A unit fraction is a fraction with a numerator of 1, such as 12, 15 or 112. It names one equal part of a whole split into that many parts. The bigger the denominator, the smaller the unit fraction, and every fraction ab is a copies of 1b.
A unit fraction is one piece of a whole. Split the whole into more parts and each piece gets smaller; count several pieces to build any other fraction.
Formal definition and notation
A unit fraction is a number of the form 1b where b is a positive whole number. It is the quantity formed by 1 part when a whole is partitioned into b equal parts. For any fraction, ab = a × 1b, the sum of a unit fractions 1b. The unit fraction 1b is the reciprocal of b.
Symbol
Read it as
Meaning
1b
one over b, or one b-th
a unit fraction with denominatorb: 18 is one eighth
A unit rate compares to 1 of something (60 miles per 1 hour). A unit fraction is one part of a whole.
Quick check: 3 questions
Question 1 of 3
Which list shows the unit fractions in order from least to greatest?
Show answer and explanation
Answer: B. Bigger denominators make smaller pieces, so 19 < 16 < 13. Choice A is greatest to least, which comes from ordering the denominators instead of the values.
Question 2 of 3
How many copies of 18 make 58?
Show answer and explanation
Answer: D.58 = 5 × 18, so 5 copies. 8 copies make a whole, 3 is the difference, and 13 adds 5 and 8.
Question 3 of 3
Which number is a unit fraction?
Show answer and explanation
Answer: A.17 has numerator 1. 71 is the whole number 7, 37 has numerator 3 (it is three unit fractions), and 0.7 is 710.
Unit Fraction: frequently asked questions
Is 1/1 a unit fraction?
By the plain definition, yes: its numerator is 1 and its denominator is a positive whole number, so 11 is the unit fraction equal to 1. In practice, school lessons focus on unit fractions with denominators of 2 or more, which name a part smaller than the whole, like 12 or 110.
Why does a bigger denominator make a smaller unit fraction?
The denominator is the number of equal parts the whole is cut into. Cut a pizza into 4 slices and each slice is 14; cut the same pizza into 8 slices and each is 18, half as big. The more pieces you make from one whole, the less each piece can hold.
How do you divide a unit fraction by a whole number?
Split the piece into that many equal parts. 13 ÷ 2 cuts a third in half, and each half is 16 of the whole. In general, 1b ÷ n = 1b × n, so the answer is another unit fraction with a bigger denominator.
Updated November 2026·Reviewed by the Effortless Math team·Standards: CCSS 3.NF.A.1, CCSS 4.NF.B.3
Cite this entry
Effortless Math Team. “Unit Fraction.” Effortless Math Dictionary. Updated November 2026. https://www.effortlessmath.com/math-dictionary/unit-fraction/
Flashcard
TermUnit Fractionnumerator is 1
DefinitionA fraction with numerator 1, naming one equal part of a whole, like 1/4. Any fraction counts copies of one: 3/4 = 1/4 + 1/4 + 1/4.
NumberA number is an amount or value used to count, measure, order and compare. We write numbers with digits (7), words (seven) or other symbols, but the number itself is the amount: 7 dots, the word seven and the numeral 7 all show the same number. Numbers include whole numbers, fractions, decimals and negatives.Number System · Kindergarten, Grade 1, Grade 2Whole NumberA whole number is 0 or a positive counting number: 0, 1, 2, 3, and so on. Whole numbers do not include fractions, decimals, or negative numbers, so 12 is a whole number but 8.5, 34 and −3 are not. Every whole number is also an integer.Number System · Grade 3, Grade 4, Grade 5FractionA fraction is a number that names equal parts of a whole, written ab. The bottom number, the denominator, tells how many equal parts make one whole; the top number, the numerator, tells how many of those parts you have. So 34 means 3 of 4 equal parts. Every fraction is also a point on the number line.Number System · Grade 3, Grade 4, Grade 5ReciprocalThe reciprocal of a number is 1 divided by that number, so the two multiply to 1. For a fraction, swap the numerator and denominator: the reciprocal of 34 is 43, and the reciprocal of 5 is 15. Zero has no reciprocal. Dividing by a fraction is the same as multiplying by its reciprocal.Number System · Grade 5, Grade 6, Grade 7DenominatorThe denominator is the bottom number in a fraction. It tells how many equal parts make one whole: in 35, the 5 means the whole is cut into 5 equal parts, and the numerator 3 counts the parts you have. The denominator can never be 0.Number System · Grade 3, Grade 4, Grade 5NumeratorThe numerator is the top number in a fraction. It tells how many parts are being counted, while the denominator below the fraction bar tells how many equal parts make one whole. In 35, the numerator is 3: three of five equal parts are taken.Number System · Grade 3, Grade 4, Grade 5MultiplicationMultiplication is the operation that combines equal groups to find a total. The fact 4 × 6 = 24 means 4 groups of 6, or 6 + 6 + 6 + 6. The numbers you multiply are called factors, and the result is the product. Order does not matter: 6 × 4 is also 24.Operations · Grade 2, Grade 3, Grade 4LengthLength is how long something is: the distance from one end of an object to the other. You measure it by counting equal units laid end to end with no gaps or overlaps, such as inches, feet, centimeters or meters. A new pencil is about 7 inches long.Measurement · Grade 1, Grade 2, Grade 3MeterA meter (m) is a metric unit of length equal to 100 centimeters. It is the base unit of length in the metric system, a little over three feet (about 3.28 feet). A doorway is about 2 meters tall, and 1,000 meters make a kilometer. To change meters to centimeters, multiply by 100.Measurement · Grade 3, Grade 4, Grade 5Equal ToEqual to means having exactly the same value or amount, and it is written with the symbol =. Numbers that look different can still be equal to each other: 12 is equal to 0.5. When two amounts are not the same, one is greater than or less than the other.Symbols and Notation · Kindergarten, Grade 1, Grade 2Proper FractionA proper fraction is a fraction whose numerator is smaller than its denominator, such as 34 or 29. Because fewer parts are taken than make one whole, a positive proper fraction is always less than 1. If the numerator is equal to or greater than the denominator, the fraction is improper instead.Number System · Grade 3, Grade 4, Grade 5Unit RateA unit rate is a rate that compares a quantity to one unit of another quantity, such as 50 miles per hour or 4 dollars per notebook. To find it, divide the first quantity by the second: 150 miles in 3 hours is 150 ÷ 3 = 50 miles per hour. The word per means for each one.Ratios and Proportions · Grade 6, Grade 7, Grade 8MileA mile (mi) is a US customary unit of length for long distances, equal to 5,280 feet or 1,760 yards. One mile is about 1.61 kilometers, so it is longer than a kilometer. Road distances and speed limits in the United States use miles and miles per hour.Measurement · Grade 4, Grade 5, Grade 6Improper FractionAn improper fraction has a numerator that is greater than or equal to its denominator, so its value is 1 or more. Examples are 74, 92 and 55. Every improper fraction can be rewritten as a whole number or a mixed number: 74 = 134. Improper does not mean wrong.Number System · Grade 4, Grade 5, Grade 6Mixed NumberA mixed number has a whole number and a fraction together, such as 234, which means 2 + 34. It represents a value greater than one whole when the whole-number part is positive. Every mixed number can be rewritten as an improper fraction, and back: 234 = 114.Number System · Grade 4, Grade 5, Grade 6Equivalent FractionsEquivalent fractions are fractions that name the same value, like 12, 24 and 36. To make one, multiply or divide the numerator and denominator by the same nonzero number. That multiplies the fraction by 1, so its value does not change, and the fractions land on the same point of a number line.Number System · Grade 3, Grade 4, Grade 5TenthsTenths are the parts you get when one whole is split into 10 equal parts; each part is one tenth, written 110 or 0.1. The tenths place is the first digit to the right of the decimal point, so in 4.7 the 7 means 7 tenths, or 0.7.Number System · Grade 3, Grade 4, Grade 5