Micrometer reading practice
To read a micrometer, take the last sleeve line the thimble edge has uncovered and add the thimble division that lines up with the reference line. The sleeve gives whole and half millimeters (or steps of 0.025 in) and the thimble adds hundredths of a millimeter (or thousandths of an inch).
Scroll sideways to see the whole scale.
Your reading
Right: 0 of 0
How to read a metric micrometer
One turn of the thimble moves the spindle 0.5 mm, and the thimble has 50 divisions, so each division is 0.01 mm. Above the reference line the sleeve has a line for each millimeter, numbered every 5 mm. Below it a second row of lines marks the half millimeters.
- Read the last millimeter line the thimble has uncovered.
- Add 0.5 mm if a half-millimeter line shows after it.
- Add the thimble division that lines up with the reference line, times 0.01 mm.
Example: the 5 mm line and the half-millimeter line after it show, and thimble line 28 sits on the reference line. The reading is 5 + 0.5 + 0.28 = 5.78 mm.
How to read an inch micrometer
The spindle has 40 threads per inch, so one turn moves it 0.025 in. The sleeve has a line every 0.025 in. Every fourth line is longer and numbered: 1 for 0.100 in, 2 for 0.200 in and so on. The thimble has 25 divisions of 0.001 in.
- Read the last numbered sleeve line as tenths of an inch.
- Add 0.025 in for each line after it that the thimble has uncovered.
- Add the thimble division on the reference line, in thousandths.
Example: the 1 line shows with three lines after it, and thimble line 3 sits on the reference line. The reading is 0.100 + 0.075 + 0.003 = 0.178 in.
Common reading mistakes
Missing the half-millimeter line
Look below the reference line before you read the thimble. When the thimble reads in the high 40s, the edge has nearly reached the next sleeve line and that line does not count yet. When it reads a low number, the edge has just passed a line and that line counts.
Reading the nearest number
On a metric thimble the numbers mark every fifth division. Find the line that sits on the reference line and count the divisions to it from the nearest number.
Parallax
Manufacturer instructions say to look straight at the reference line. Seen from above or below, a thimble line can seem to sit on the reference line when it does not.
Zero error
Before you measure, wipe the faces, close them with the ratchet or friction thimble and check that the thimble zero meets the reference line. Do it two or three times. A micrometer that reads 0.02 mm with the faces closed reads 0.02 mm high until its zero is reset. On a micrometer whose range starts above zero, check it on its setting standard.
Common questions
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How do you read a micrometer in millimeters?
Take the last millimeter line showing on the sleeve and add 0.5 mm if a half-millimeter line shows after it. Then add the thimble division on the reference line times 0.01 mm, for example 5 + 0.5 + 0.28 = 5.78 mm.
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How do you read a micrometer in inches?
Read the last numbered sleeve line as tenths of an inch and add 0.025 in for each line after it. Then add the thimble division on the reference line in thousandths, for example 0.100 + 0.075 + 0.003 = 0.178 in.
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What is the least count of a micrometer?
A standard metric micrometer reads to 0.01 mm: one turn moves the spindle 0.5 mm and the thimble has 50 divisions. A standard inch micrometer reads to 0.001 in: one turn is 0.025 in and the thimble has 25 divisions. A micrometer with a vernier on the sleeve reads one decimal place further.
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What if the reference line falls between two thimble lines?
Read the lower of the two numbers. A micrometer with a vernier on the sleeve then gives the next decimal place from the vernier line that meets a thimble line.
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How do you check a micrometer for zero error?
Wipe the measuring faces, close them with the ratchet or friction thimble and see whether the thimble zero meets the reference line. Repeat it two or three times. If it misses, reset the zero as the maker's instructions describe before you measure.
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