How to read a micrometer
A micrometer measures by turning a very fine screw. In an inch micrometer the screw has 40 threads to the inch, so one full turn of the thimble moves the spindle exactly 0.025 inch. The reading comes from three places that you add together: the sleeve (the fixed barrel with the lines along it), the thimble (the turning part with the ring of lines), and, on some micrometers, the vernier (ten short lines on the sleeve). The picture above shows all three, and Count it for me lights them up one at a time.
Reading an inch micrometer to a thousandth
The sleeve is marked in 0.025 inch steps. Every fourth line is longer and numbered, and each number stands for 0.100 inch. The thimble has 25 lines, and each one is 0.001 inch, so 25 of them make one turn of 0.025 inch.
- Read the sleeve: the last numbered line you can see, in tenths of an inch.
- Count the extra lines past it. Each is 0.025 inch.
- Read the thimble line that sits on the long line along the sleeve. Each is 0.001 inch.
Example: the sleeve shows 1 (0.100), three more lines are showing (3 x 0.025 = 0.075), and thimble line 3 is at the long line (0.003). The reading is 0.178 inch.
The vernier: reading to ten-thousandths
On a vernier micrometer, ten short lines run along the sleeve. They cover the same length as nine thimble lines, so each vernier line is 0.0001 inch away from lining up with a thimble line. After you read the thousandths, find the one vernier line that is exactly in line with a thimble line. Its number is the ten-thousandths digit.
Example: the sleeve shows 2 (0.200) and two more lines (0.050), the thimble is on 0 (0.000), and vernier line 7 lines up (0.0007). The reading is 0.2507 inch.
Reading a metric micrometer
A metric micrometer has a 0.5 millimeter pitch, so one turn of the thimble moves the spindle 0.5 mm. The thimble has 50 lines, and each one is 0.01 mm. The sleeve shows whole millimeters on one side of the long line and half millimeters on the other side.
Add the last whole millimeter line you can see, 0.50 mm if the half-millimeter line has also come into view, and the thimble line times 0.01. Example: 6 mm, plus the half-millimeter line, plus thimble line 43 (0.43 mm), is 6.93 mm. A metric vernier adds a third decimal place: its lines are 0.001 mm each, so if the thimble reading is 3.55 mm and vernier line 3 lines up, the reading is 3.553 mm.
Sizes, and what to add
A micrometer usually measures a range of one inch or 25 millimeters. A 0-1 inch micrometer reads from zero. A 1-2 inch micrometer reads from one inch, so you add 1 inch to what you read on its scale. The Read tab has the four sizes, and the steps include the extra inches. The reading also tells you how many full turns of the thimble the spindle has made: 0.2507 inch is 10.028 turns.
Micrometer or caliper?
A caliper is quicker to use and opens wider, so it is the tool for a fast check on a board, a bolt or a pipe. A micrometer reads finer. When you need a thousandth of an inch or a hundredth of a millimeter, or the part is small, use the micrometer. Both are read the same way: whole lines first, then the finer scale. To read one, see how to read a caliper.
Tips and common mistakes
- Close the micrometer on the part with the ratchet, not by hand, so you apply the same light force every time.
- Read straight on. Looking from the side moves the thimble line against the long sleeve line.
- On the inch side, count every 0.025 line after the number. Forgetting one is an error of 0.025 inch.
- On the metric side, look for the half-millimeter line. Missing it is an error of 0.5 mm.
- If the thimble line is close to the sleeve line, check the line count on the sleeve again before you trust the thimble.
Where the numbers come from
- 40 threads per inch, 0.025 inch sleeve lines (every fourth numbered as 0.100 inch), 25 thimble lines of 0.001 inch, and the vernier of ten lines over nine thimble lines: Starrett's micrometer guide and MSC Industrial's guide. The examples 0.178 and 0.2507 are Starrett's.
- Metric: 0.5 mm pitch, 50 thimble lines of 0.01 mm, and half-millimeter sleeve lines: RMIT's workshop measuring guide and Penn Tool's guide. The 0.001 mm vernier with ten lines is from a manufacturer's micrometer manual.
- One inch is exactly 25.4 millimeters (NIST SP 811). Switching between Imperial and Metric above changes which micrometer opens first and every unit shown. The conversions use that exact factor, with no rounding until the last digit shown.
- The tips are common workshop practice, not a standard. This page does not tell you how accurate your micrometer is.
Questions
How do you read a micrometer?
Add the sleeve reading, the thimble reading and, if there is one, the vernier reading. On an inch micrometer that is the last numbered line (0.100 each), the extra lines (0.025 each), the thimble line (0.001 each) and the vernier line (0.0001 each).
How do you read a micrometer in thousandths?
Use the sleeve and the thimble only. In the example 0.178 inch is 0.100 + 0.075 + 0.003.
How do you read a vernier micrometer?
Read to the thousandth first, then find the vernier line that lines up exactly with a thimble line and add its number as ten-thousandths. A 7 adds 0.0007 inch.
How do you read a metric micrometer?
Add the whole millimeters on the sleeve, 0.5 mm if the half-millimeter line is showing, and the thimble line times 0.01 mm.
Can I use metric?
Yes. The Imperial or Metric switch at the top of the page picks the micrometer that opens first, and the Convert tab shows any reading in both units. One inch is exactly 25.4 millimeters.
Is my reading sent anywhere?
No. Share saves only the picture's reading position, the system and the size for 90 days so a friend can open the same puzzle. The answer is not in the picture.