Common Denominator Calculator
commondenominatorcalculator.com • Fractions
Fraction form
1st fraction ▾
2nd fraction ▾
Common denominator results
Least common denominator
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1st fraction as /LCD
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2nd fraction as /LCD
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How the Common Denominator Calculator works

Type your fractions — either simple (n/d) or mixed (W n/d). We instantly convert any mixed numbers to improper fractions, compute the least common denominator (LCD) of the denominators, and scale the numerators so both fractions share the same denominator. Toggle the step‑by‑step to see the working.

How to find the least common denominator

  1. Prime‑factor each denominator.
  2. Take the highest power of every prime that appears in any denominator — this product is the LCD.
  3. Multiply each numerator by LCD ÷ its original denominator.

What are the common denominators?

Any multiple of the LCD is a common denominator. For example, if the LCD is 12 then 24, 36, 48… are all common denominators. The LCD is the smallest positive common denominator and keeps numbers simplest.

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FAQs

How do I find the LCD of 1/3 and 2/7?
Factor 3 and 7 → LCD = 21. 1/3 = 7/21; 2/7 = 6/21.
What is the least common denominator of 2/3 and 5/8?
LCM(3, 8) = 24 → 2/3 = 16/24 and 5/8 = 15/24.
What's the least common denominator of 3/4, 4/5, and 2/3?
LCM(4, 5, 3) = 60.
How do you find the common denominator?
Use prime factors and take the highest power of each prime → the LCM. Multiply numerators by LCD ÷ original denominator.
What is the common denominator of 1/3 and 4/9?
LCM(3, 9) = 9 → 1/3 = 3/9; 4/9 stays 4/9.
What is the common denominator of 1/4 and 2/7?
LCM(4, 7) = 28 → 1/4 = 7/28; 2/7 = 8/28.