How to Read a Ruler

The Line Ladder

On a standard imperial ruler there are only five line heights between one inch and the next, and each height means exactly one thing.

Reading a Measurement in Three Questions

Suppose the edge of your workpiece lands on a short line somewhere past the 4-inch mark. Do not count from zero. Ask three questions instead.

Which whole inch have I passed? Four.

Am I past the half-inch mark? Say yes, so you are at least 4½.

How many small marks past it? One eighth-mark past the half is 5/8. One more sixteenth beyond that is 11/16.

The reading is 4 and 11/16 inches. Three decisions, none of them involving counting past four.

• Reduce your fractions. 8/16 is a half and 4/16 is a quarter. Writing 4-8/16 on a board is a good way to misread it later.
• Look straight down. Reading a ruler at an angle shifts the apparent mark by a sixteenth or more. Parallax is a real source of error on thick rulers in particular.
• Mark with a V rather than a line. A pencil line is itself about a sixteenth wide. The point of the V is your measurement.

Metric Is Simpler, and Worth Using

A metric ruler has numbered marks for centimetres and ten small divisions between them, each one millimetre. Some rulers make the 5 mm mark slightly longer as a visual aid. There are no fractions and nothing to reduce.

A reading of 3 centimetres plus 7 small marks is 3.7 cm, or 37 mm. Most trades outside the US work in millimetres alone precisely because it removes the arithmetic, 37 is easier to add and halve than 1-7/16.

If a project involves much adding, subtracting or dividing of measurements, switching to millimetres for that project is often faster and less error-prone, even in an imperial workshop.

Where Rulers Differ from Tape Measures

Two differences matter in practice.

Rulers usually start at the very end of the material. That edge wears, and a ruler with a rounded or chipped end reads short. Precision work is often measured from the 1-inch line instead, subtracting an inch at the end. The same trick machinists use to avoid a worn zero.

A tape measure has a sliding hook that compensates for its own thickness so inside and outside measurements both read true. A ruler has no such compensation, so butting it into a corner and reading from the end introduces whatever error the end has.

Finer Graduations and Specialist Scales

Some rules divide further or differently, and reading them starts with working out what scale you are holding.

• Thirty-seconds and sixty-fourths appear on machinist rules, usually on one edge only. The rule stays the same: the new shortest line is the smallest fraction.
• Decimal inch rules divide the inch into tenths and hundredths rather than halves and quarters. Mixing one of these up with a fractional rule produces confident, wrong numbers.
• Engineer and architect scales are triangular and are not for measuring real distance at all. Each edge is a drawing scale, such as 1/4 inch to the foot, for reading plans.
• Combination squares carry a rule that slides, which lets you set a repeatable depth or marking distance rather than reading a measurement each time.

Adding and Halving Fractions Without Arithmetic

The awkward part of imperial measuring is not reading the ruler, it is doing sums with what you read. Two tricks remove most of the pain.

To add fractions, convert both to sixteenths first, because every fraction on the rule divides into sixteenths cleanly. A half is 8/16, a quarter is 4/16, an eighth is 2/16. So 3/8 plus 1/4 becomes 6/16 plus 4/16, which is 10/16, which reduces to 5/8. Working in a common denominator you can see on the rule turns fraction arithmetic into counting.

To halve a measurement, finding the center of a board, for instance, do not calculate at all. Lay the rule diagonally across the board so that a conveniently divisible number lands exactly on each edge. Set zero on one edge and 9 inches on the other, and the midpoint is simply the 4½ mark, whatever the actual width happens to be. The same trick divides a board into any number of equal parts: choose a number divisible by however many pieces you want.

Small Habits That Keep You Accurate

Most measuring error is procedural rather than a misread scale.

• Use one rule for a whole project. Two rules can genuinely disagree by a sixteenth, and mixing them mid-job puts that error into your joints.
• Measure twice from the same reference. Re-measuring from a different edge finds a different answer and tells you nothing.
• Keep the rule flat against the work. A rule held at an angle across a curved or uneven surface measures the hypotenuse rather than the distance you want.
• Do not measure from a damaged end. Start at 1 and subtract.
• Write the fraction down. Remembering 11/16 across a workshop is how it becomes 13/16.

Common questions

What are the little lines on a ruler?

Fractions of an inch, sized by line height. Tallest is the whole inch, then halves, quarters, eighths, and the shortest marks are sixteenths. Some rules add thirty-seconds on one edge.

How many sixteenths are in an inch?

Sixteen. Between two inch marks there are fifteen shorter lines, and the shortest of them fall on the sixteenths that are not also eighths, quarters or halves.

How do I read 3/8 on a ruler?

Find the quarter-inch mark, then count one eighth-mark past it. Working down from the nearest big fraction is faster and far less error-prone than counting sixteenths up from the inch line.

What is the difference between a ruler and a tape measure?

A tape measure has a sliding hook that shifts by its own thickness so inside and outside measurements both read correctly. A ruler has a fixed end that wears, which is why precision work is often measured from the 1-inch line instead.

How do you read a metric ruler?

Numbered marks are centimetres and the ten small divisions between them are millimetres. Three centimetres plus seven small marks is 3.7 cm, or 37 mm. There are no fractions to reduce.

Related

• How to Read a Tape Measure