Ruler Measurements

Every Mark on an Inch Ruler

Between any two whole-inch numbers, a standard imperial ruler contains fifteen graduation lines that divide the span into sixteen equal parts. The height of each line indicates the size of fraction it represents. Inside the inch, the 1/2 mark is the tallest line. The next tallest lines mark the quarters at 1/4 and 3/4 inch. Eighth-tier lines mark the odd eighths, and the shortest lines mark the odd sixteenths.

Every mark belongs to the tallest tier whose fraction it reduces to in lowest terms. For example, 6/16 inch reduces to 3/8 inch, which means it receives an eighth-tier line height rather than a sixteenth-tier line height. Similarly, 12/16 inch reduces to 3/4 inch and stands at quarter-tier height.

Locating Common Fractional Marks

Each of these marks sits halfway between two taller landmark lines.

• Where is 3/8 on a ruler: The 3/8 mark is the eighth-tier line located halfway between the 1/4 line and the 1/2 line. It sits exactly 6 sixteenth-steps past the preceding whole inch.
• Where is 1/8 on a ruler: The 1/8 mark is the first eighth-tier line after the whole-inch line. It sits halfway between the whole-inch mark and the 1/4 line, exactly 2 sixteenth-steps past the inch.
• Where is 5/8 on a ruler: The 5/8 mark is the eighth-tier line located halfway between the 1/2 line and the 3/4 line. It sits exactly 10 sixteenth-steps past the preceding whole inch.
• Where is 3/4 on a ruler: The 3/4 mark is the quarter-tier line located halfway between the 1/2 line and the subsequent whole-inch mark. It sits exactly 12 sixteenth-steps past the whole inch.
• Where is 7/8 on a ruler: The 7/8 mark is the eighth-tier line located halfway between the 3/4 line and the next whole-inch mark. It sits 14 sixteenth-steps past the inch, or 2 sixteenths before the next whole number.

Finding a Mixed Measurement

A mixed measurement combines a whole number with a fractional remainder. To locate a mixed measurement on a ruler, first locate the whole-inch numeral along the blade, then locate the fraction within the subsequent inch span.

Take 10 5/8 inches as an example. First find the numbered 10 mark on the ruler. Next, scan past the 10 1/2 line into the second half of that inch. The 5/8 mark sits on the eighth-tier line located halfway between 10 1/2 and 10 3/4 inches.

In decimal form, 10 5/8 inches equals 10.625 inches. Expressed as an improper fraction, it represents 170/16 inches. In metric units, it converts to 269.875 millimeters, which rounds to 269.9 millimeters. The mark also sits exactly 3/8 inch short of the whole 11 mark.

The same process applies to any mixed dimension. For 4 3/8 inches, find the whole number 4, locate the 1/4 line, and step one eighth-tier mark to the right, which lands halfway between 4 1/4 and 4 1/2. In decimal form, 4 3/8 equals 4.375 inches, or approximately 111.1 millimeters.

Converting Decimals to Ruler Marks

Blueprints, digital calipers, and calculators frequently yield decimal dimensions rather than fractions. To locate a decimal value on an imperial ruler, multiply the decimal portion by 16 and round to the nearest whole integer to find the count of sixteenths.

As an example, if a calculation produces 0.625, multiplying 0.625 by 16 yields exactly 10. That corresponds to 10/16, which reduces to 5/8 inch on the blade. If an engineering drawing specifies 0.380 inches, multiplying 0.380 by 16 gives 6.08 sixteenths, identifying 6/16 (or 3/8 inch) as the nearest mark.

For 32nds and 64ths, use the Fraction to Decimal Chart. Standard tape measures also follow these exact sixteenth divisions, as detailed in our guide on How to Read a Tape Measure.

Metric Ruler Measurements and Metric Conversion

Rulers often feature an imperial edge and a metric edge on the same blade. The metric scale divides length into centimeters (cm) and millimeters (mm), where 10 millimeters equal 1 centimeter.

The imperial inch is linked directly to metric standards. Per the National Institute of Standards and Technology (NIST), the inch is defined as exactly equivalent to 25.4 millimeters. That standard took effect July 1, 1959, under Federal Register Notice 59-5442, following an international agreement among English-speaking nations to anchor length standards to metric measurements.

One sixteenth of an inch is 1.5875 millimeters, so none of the sixteenth marks between 0 and 1 inch falls on a whole millimeter. The table below matches common millimeter values to their nearest sixteenth-inch mark, indicating the variance for layout reference.

Common questions

Where is 3/8 on a ruler?

The 3/8 mark sits on the eighth-tier line halfway between 1/4 inch and 1/2 inch. It is located exactly 6 sixteenth-steps past the whole inch.

Where is 5/8 on a ruler?

The 5/8 mark is the eighth-tier line halfway between 1/2 inch and 3/4 inch. It sits exactly 10 sixteenth-steps past the preceding whole inch.

Where is 7/8 on a ruler?

The 7/8 mark is the eighth-tier line halfway between 3/4 inch and the next whole-inch mark. It sits 14 sixteenth-steps past the inch, or two sixteenths before the next whole number.

Where is 10 5/8 on a ruler?

Find the numbered 10 mark, scan past the 10 1/2 line, and locate the eighth-tier line halfway between 10 1/2 and 10 3/4. That mark is 10 5/8 inches, which equals 10.625 inches or 269.9 millimeters.

What does 1 inch look like on a ruler?

One inch equals exactly 25.4 millimeters. On a standard ruler, that span sits between two tall numbered marks, with fifteen shorter lines between them in height tiers for halves, quarters, eighths, and sixteenths.

How do you read the measurements on a ruler?

Identify the whole number you have passed, observe the height of the line at your measurement point to determine its fractional tier, and read the reduced fraction. For full instruction on line tiers and measurement technique, consult our guide on How to Read a Ruler.

Related

• How to Read a Ruler
• How to Read a Tape Measure
• Fraction to Decimal Chart