Roman Numerals for Days: I to XXXI

For example:

29th day
→ day number 29
→ XXIX

This guide covers day numbers within a month. It does not convert days of the week, such as Monday, or numbered days of the year.

Key takeaways

• Calendar day numbers run from I to XXXI.
• Convert the day number, not the ordinal ending such as “st,” “nd,” “rd” or “th.”
• Only I, V and X are needed for day numbers from 1 to 31.
• Days ending in 4 or 9 use the standard forms IV and IX.
• Leading zeroes such as 04 or 09 are not converted separately.
• A valid Roman numeral day does not prove that the complete calendar date exists.
• Values from I to XII may represent either a month or an early day.
• Centred dots should separate day, month and year in complete date examples.

Days of the month in Roman numerals

The sequence begins with I for day 1 and ends with XXXI for day 31.

What “day” means in this guide

This guide covers numbered days within a month. A numeral such as XVII means 17 and represents day 17 only when used as the day component of a date. It does not convert weekday names such as Monday.

How the values I to XXXI are formed

The day numbers follow four simple groups.

For day 14:

14 = 10 + 4
14 = X + IV
14 = XIV

For day 29:

29 = 20 + 9
29 = XX + IX
29 = XXIX

Only three symbols are required:

I = 1
V = 5
X = 10

No calendar day number is high enough to require L, C, D or M.

Days ending in 4 and 9

The standard four-form and nine-form repeat in each group:

4 = IV
9 = IX
14 = XIV
19 = XIX
24 = XXIV
29 = XXIX

The tens appear first, followed by IV or IX. Use IV rather than IIII, IX rather than VIIII, and XXIX rather than XXVIIII. IIII appears on some clocks, but IV is the standard modern date form.

Leading zeroes in day numbers

Digital dates often write early days as 01, 04 or 09. The zero is only a formatting device:

01 → I
04 → IV
09 → IX

It contributes no Roman numeral symbol.

A valid numeral is not always a valid calendar date

A correctly written day numeral may still be impossible in a particular month or year. XXXI means 31, but day 31 does not occur in April, June, September or November. Check the month and year as well as the numeral.

Month-length reference

Therefore, 31 April is invalid, while 29 February is valid only in a leap year. The day numeral alone cannot confirm the complete date.

Leap-day example

29 February 2024 is a valid leap-day date.

Day 29 = XXIX
February = II
2024 = MMXXIV

In day-month-year order:

XXIX · II · MMXXIV

By contrast, 29 February 2023 does not exist. XXIX remains the correct numeral for day 29, but the complete calendar date is invalid.

Day numerals inside complete dates

For 4 July 2026:

Day 4 = IV

DMY: IV · VII · MMXXVI
MDY: VII · IV · MMXXVI

For 31 December 1999:

Day 31 = XXXI

DMY: XXXI · XII · MCMXCIX
MDY: XII · XXXI · MCMXCIX

The day value stays the same; only its position changes.

Day and month ambiguity

Every month number lies between I and XII. Those same values can also represent days 1 to 12.

Consider:

IV · VII · MMXXVI

In DMY order, this is 4 July 2026. In MDY order, it is April 7, 2026.

A day above XII cannot represent a month, but the intended order should still be stated clearly.

Common day-number mistakes

These errors usually come from repeating I instead of using IV or IX endings.

How to check a Roman numeral day

Check the decimal day, convert its tens and units, read the numeral back, and then confirm that the day exists in the chosen month and year.

For day 29:

29 → XXIX
XXIX = XX + IX
XXIX = 20 + 9
XXIX = 29

The numeral is correct, but the complete date still requires a valid month and year.

A simple way to remember I to XXXI

Learn the sequence by tens:

1–9: I to IX
10–19: X to XIX
20–29: XX to XXIX
30–31: XXX and XXXI

The units pattern repeats in each group. Add X for the teens and XX for the twenties.

Continue learning

Use the related guides for more detail:

• How to Write Dates in Roman Numerals
• Roman Numeral Date Formats
• Roman Numerals for Years
• Roman Numerals for Months
• Valid and Invalid Roman Numerals
• Common Roman Numeral Mistakes