The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 8, 'Dates, Numbers, Currency, and Weights and Measures'
Numbers up to and including one hundred, including ordinals, should be written in words when the context is not statistical. Figures should be used for volume, part, chapter, and page numbers; but note:
The second chapter is longer than the first.
Figures are also used for years, including those below one hundred (see 8.1). However, numbers at the beginning of sentences and approximate numbers should be expressed in words, as should ‘hundred’, ‘thousand’, ‘million’, ‘billion’, etc., if they appear as whole numbers:
Two hundred and forty-seven pages were written.
The fire destroyed about five thousand books.
She lived and wrote a thousand years ago.
Words should be preferred to figures where inelegance would otherwise result:
He asked for ninety soldiers and received nine hundred and ninety.
In expressing inclusive numbers falling within the same hundred, the last two figures should be given, including any zero in the penultimate position:
13–15, 44–47, 100–22, 104–08, 1933–39
Where four-digit numbers do not fall within the same hundred, give both figures in full:
Dates of lifespans should be given in full, e.g. 1913–1991. Datespans before the Christian era should be stated in full since the shorter form could be misleading:
The First Punic War (264–241 bc) (not 264–41 bc)
Numbers up to 9999 are written without a comma, e.g. 2589; those from 10,000 upwards take a comma, e.g. 125,397; those with seven or more digits take two or more commas, separating groups of three digits counting from the right, e.g. 9,999,000,000. However, where digits align in columns, in copy such as tables or accounts, commas must be consistently included or omitted in all numbers above 999.