Numbers, Dates & Times Quick Rules
You don't want a lecture here — you want to know whether it's 5/3 or 3/5, and whether the train leaves at 9.30 or 9:30. So here's the quick version, side by side, with a link home whenever you'd like the fuller story.
A word on the Type column: rule means it's fixed, tendency means a strong preference either side, and variant means both forms are genuinely in use. Where UK and US do the same thing, I've said so plainly — no invented differences to pad a table out.
Spelling out numbers & ordinals
Mostly shared ground, this — the choices are driven by house style, not by which side of the Atlantic you're on.
| Point | UK | US | Type | Home |
|---|---|---|---|---|
| Small numbers in running text | Words for one–nine, numerals from 10 | Same | tendency (shared) | P8 |
| Number opening a sentence | Spell out, or recast the sentence | Same | rule | P8 |
| Ordinal forms | 1st, 2nd, 3rd… / first, second, third… | Same | rule (shared) | P8; P6 (hyphens, e.g. twenty-first) |
| Ordinal inside a date | 5 May or 5th May both seen | May 5 usual; May 5th more informal | tendency | P8 |
| Percent | per cent (two words) traditional, percent rising; symbol 15% | percent (one word); symbol 15% | variant | P8 |
Dates
This is where the two really do part company — enough that a bare 5/3/2026 can mean two different days.
| Point | UK | US | Type | Home |
|---|---|---|---|---|
| Numeric order | dd/mm/yyyy → 05/03/2026 = 5 March | mm/dd/yyyy → 03/05/2026 = 5 March | rule (fixed each side) | P8; Master UK/US Index |
| Written long form | 5 March 2026 (no comma) | March 5, 2026 (comma after the day) | tendency | P6 (commas); P8 |
| ISO / sortable form | 2026-03-05 | 2026-03-05 | rule — same both sides | P8 |
| "Of" in the date | the 5th of May common in speech and traditional prose | of rarer; May 5 is standard | variant | P8 |
Common Mistake: copying a numeric date straight across the Atlantic. 4/7 is 4 July to a UK reader and April 7 to a US one — if it matters, spell the month.
Times
Both sides read both clocks; the habits differ.
| Point | UK | US | Type | Home |
|---|---|---|---|---|
| 12-hour separator | 9.30 (full stop [US: period]) or 9:30 | 9:30 (colon) | tendency | P6 (punctuation in numbers) |
| am / pm | lower-case am / pm or a.m. / p.m. | a.m. / p.m. or AM / PM | variant | P8; P6 |
| 24-hour clock | common in timetables, transport, official notices (15:30) | mainly military, aviation and technical use; rare in everyday writing | tendency | P8; Master UK/US Index |
Pro-Tip: writing for readers on both sides at once? Spell the month and pick one clock — 5 March 2026, 3.30 pm leaves nothing to guess at.
Decimals & separators
No divergence to report here — the honest answer is that UK and US agree.
| Point | UK | US | Type | Home |
|---|---|---|---|---|
| Decimal marker | point → 3.5, 0.75 | point → 3.5, 0.75 | rule — same both sides | P6 |
| Thousands separator | comma → 1,234.56 | comma → 1,234.56 | rule — same both sides | P6 |
Links home
When you want the teaching rather than the reminder:
- P6 · Punctuation — punctuation in numbers (commas, colons, full stops, hyphens and dashes).
- P8 · Spelling, Morphology & Word Choice — formatting conventions, UK/US vocabulary, per cent vs percent.
- Master UK/US Index (piece 10) — every regional difference in one place.