On most inch tapes, each inch is divided into 16 parts. The longest line is the inch, then come the 1/2, the 1/4, the 1/8 and, shortest, the 1/16 marks. To read a length, count the full inches, then the marks past the last inch: six 1/16 marks past 5 in is 5 6/16, which is 5 3/8 in.
The marks, from longest to shortest
Johnson Level, a tape maker, puts it simply: the bigger the mark, the bigger the fraction. Each mark is half the one before it.
| Mark | Length on the tape | Decimal | Millimeters |
|---|---|---|---|
| 1 in | Longest, with a number | 1.0 | 25.4 |
| 1/2 in | Second longest, halfway | 0.5 | 12.7 |
| 1/4 in | Third longest | 0.25 | 6.35 |
| 1/8 in | Fourth longest | 0.125 | 3.18 |
| 1/16 in | Shortest | 0.0625 | 1.59 |
Some tapes go one step further, to 1/32 in, as Johnson Level notes.
Reading a length, step by step
- Hook the end on the edge of the board, or push the end against the inside of what you’re measuring.
- Read the last full inch before your mark: the numbered long line.
- Count the small marks from that inch to your mark, in sixteenths. Then reduce the fraction: 4/16 is 1/4, 6/16 is 3/8, 12/16 is 3/4.
- Add them up. 5 inches and 6/16 is 5 3/8 in.
A shortcut for an unlabeled mark, from Johnson Level: find the two labeled marks on either side and take the one halfway between. Halfway between 3/4 (12/16) and 7/8 (14/16) is 13/16.
Every sixteenth, as a fraction, decimal and millimeters
| Sixteenths | Reduced | Decimal | mm |
|---|---|---|---|
| 1/16 | 1/16 | .0625 | 1.59 |
| 2/16 | 1/8 | .125 | 3.18 |
| 3/16 | 3/16 | .1875 | 4.76 |
| 4/16 | 1/4 | .25 | 6.35 |
| 5/16 | 5/16 | .3125 | 7.94 |
| 6/16 | 3/8 | .375 | 9.53 |
| 7/16 | 7/16 | .4375 | 11.11 |
| 8/16 | 1/2 | .5 | 12.70 |
| 9/16 | 9/16 | .5625 | 14.29 |
| 10/16 | 5/8 | .625 | 15.88 |
| 11/16 | 11/16 | .6875 | 17.46 |
| 12/16 | 3/4 | .75 | 19.05 |
| 13/16 | 13/16 | .8125 | 20.64 |
| 14/16 | 7/8 | .875 | 22.23 |
| 15/16 | 15/16 | .9375 | 23.81 |
The 16 in marks and the black diamonds
Framing tapes mark two spacings so you can lay out a wall or a floor without adding up numbers:
- Every 16 in (16, 32, 48 …): Johnson Level marks these because 16 in is the industry standard spacing for studs in a house wall. On our drawn tape they are boxed in red.
- Every 19.2 in (19.2, 38.4, 57.6 …): a small black diamond. Stanley explains it is for the spacing of engineered joists, and that 19.2 in lets five spaces end at 8 ft, so full sheets of plywood break on a joist.
The diamonds don’t fall on a sixteenth: 19.2 in is a hair past 19 3/16 in. Use the diamond itself, not the nearest line, when laying out joists.
Why the hook moves
The metal hook at the end of the tape slides a little on purpose. Stanley calls its design the Tru-Zero hook: it slides to allow for inside and outside measurements, so the hook’s own thickness doesn’t throw the reading off. Pulled against the edge of a board, it slides out by its thickness; pushed against a wall, it slides in. Either way, zero is where it should be.
If a hook is bent or its rivets are worn, the reading is off by that amount for every measurement. For a precise cut, measure from the 1 in mark instead and subtract an inch, a trick carpenters call “burning an inch.”
Reading a metric tape
A metric tape is simpler: every line is a millimeter, the half-centimeter (5 mm) lines are a little longer, and each centimeter is numbered. A meter is 100 cm. Switch the tool above to Metric tape to practice. To convert, an inch is exactly 25.4 mm.
Questions people ask
What are the smallest marks on a tape measure?
On most inch tapes, 1/16 in, about 1.6 mm. Some tapes go down to 1/32 in.
How do you read 5/8 on a tape measure?
It is the tenth 1/16 mark after the inch, and the same mark as 5/8: count 10 sixteenths, or find the 1/8 mark halfway between 1/2 and 3/4.
What is the red mark every 16 inches?
A stud mark. 16 in is the standard spacing for studs in a house wall, so framing tapes highlight every 16 in.
What do the black diamonds on a tape measure mean?
They mark every 19.2 in, for engineered joist spacing: five spaces make 8 ft, the length of a sheet of plywood.
Why is the end of my tape measure loose?
On purpose. The hook slides by its own thickness so inside and outside measurements both start at a true zero.
How many sixteenths are in an inch?
Sixteen. Two sixteenths are 1/8, four are 1/4 and eight are 1/2.
Sources
- Johnson Level & Tool, How to Read a Tape Measure and the 1804-0025 Power Tape Measure page (16 in stud marks, 19.2 in diamonds)
- Stanley, What does the black diamond mean on the measuring tapes?, Why does the hook on my tape measure move? and the 12 ft PowerLock tape (Tru-Zero hook)
- National Institute of Standards and Technology, NIST SP 811, Appendix B.9 (1 in = 25.4 mm exactly)
Checked October 12, 2026.