How to write out numbers in words in (US) American English
1. How to convert natural numbers (positive integers) to (US) American English words, how to write them out (spell them out)?
1.1. To know how to write a number in words we must know the place value of each digit.
For example, the number 12,345 has a 1 in the ten 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 ones place.
12,345 in words =
= one ten thousands (10,000) + two thousands (2,000) + three hundreds (300) + four tens (40) + five ones
= ten thousands (10,000) + two thousands (2,000) + three hundreds (300) + four tens (40) + five ones
= ten thousand + two thousand + three hundred + forty + five
= (ten + two) thousand + three hundred + forty-five
= twelve thousand + three hundred + forty-five
= twelve thousand three hundred forty-five.
1: Note the hyphen (or the minus sign) in "thirty-four" above. Technically, it's correct to hyphenate all compound numbers from twenty-one (21) through ninety-nine (99).
2: In American English, when writing out natural numbers of three or more digits, the word "and" is not 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. In British English, the word "and" is used after "hundred" or "thousand" in numbers of three or more digits.
3. Do not use commas when writing out numbers above 999: so it is "one thousand two hundred thirty-four" and not "one thousand, two hundred thirty-four".
4. For clarity, use commas when writing figures of four or more digits: 1,234, 43,290,120, etc. In other countries a point is used to group digits by 3 and a comma to separate the decimals, ex: 1.234,55, 43.290.120,84. In some other countries a space is used to group digits by 3, ex: 1 234, 43 290 120.
Spell out all numbers beginning a sentence, "Forty years ago today,..." Not "40 years ago today,...".
The Chicago Manual of Style calls for the numbers zero through one hundred to be written out - this would include forms like "one hundred million".
Using words to write short numbers makes your writing look clean and classy. In handwriting, words are easy to read and hard to mistake for each other. Writing longer numbers as words isn't as useful, but it's good practice while you're learning.
Otherwise, clarity should matter, for example when two numbers are used in a row allways spell one out: "They needed five 2-foot copper pipes to finish the job. There were 15 six-foot tall men on the basketball team roster.".
Be consistent within a sentence, phrase... Do not write "... one million people..." and "... 1,000,000 cars..."; stick to one or another, not both.