How to Read a Ruler (Inches and Centimeters)

A ruler looks busy because it packs many lines into a small space, but it follows a simple pattern: the longer the line, the bigger the fraction. Once you see that pattern, you can read any US or metric ruler in seconds.

Reading inches

On a standard US ruler, the numbers mark whole inches. The space between two numbers is split into 16 equal parts, and the line lengths tell you which fraction you’re looking at:

Line	Value	Also equals
Longest (numbered)	1 inch	16/16
Second longest	1/2 inch	8/16
Third longest	1/4 and 3/4 inch	4/16, 12/16
Fourth longest	1/8, 3/8, 5/8, 7/8 inch	2/16, 6/16, 10/16, 14/16
Shortest	1/16, 3/16, 5/16 … inch	—

Worked example

Say an object ends 5 short lines after the 2:

1. Count whole inches first: 2 inches.
2. Count the small lines after the 2: 5 sixteenths, so 5/16 inch.
3. Put them together: 2 5/16 inches.

If the count is even, simplify it: 6/16 is 3/8, 8/16 is 1/2, and 12/16 is 3/4. You can skip the counting by recognizing line lengths: the third-longest line past the 2 is 2 1/4 inches.

Inch fractions as decimals

Plans and product listings often use decimals instead of fractions. The most common ones are worth memorizing: 1/8 = 0.125, 1/4 = 0.25, 3/8 = 0.375, 1/2 = 0.5, 5/8 = 0.625, 3/4 = 0.75 and 7/8 = 0.875. For everything else, use our fraction to decimal inch chart.

Reading centimeters and millimeters

The metric side is simpler because everything is based on 10:

• Numbered lines are centimeters (cm).
• Each small line is 1 millimeter (mm). There are 10 mm in 1 cm.
• The slightly longer line halfway between two numbers is 5 mm, or half a centimeter.

To read a length, take the last number you passed, then count the small lines after it. An object that ends 3 lines past the 2 is 2.3 cm, which is also 23 mm.

Tips for accurate measuring

• Start at the 0 line, not the ruler’s edge. Many rulers have a short blank area before 0.
• Look straight down at the mark. Viewing at an angle (parallax) can shift your reading by half a line or more.
• Hold the ruler flat against the object, not tilted.
• For small parts, use millimeters. A 1 mm line is smaller than a 1/16-inch line (1.59 mm), so metric gives finer readings.
• Measure twice. If the two readings disagree, measure a third time.

Converting between inches and centimeters

One inch is exactly 2.54 cm (25.4 mm). To convert inches to centimeters, multiply by 2.54; to convert centimeters to inches, divide by 2.54. For example, 6 inches × 2.54 = 15.24 cm, and 30 cm ÷ 2.54 ≈ 11.81 inches. For longer lengths in inches, the inches to feet converter shows, for example, how many feet is 65 inches.

No ruler at hand? Our online ruler shows real sizes on your screen once calibrated, and the printable ruler gives you a paper one in a minute.

Practice worksheets

Want practice? Our free ruler worksheets cover inches to the quarter and the sixteenth, centimeters and millimeters, and measuring real lines, each with an answer key. To practice on screen, try the ruler reading practice quiz, which explains every answer.