes th and becomes 112th. A round hundred like 100 takes th and reads as one hundredth in words, while 1000 becomes one thousandth. When you spell these out, British English inserts and before the final part, giving one hundred and first for 101 and one hundred and twenty-third for 123. American English often drops the and, writing one hundred first, though both are widely understood.
Where ordinals show up in real writing
Ordinals appear constantly: dates (the 1st of May, her 21st birthday), rankings (he finished 3rd), floors (the 42nd floor), and ordered lists or steps. In formal writing many style guides prefer the spelled-out word for small ordinals, so first through ninth are written as words while 10th and above use figures with a suffix. In casual or technical writing the numeric form is common throughout. Superscript styling, as in 1st, is optional and mostly cosmetic; plain 1st is perfectly correct and easier to copy between systems.