ruler — Definition, Formula & Examples
A ruler is a flat, straight tool marked with evenly spaced units (such as centimeters or inches) that you use to measure the length of objects and to draw straight lines.
A ruler is a graduated measuring instrument with a linear scale, typically 30 cm (12 inches) in length, used to determine the distance between two points along a straight path.
How It Works
Place one end of the object you want to measure at the zero mark on the ruler. Look at where the other end of the object lines up with the ruler's markings. The number at that point tells you the length. On the metric side, the small lines represent millimeters and the numbered lines represent centimeters. On the imperial side, the numbered lines represent inches, and the smaller lines between them represent fractions of an inch.
Worked Example
Problem: A pencil is placed against a ruler with one end at the 0 cm mark. The other end lines up with the 14 cm mark and 5 small lines past it. How long is the pencil?
Step 1: Read the large numbered mark closest to the pencil's end without going past it.
Step 2: Count the small lines past the 14 cm mark. Each small line equals 1 mm, and there are 5 small lines.
Step 3: Add the centimeters and the extra millimeters together.
Answer: The pencil is 14.5 cm long.
Why It Matters
Reading a ruler accurately is one of the first measurement skills you practice in math and science classes. It also shows up in everyday tasks like crafts, building projects, and cooking, where you need to cut or size materials to a specific length.
Common Mistakes
Mistake: Starting the measurement from the edge of the ruler instead of the zero mark.
Correction: On many rulers the zero mark is slightly inward from the edge. Always align your object with the 0, not the physical end of the ruler.