Should numbers be written as words or digits?

The most widely used convention is to spell out numbers one to nine and use digits from 10 upward. Some style guides set the cut-off at one hundred instead, so the first thing to establish is which guide you are following.

Beyond that threshold, a set of consistent conventions applies regardless of which cut-off you use.

Never start a sentence with a digit. Either spell it out or rewrite. Twenty-three people attended — or better, A total of 23 people attended, since spelling out large numbers is awkward.

Use digits for measurements, percentages, money, ages, dates and times, even below the threshold. 5km, 3%, £4, 6 years old, 2 May, 7pm.

Be consistent within a comparison. If two numbers are being contrasted and one requires digits, use digits for both. We received 8 complaints and 14 compliments, not eight complaints and 14 compliments.

Hyphenate compound numbers when spelled out: twenty-three, forty-seven.

Very large round numbers often read better as a mix: 2 million rather than 2,000,000.

Avoid two numbers adjacent to each other. 12 20-page reports is hard to parse; write twelve 20-page reports.

On separators, English uses a comma for thousands and a full stop for decimals — 1,234.56 — while much of Europe reverses both. In international writing a thin space is sometimes used instead to avoid ambiguity.

Ordinals follow the same threshold rule: third, but 21st.

As with most style questions, consistency matters more than which convention you pick.

For accessibility, prefer digits in interface text and instructions. Screen reader users and people scanning a page find 3 steps faster to parse than three steps, and readers with dyslexia generally report digits as easier. Prose is where the spelled-out convention earns its place.