How to read a tape measure in inches, and turn the reading into mm

How an inch tape is divided

A standard inch tape divides each inch into sixteen equal steps, and the length of the line shows the size of the step. The longest line marks the whole inch and carries the number. The next longest is the half inch, then the quarters, then the eighths, and the shortest lines are the sixteenths. Some tapes go finer, to 1/32, but sixteenths are what most household and workshop tapes show.

The simplest way to read any position is to count the ticks past the last whole inch and treat each tick as 1/16. If you land on the ninth tick, the reading is 9/16, and the fraction reduces only if the number of ticks is even: 8 ticks is 1/2, 12 ticks is 3/4. A step is 1.5875 mm wide, so you can also read the position in millimeters directly by multiplying the tick count by that figure.

The conversion itself is exact. One inch is 25.4 millimeters by international agreement in 1959, listed in NIST SP 811 and by the BIPM. There is no rounding in the rule, only in how many digits you keep. For other values use the mm to inches converter, and see the fractions of an inch guide for the full 1/64 to 1 inch chart.

Every tick between 0 and 1 inch

Here are all fifteen ticks in one inch. The last two columns show the nearest whole millimeter and how far it is from the true value, which matters if you work from the metric edge of a dual tape.

What the table tells you

Across the fifteen ticks, the closest to a whole millimeter is 3/4 inch at 19.05 mm, only 0.05 mm from 19 mm. The furthest is 3/8 inch at 9.53 mm, a gap of 0.48 mm. Because one sixteenth is 1.59 mm and a millimeter tick is 1 mm, the two scales slip against each other along the blade. They line up exactly only at the start of the tape, and again at every 127 mm, which is 5 inches.

Three readings worked end to end

Each example counts the ticks, writes the fraction, converts to decimal inches and then multiplies by 25.4.

Long measurements: feet and inches

Tapes also carry feet. A length of 5 ft 3 7/16 in is 5 × 12 + 3 + 7/16 = 63.4375 inches, which is 161.13 cm. The same arithmetic is what the feet and inches to cm converter performs, and the inches to cm chart lists the whole-inch values. If you only need a body measurement rather than a workshop one, the post on how to measure your height accurately covers posture and the surface to use, and the cm to feet and inches cheat sheet goes the other way.

Reading the metric edge of a dual tape

Many tapes have inches on one edge and centimeters on the other. Going from the metric edge to the inch edge, a whole centimeter lands between ticks, so here is the nearest sixteenth for each of the first twelve centimeters. The last column is how far the sixteenth is from the exact length.

How accurate is a read-off?

If you read to the nearest sixteenth, the largest error you can make from the tick spacing alone is half a tick, 1/32 inch, which is 0.79 mm. In practice the reading is rougher than that, because the hook at the end of a retractable tape is loose by design, to allow for its own thickness when you measure inside or outside a corner. Two things help: measure from the 1 inch mark instead of the hook when precision matters, then subtract 25.4 mm, and read the tape straight on to avoid parallax.

For marking and cutting, remember that a saw blade removes material, so a line at 40.64 cm (16 inches, a common spacing for wall studs) is only the starting point. Studs are also often laid out at 24 inches, which is 60.96 cm.

Habits that prevent most misreads

• Count from the last whole inch, not from the previous fraction. Counting from the wrong line is the most frequent mistake.
• Say the fraction as a number of sixteenths first (11 of 16), then reduce it (11/16 does not reduce) only when the number is even.
• Write the unit with every number. A bare "7" next to a reading is how inches and centimeters get mixed in a drawing.
• Measure twice from the same end. Starting from a different end of a board shifts the reading by the hook play.
• Keep the tape flat. A tape sagging across a gap reads longer than the true distance.

Where this fits in the series

This is the third post in the tools series. The first, on drill bit sizes in mm and inches, uses the same sixteenth ladder to match bits, and the second, on socket and wrench sizes, shows which close pairs round a bolt. For a quick conversion of any single reading, the inches to cm converter gives the answer in one step. Paper is the other everyday inch-versus-mm case: see A4 vs US Letter in inches and cm.