How to Write Out Ordinal Numbers in Words in (US) American English, Rules, How to Spell Out, Exceptions, Comparison to (UK) British English.
1. What are the ordinal and the cardinal numbers?
1.1. Ordinal numbers - are those numbers defining a thing's position in a series:
1st - first - as a number, preceding all others in time, order, or importance; ex: They sat in the first row. He succeeded on his first attempt.
2nd - second - as a number, next to the first in place, time, or importance; ex: Our second option is to take a taxi. The second problem is the lack of global financing.
3rd - third - as a number, constituting number three in a sequence; ex: He came third in the competition. Our house is the third from the end on the left.
4th - fourth - as a number, constituting number four in a sequence; ex: He could do no better than finish fourth. Take the elevator to the fourth floor.
5th - fifth - as a number, constituting number five in a sequence; ex: May is the fifth month of the year. Coming in fifth place is better than finishing last.
1.2. Cardinal numbers - are those numbers denoting quantity, as opposed to ordinal numbers.
1 - one - as a number, the lowest cardinal number; half of two; ex: They have many daughters but only one son.
2 - two; - equivalent to the sum of one and one; one less than three; ex: The number that comes before three is two.
3 - three - equivalent to the sum of one and two; one more than two; ex: The bag contains three ounces of chocolate.
4 - four - equivalent to the product of two and two; one more than three; ex: Four people in five preferred this brand.
5 - five - equivalent to the sum of two and three; one more than four; ex: I am ten years old and my brother is five.
2. How to write out cardinal numbers (those denoting quantity)?
First, to know how to write out a cardinal number in words it's important to know the name of the place where each digit is.
For example, the number 12,345 has a 1 in the tens thousands place, a 2 in the thousands place, a 3 in the hundreds place, a 4 in the tens place, and a 5 in the units place.
12,345 in words is: ten thousand (10,000) + two thousand (2,000) + three hundred (300) + four tens (40) + five units (5) = ten thousand + two thousand + three hundred + forty + five = twelve thousand three hundred forty-five.
2.1. Useful notes / guidelines:
1: Note the hyphen (or the minus sign) in "twenty-three" above. Technically, it's correct to hyphenate all compound numbers from twenty-one, 21, to ninety-nine, 99. Using a hyphen also applies to ordinal numbers between 21st and 99th (notice the "st" and "th" after the cardinal numbers).
2: "And" placement - in (US) American English, do not use the word "and" after "hundred" or "thousand" (In British English, the word "and" is used after "hundred" or "thousand"). So it is "one hundred twenty-three" and not "one hundred and twenty-three", though you may hear a lot of people using the last, informally. The word "and" is used only when expressing the decimal points in decimal numbers.
3: With figures of four or more digits, use commas, ex: 1,154, 12,345.
4: When writing out numbers above 999, in American English, do not use commas. Incorrect (it is correct in British English): one thousand, one hundred fifty-four (1,154). Correct: one thousand one hundred fifty-four (1,154).
3. How to spell ordinal numbers (those denoting order)?
Most ordinal numbers end in "th", so just add "th" to the end of the cardinal number:
four - 4th - fourth
six - 6th - sixth
seven - 7th - seventh
ten - 10th - tenth
eleven - 11th - eleventh
nineteen - 19th - nineteenth
one hundred - 100th - (one) hundredth
one thousand - 1,000th - (one) thousandth
one million - 1,000,000th - (one) millionth
one - first - 1st
two - second - 2nd
three - third - 3rd
five - fifth - 5th
eight - eighth - 8th
nine - ninth - 9th
twelve - twelfth - 12th
twenty - twentieth - 20th
thirty - thirtieth - 30th
forty - fortieth - 40th
fifty - fiftieth - 50th
sixty - sixtieth - 60th
seventy - seventieth - 70th
eighty - eightieth - 80th
ninety - ninetieth - 90th
3.2. In compound ordinal numbers only the last digit is written as an ordinal:
27 = twenty-seven - 27th = twenty-seventh
123 = one hundred twenty-three - 123rd = one hundred twenty-third
1,213,498 = one million two hundred thirteen thousand four hundred ninety-eight - 1,213,498th = one million two hundred thirteen thousand four hundred ninety-eighth
3.3. When expressing ordinal numbers as figures, the last two letters of the written word are added to the ordinal number:
Since it's ok to write ordinal numbers in figures (1st, 2nd, 3rd, ...) in informal writing to friends, colleagues, etc, we need to write them in words in formal writing - business, job applications, etc.
When beginning a sentence, "Twentieth year of her life... " and not "20th year of her life...".
The Chicago Manual of Style recommends spelling out the numbers zero through one hundred and using figures thereafter - except for whole numbers used in combination with hundred, thousand, hundred thousand, million, billion, and beyond (e.g., two hundredth; twenty-eight thousandth; three hundred thousandth; one millionth, one hundred millionth).