Roman Numerals Reference Cheat Sheet

Key Facts

• The seven Roman numeral symbols are $I = 1$, $V = 5$, $X = 10$, $L = 50$, $C = 100$, $D = 500$, and $M = 1000$.
• When symbols go from greatest to least value, add them, such as $XVII = 10 + 5 + 1 + 1 = 17$.
• When a smaller symbol comes before a larger symbol, subtract it, such as $IV = 5 - 1 = 4$.
• The main subtraction pairs are $IV = 4$, $IX = 9$, $XL = 40$, $XC = 90$, $CD = 400$, and $CM = 900$.
• Only $I$, $X$, $C$, and $M$ may repeat, and they should not repeat more than three times in a row.
• The symbols $V$, $L$, and $D$ are not repeated, so $VV$ is not the standard way to write $10$.
• To write a number in Roman numerals, break it into place values, such as $348 = 300 + 40 + 8 = CCC + XL + VIII = CCCXLVIII$.
• To read a Roman numeral, scan left to right and combine each add or subtract group, such as $MCMXCIV = 1000 + 900 + 90 + 4 = 1994$.

Vocabulary

Roman numeral

A Roman numeral is a number written with letters such as $I$, $V$, $X$, $L$, $C$, $D$, and $M$.

Symbol value

A symbol value is the number represented by one Roman numeral letter, such as $X = 10$.

Additive rule

The additive rule says to add values when Roman numeral symbols are written from larger to smaller, such as $LX = 50 + 10 = 60$.

Subtractive rule

The subtractive rule says to subtract a smaller value placed before a larger value, such as $IX = 10 - 1 = 9$.

Place value

Place value is the value of a digit based on its position, such as hundreds, tens, and ones in $426 = 400 + 20 + 6$.

Standard form

Standard form is the usual digit form of a number, such as writing $XLII$ as $42$.