The Least Common Denominator (LCD) is the smallest denominator that two or more fractions share when they are written with equivalent denominators. It allows you to compare, add, or subtract fractions easily.
The LCD is found by finding the least common multiple (LCM) of the denominators.
To find the LCD, list the multiples of each denominator until you find the smallest one that they have in common.
It can help to know your multiplication facts well—this makes finding the LCD much faster.
Before adding or subtracting fractions, you must make sure the denominators are the same. Use the LCD to rewrite the fractions with equivalent denominators.
The denominator does not change when adding or subtracting fractions—it stays the same after you find the LCD.
Sometimes you can find the LCD by using simple patterns or multiplication facts instead of listing all multiples.
Understanding both the greatest common factor (GCF) and the least common multiple (LCM) helps you find the LCD faster and check your work.