Fraction Calculator

Addition needs a common denominator; multiplication does not

This asymmetry is the source of most fraction errors. To add or subtract, both fractions must describe parts of the same size, so you convert to a common denominator first — a half plus a third is not two fifths, because halves and thirds are different-sized pieces. To multiply, you simply multiply the numerators and the denominators; no conversion is needed. Division is multiplication by the reciprocal: flip the second fraction and multiply.

Least common denominator versus any common denominator

Multiplying the two denominators always produces a workable common denominator, and it always works. Using the least common multiple keeps the numbers smaller and reduces how much simplifying is needed afterwards. For 1/4 plus 1/6, multiplying gives 24 while the LCM is 12 — both reach the same final answer, one with less arithmetic.

Reducing to lowest terms

Divide numerator and denominator by their greatest common divisor. 18/24 reduces to 3/4 because both share a factor of 6. A result left unreduced is not wrong, but it is not conventionally complete, and most marking schemes require it. Euclid's algorithm finds the GCD quickly and is what any calculator uses internally.

Mixed numbers and improper fractions

2 3/4 and 11/4 are the same value written differently. Convert to improper form before calculating — multiply the whole number by the denominator and add the numerator — then convert back at the end if the context wants it. Trying to add mixed numbers directly, whole parts and fractional parts separately, works until the fractional parts sum past one, which is exactly where the errors appear.

Why fractions beat decimals for exactness

One third is 0.333… and no finite decimal represents it. Adding three thirds as decimals gives 0.999… and requires a rounding decision; adding them as fractions gives exactly 1. This matters wherever repeated arithmetic accumulates error, which is why computer algebra systems, engineering tolerances and financial rounding rules keep values in rational form for as long as possible.

Negative signs belong at the front

Minus 3/4, 3 over minus 4, and minus (3/4) are all equal. The convention is to place the sign before the whole fraction. When subtracting fractions, it is worth converting subtraction into addition of a negative before finding the common denominator, since sign errors during conversion are the second most common mistake after forgetting the denominator entirely.