Over the years of teaching English as a Second Language (ESL), English as a Foreign Language (EFL) and English (native speaker level), I have run across two different rules for when to use a numeral versus spelling it out. One rule stipulated that numerals up to ten need had to be written in words. Another rule said, all numerals up to twelve need to be written. Below is a guide I can recommend, finally.

The Style Guide from BBC offers the following rules:

Write one to nine in words. Use numerals for numbers from 10 upwards and for all numbers that include a decimal point or fraction. The same applies to ordinals -‘first’, ‘second’, and so on. However, use words – even for numbers over nine – in the following cases:

• when a number comes at the beginning of a sentence
• for approximate numbers: about thirty people attended

NB: numbers twenty-one to ninety-nine are hyphenated. Use numerals – even for numbers one to nine – in the following cases:

• when the number is an exact measurement: 5 metres, 4 tonnes
• when it is followed by million or billion: 2 million
• for page references: see page 6
• where there are two numbers in a range and one is over ten: between the ages of 4 and 11

If the number is followed by an abbreviation, don’t put a space between them:

• 35mm, 10kg, 128MB, 11am

Write percentages in numerals and with the % sign, with no space between them:

• 2%, 33.3%

Write fractions less than one in words, with a hyphen where appropriate:

• one-third, three-quarters, a twentieth

Large numbers

Always include commas in numbers from 1,000 upwards. Write out ‘million’ and ‘billion’ in lower case, with a space after the number except in amounts of money.

• 10,000, 15 million viewers, £15billion

Special thanks to Graham Tappenden for sending me this link and helping me clear up this mystery numeral world.