Roman Numerals

The Roman numeral system uses seven letters:

	1		5		10		50		100		500		1000
	I		V		X		L		C		D		M

If you have difficulty remembering the letters, here's a mnemonic:

I

Value

Xylophones

Like

Cadbury's

Dairy

Milk

1 to 20

	1		2		3		4		5		6		7		8		9		10
	I		II		III		IV		V		VI		VII		VIII		IX		X

	11		12		13		14		15		16		17		18		19		20
	XI		XII		XIII		XIV		XV		XVI		XVII		XVIII		XIX		XX

10 to 100 (in tens)

	10		20		30		40		50		60		70		80		90		100
	X		XX		XXX		XL		L		LX		LXX		LXXX		XC		C

91 to 100

	91		92		93		94		95		96		97		98		99		100
	XCI		XCII		XCIII		XCIV		XCV		XCVI		XCVII		XCVIII		XCIX		C

100 to 200 (in tens)

	110		120		130		140		150		160		170		180		190		200
	CX		CXX		CXXX		CXL		CL		CLX		CLXX		CLXXX		CXC		CC

100 to 1000 (in hundreds)

	100		200		300		400		500		600		700		800		900		1000
	C		CC		CCC		CD		D		DC		DCC		DCCC		CM		M

1900 to 1990 (in tens)

	1900		1910		1920		1930		1940
	MCM		MCMX		MCMXX		MCMXXX		MCMXL

 

	1950		1960		1970		1980		1990
	MCML		MCMLX		MCMLXX		MCMLXXX		MCMXC

1990 to 2000

	1991		1992		1993		1994		1995
	MCMXCI		MCMXCII		MCMXCIII		MCMXCIV		MCMXC

 

	1996		1997		1998		1999		2000
	MCMXCVI		MCMXCVII		MCMXCVII		MCMXCIX		MM

For numbers that aren't on the table:

Converting from Arabic numerals to Roman

Consider each digit separately.

For example: 1066 = 1000 + 0 + 60 + 6 = 'M' + '' + 'LX' + 'VI' = MLXVI

Converting from Roman numerals to Arabic

Simply add up all the numbers, but if there's a 'CM', 'CD', 'XC', 'XL', 'IX', or 'IV', count these pairs together (CM = 900, CD = 400, XC = 90, XL = 40, IX = 9, IV = 4).

Example: MCMLXXXIV is M + CM + L + X + X + X + IV: 1000 + 900 + 50 + 10 + 10 + 10 + 4 = 1984.

Final note: When writing down Roman numerals, there are no hard and fast rules. For example, some experts say that 90 could be written as LXXXX, and 99 as LXXXXVIIII; conventionally, however, they are written as XC and XCIX. Similarly, 999 could be written as DCCCCLXXXXVIIII; but it's easier to write CMXCIX.

Roman numerals: a bar (line) above a number

Multiplies it by 1,000

© Haydn Thompson 2017