How to read a ruler

To read a ruler, line up the zero mark with one end of the object, then read the last numbered line before the other end and count the small lines past it. On the inch side, each small line is usually 1/16 inch. On the metric side, each small line is 1 millimeter, and 10 of them make a centimeter.

The ruler above has both scales. Point at any spot to see the reading in inches and centimeters.

The two sides of a ruler

A standard school or desk ruler is 12 inches long, and many carry a 30 cm scale along the other edge. The inch side is for US customary measurements, and the metric side is for centimeters and millimeters. Check which edge you are reading before you start, because a centimeter line and an inch fraction can look alike at a glance.

How to read a ruler in inches

Each inch is split into smaller parts, and the length of each line tells you which part it marks. Most rulers divide the inch into 16. Some cheaper or children’s rulers stop at 1/8, and steel rules for machining often go down to 1/32 or 1/64.

• Longest lines are the whole inches, with a number beside each.
• The 1/2-inch line is the next longest, halfway between two numbers.
• 1/4-inch lines are a little shorter, at the 1/4 and 3/4 points.
• 1/8-inch lines are shorter again.
• 1/16-inch lines are the shortest.

To read a length, note the last whole inch, then count sixteenths to the end of the object. Reduce the fraction by halving the top and bottom while the top number is even. Six sixteenths is 3/8, and twelve sixteenths is 3/4.

Inch fractions chart

Fraction	Sixteenths	Decimal inch	Line on the ruler
1/16	1	0.0625	Shortest
1/8	2	0.125	Eighth
3/16	3	0.1875	Shortest
1/4	4	0.25	Quarter
5/16	5	0.3125	Shortest
3/8	6	0.375	Eighth
7/16	7	0.4375	Shortest
1/2	8	0.5	Half (long)
9/16	9	0.5625	Shortest
5/8	10	0.625	Eighth
11/16	11	0.6875	Shortest
3/4	12	0.75	Quarter
13/16	13	0.8125	Shortest
7/8	14	0.875	Eighth
15/16	15	0.9375	Shortest

The same system is used on tape measures, so once you can read a ruler you can read a tape. The tape measure guide adds feet, and the tape measure markings guide covers the extra layout marks.

How to read a ruler in cm and mm

The metric side is easier because it counts in tens.

• Numbered lines are centimeters: 1, 2, 3 and up to 30 on a 30 cm ruler.
• Small lines between them are millimeters. There are 10 mm in each centimeter.
• The middle line, slightly longer, marks 5 mm, halfway between two numbers.

To read a metric length, take the last centimeter number and count the millimeter lines past it. If an object ends 7 lines past the 4, it is 4.7 cm, which is the same as 47 mm. Most people working in metric give small sizes in millimeters, so a screw is 38 mm long and a sheet is 210 mm wide.

A mm ruler is any ruler with millimeter lines. Some engineering rulers and steel rules split the millimeter in half for 0.5 mm readings.

Millimeter to inch chart

One inch is exactly 25.4 mm. The table gives decimal inches and the nearest common fraction.

Millimeters	Inches (decimal)	Nearest fraction
1 mm	0.039 in	about 1/32 in
3 mm	0.118 in	about 1/8 in
5 mm	0.197 in	about 3/16 in
6 mm	0.236 in	just under 1/4 in
10 mm (1 cm)	0.394 in	just over 3/8 in
13 mm	0.512 in	about 1/2 in
16 mm	0.630 in	about 5/8 in
19 mm	0.748 in	about 3/4 in
25 mm	0.984 in	just under 1 in
25.4 mm	1.000 in	exactly 1 in
50 mm (5 cm)	1.969 in	just under 2 in
100 mm (10 cm)	3.937 in	about 3 15/16 in
300 mm (30 cm)	11.811 in	about 11 13/16 in

So a 30 cm ruler is about 3/16 inch shorter than a 12-inch ruler. Keep that in mind when a ruler shows both scales side by side.

Worked examples

A pencil, in inches. Set the flat end of the pencil on the zero line. The tip passes the 5 and stops at the medium-length line between 5 1/2 and 6. That is the 3/4 line, so the pencil is 5 3/4 inches long. On the metric side the same pencil reads about 14.6 cm.

A screw, in millimeters. Put the underside of the head on the zero line. The tip ends 8 small lines past the 3. That is 3.8 cm, written as 38 mm.

A small part, in sixteenths. A washer’s outside edge stops past the 2 inch line, three of the shortest steps along. Three sixteenths past 2 is 2 3/16 inches, about 55.6 mm. Three is odd, so the fraction is already in its simplest form.

Starting at zero vs the ruler edge

On many rulers the zero line sits a few millimeters in from the end. The blank strip protects the scale, because a ruler’s end gets knocked and worn. If you line your object up with the edge of the plastic instead of the zero line, every reading comes out short or long by that gap.

Look for the first line on the scale, usually marked 0 or left unlabelled at the end of the numbers, and line the object up with that. If the end of your ruler is chipped or the zero line is hard to see, start at the 1-inch or 1 cm line and subtract that amount from your reading.

Two other habits keep readings accurate:

• Look straight down at the mark. Viewing the scale at an angle shifts the line you think it touches, especially on a thick ruler.
• Keep the ruler flat and parallel. Lay the ruler right along the edge you are measuring. A slight slant makes the reading longer than the real size.

For anything longer than your ruler, switch to a tape measure. If you have neither, our guide to measuring without a tape measure lists common objects with known sizes, and the AR ruler app estimates widths with your iPhone camera. Treat AR readings as close estimates and confirm with a real ruler or tape before cutting.