Grammatical rules for writing numbers in English include spelling out numbers less than ten and all numbers at the beginning of a sentence, hyphenating compound numbers and fractions, using comma separators for large numbers and using digits for time of the day and measurements. Always spell out rounded numbers.
When writing numbers in English, figures less than ten and one-word numbers are spelt out, but larger and two-word numbers are written in digits. In the USA and UK, a comma is used to the left of every three digits for numbers with four or more digits, in the form 1,000. Compound numbers and spelled-out fractions are hyphenated. Also, numbers are always spelled out when used at the beginning of a sentence.
When writing decimals, percentages, times of day and measurements in recipes, digits are used. For time of the day, the abbreviations AM or PM are used, with or without periods in between depending on the chosen style. Decimal numbers are indicated by using a period as a decimal point. Decades and centuries are spelled out without capitalization.
Approximate and rounded numbers are spelled out. If two numbers are adjacent to each other in a sentence, at least one of them should be spelled out, for clarity. When writing several numbers in the same sentence, consistency must be maintained.