Length 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.
When an object starts at 0, the end mark is its length. When it starts somewhere else, subtract the start mark from the end mark.
Formal definition and notation
The length of an object or a line segment is the number of identical length units that span it end to end, with no gaps or overlaps, including any fraction of a unit left over. Length is one-dimensional: it has a number and a unit, but no area. In a rectangle, length usually names the longer side and width the shorter side; for a curved path, length is measured along the curve.
Symbol
Read it as
Meaning
ℓ or L
ell, or capital L
a variable for length in formulas, as in A = ℓ w for the area of a rectangle
in, ft, yd, mi
inches, feet, yards, miles
customary (US) units of length
mm, cm, m, km
millimeters, centimeters, meters, kilometers
metric units of length
Key facts
To measure with a ruler, line one end of the object up with the 0 mark and read the mark at the other end.
If an object does not start at 0, subtract: an edge from the 3 cm mark to the 10 cm mark is 10 − 3 = 7 cm long.
A length always needs a unit. “5” alone is not a length; 5 feet and 5 meters are very different.
Area counts square units inside a flat shape, so its unit is squared (cm²). Length counts units along a line (cm).
Quick check: 3 questions
Question 1 of 3
A key lies on a ruler from the 4 cm mark to the 10 cm mark. How long is the key?
Show answer and explanation
Answer: B. Subtract the marks: 10 − 4 = 6 cm. 10 cm reads only the end mark, 14 cm adds the marks, and 7 cm counts the tick marks instead of the spaces between them.
Question 2 of 3
Which is the longest length?
Show answer and explanation
Answer: A. Change all to inches: 3 feet = 36 in, 1 yard = 36 in, and 100 cm is about 39.4 in (100 ÷ 2.54). So 40 inches is the longest. Comparing only the numbers (100 is biggest) ignores the units.
Question 3 of 3
A rope is 5 feet 4 inches long. What is its length in inches?
Show answer and explanation
Answer: C. 5 feet is 5 × 12 = 60 inches, plus 4 inches gives 64 inches. 54 writes the digits side by side, 9 adds 5 + 4, and 20 multiplies 5 by 4.
Length: frequently asked questions
Is length the same as distance?
Nearly. Both measure how far it is from one point to another, in the same units. We usually say length for an object (a 2-meter board) and distance for the space between two places or points (the distance from home to school). On a number line, the distance between 3 and 11 is 11 − 3 = 8, which is also the length of that segment.
Which side of a rectangle is the length?
By habit, the length is the longer side and the width is the shorter side, so a 12 by 5 rectangle has length 12 and width 5. This is a naming convention, not a rule of math: the area ℓ × w and the perimeter come out the same if you swap the names. When a problem names the sides, follow its labels.
What units are used to measure length?
In the US customary system: inches, feet, yards and miles. In the metric system: millimeters, centimeters, meters and kilometers. Pick a unit that fits the object: millimeters for an ant, centimeters or inches for a book, meters or feet for a room, kilometers or miles for a trip.
Updated November 2026·Reviewed by the Effortless Math team·Standards: CCSS 1.MD.A.2, CCSS 2.MD.A.1, CCSS 2.MD.A.3
Cite this entry
Effortless Math Team. “Length.” Effortless Math Dictionary. Updated November 2026. https://www.effortlessmath.com/math-dictionary/length/
Flashcard
TermLengthhow long, end to end
DefinitionThe distance from one end of an object to the other, counted in equal units such as inches or centimeters. A pencil from 0 to 7 on a ruler is 7 inches long.
Line SegmentA line segment is a straight part of a line with two endpoints. It starts at one point, stops at another, and includes every point between them. A segment with endpoints A and B is written AB. Unlike a line, which goes on forever, a segment has a definite length that you can measure.Geometry · Grade 4, Grade 5, Grade 6NumberA 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 2AreaArea is the amount of surface a flat shape covers, measured in square units. It counts how many unit squares fit inside the shape, which is why the units are squared. A rectangle’s area is length times width, so a 5 by 3 rectangle has area 15 square units.Geometry · Grade 3, Grade 4, Grade 5SideA side is one of the straight line segments that form the outline of a polygon. Two sides meet at each corner, or vertex. A triangle has 3 sides and a square has 4, and adding the lengths of all the sides gives the perimeter.Geometry · Kindergarten, Grade 1, Grade 2WidthWidth is the distance across a shape or object from side to side. In a rectangle, width is one of the two side measurements used with length, usually the shorter one, and multiplying length by width gives the area of a rectangle. Width is measured in length units such as inches, feet, centimeters or meters.Measurement · Grade 3, Grade 4, Grade 5MeterA 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 5InchAn inch is a US customary unit of length equal to one twelfth of a foot, about the width of an adult thumb. Its abbreviation is in. Rulers split each inch into halves, quarters, eighths and sixteenths. One inch is exactly 2.54 centimeters, and 12 inches make 1 foot.Measurement · Grade 2, Grade 3, Grade 4FootA foot is a US customary unit of length equal to 12 inches, about the length of a standard school ruler. Its abbreviation is ft, and its plural is feet. There are 3 feet in a yard and 5,280 feet in a mile, and 1 foot is exactly 30.48 centimeters.Measurement · Grade 2, Grade 3, Grade 4YardA yard (yd) is a US customary unit of length equal to 3 feet or 36 inches. It is a little shorter than a meter: 1 yard is exactly 0.9144 meter. Yards measure fabric, rooms, gardens and the length of a football field, which is 100 yards between the goal lines.Measurement · Grade 2, Grade 3, Grade 4CentimeterA centimeter is a metric unit of length equal to one hundredth of a meter, about the width of a fingernail. Its symbol is cm. There are 100 centimeters in 1 meter and 10 millimeters in 1 centimeter, and 1 inch equals 2.54 centimeters.Measurement · Grade 3, Grade 4, Grade 5HeightHeight is how tall something is: the distance from its bottom to its top, measured straight up and down. In geometry, the height of a shape is the perpendicular distance from a base to the opposite vertex or side, so it always meets the base at a right angle. Area and volume formulas use it.Measurement · Grade 2, Grade 5, Grade 6DifferenceA difference is the result of subtracting one number from another. In 12 − 7 = 5, the difference is 5. It tells how much bigger one number is than another, or how far apart they are on a number line. “The difference of a and b” means a − b, in that order.Operations · Grade 1, Grade 2, Grade 3PerimeterPerimeter is the total distance around a two-dimensional shape. To find it, add the lengths of all the sides. A rectangle with sides 5 and 3 has perimeter 5 + 3 + 5 + 3 = 16. Perimeter is a length, so it uses length units such as inches or meters, never square units.Geometry · Grade 3, Grade 4, Grade 5SubstituteSubstitute means to replace a variable with a number or with an equal expression. In 4x − 1, substituting 3 for x gives 4(3) − 1. You substitute to evaluate expressions, to check whether a number is a solution, and to solve systems of equations by swapping in an equal expression.Algebra · Grade 6, Grade 7, Grade 8SquareA square is a quadrilateral with four equal sides and four right angles. It is a special rectangle and a special rhombus at the same time. For a square with side s, the perimeter is 4s, the area is s2, and each diagonal is s√2.Geometry · Kindergarten, Grade 2, Grade 3Number LineA number line is a straight line with numbers placed at equal distances along it, so every point stands for a number. Zero sits in the middle, positive numbers go to the right and negative numbers to the left. Numbers increase from left to right, so a number line shows order, distance and adding as moving.Number System · Grade 2, Grade 3, Grade 6Unit ConversionUnit conversion is rewriting a measurement in a different unit without changing the amount, such as writing 3 feet as 36 inches. You multiply by a conversion factor, a fraction equal to 1 such as 12 in1 ft. Converting to a smaller unit gives a larger number; converting to a larger unit gives a smaller number.Measurement · Grade 4, Grade 5, Grade 6