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When do you write out numbers and when do you use the actual numerals?
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What I learnt at school is that you'd usually write out numbers in texts up to incl. 12 as the following numbers too long and complex.
Comments  
In general, numbers that can be stated in only one or two words are spelled out.
There were twelve girls and twenty-six boys there.
Any numeral at the beginning of a sentence should be spelled out. If this is awkwardly long or difficult to read, rewrite the sentence to avoid beginning with a numeral.
Three hundred and sixty students attended the lecture.
Six thousand dollars was stolen from the safe.
Spell out ordinal numbers whenever possible.
sixteenth century, Eighty-second Congress, Third Assembly District, Fifth Avenue,
Third Republic, Twenty-third Psalm
It is important to be consistent in the treatment of numbers when they appear in the same series or in the same sentence or paragraph. When they are in close sequence, do not spell some out and use figures for others.
The three chairs are 36, 72, and 122 years old.
He spent $800 on rent, $90 on food, and $265 on clothes.
When one number immediately follows another, spell out one and use figures for the other:
three 100-meter events, 60 four-poster beds, 20 five-cent stamps
Generally, use figures for dates, decades, dimensions, fractions,decimals, percentages, measures, scores, statistics and other numerical results, exact amounts of money, divisions of plays and books, pages, designations of time when followed by A.M. or P.M., and addresses.
four o'clock in the afternoon OR 4:00 PM
Note: In technical and some business writing, figures are preferred even when spellings would be brief, but usage varies. A common rule used by many magazines and newspapers is to spell out only numbers from one through ten and larger round or approximate numbers.