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How to use an outside micrometer

Updated: 22 October 2026

The part has stopped and you need one outside size written down with the unit. How to use an outside micrometer is a short sequence: clean the faces, close on the ratchet, read the sleeve including the half-millimetre line, add the thimble, and record mm. Most wrong home readings are force or that half line. Which frame to own is the [outside micrometer buying guide](/hub/measuring-and-marking-tools/outside-micrometer/sub/outside-micrometer-buying-guide). The tools are the [outside micrometer comparison](/hub/measuring-and-marking-tools/outside-micrometer). Peter F. used amazon.co.uk notes plus Wonkee Donkee and National Physical Laboratory Guide 40, on 20 October 2026. No part was measured here. A qualifying purchase can earn a commission at no extra cost to you.

The part has stopped and you need one outside size written down with the unit.

Our top picks

Estink Estink, 25–50 mm

Best overall

Estink Estink, 25–50 mm
7.5/10about £22.58
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Proster Proster PST185, 0–25 mm

Rank 2

Proster Proster PST185, 0–25 mm
8.4/10about £14.95
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GEU GEU, 0–25 mm

Preis-Tipp

GEU GEU, 0–25 mm
7.2/10about £9.99
View offer
On this page
  1. 1.Clean, seat, and click
  2. 2.Read the sleeve, then the thimble
  3. 3.Check zero before you trust it
  4. 4.When the number is not usable

Clean, seat, and click

Hold the frame so your palm is not wrapped around the spindle, and keep the part still. Wipe both faces. A speck of swarf reads as metal. Seat the part square, not on a burr. Bring the spindle in with the thimble until you are close, then turn the ratchet until it clicks and the spindle stops. Wonkee Donkee’s use page is the same order: ratchet until the spindle stops, then the lock only if you must lift the tool to see the lines. The thimble is the approach. The ratchet is the force. Past the click, you spring the frame and the next reading walks. Estink and Ejoyous do not state a ratchet. On those, stop at a light contact you can repeat. The lock freezes a reading. It does not set the force. Leave a hot part. Guide 40 says not to measure work that is still hot, because the drawing size is referred to 20 °C. Do not measure a rotating part, and do not measure it in the machine.

Read the sleeve, then the thimble

On a typical metric micrometer one turn of the thimble is 0.5 mm, and the thimble has 50 lines of 0.01 mm. Read the sleeve first, including the half-millimetre line, then add the thimble. A teaching example, not a part we measured: the sleeve shows 5 mm, the half line is visible, and the thimble reads 0.28 mm. That is 5 + 0.5 + 0.28 = 5.78 mm. Miss the half line and you write 5.28 mm, fifty steps of the thimble, and the part will not match the drawing you thought you had checked. Read square-on. A high or low glance is parallax. Write mm beside the number. Most mechanical listings here are the 0.01 mm thimble. The NICCOO set is in inches. Do not run this millimetre sum on it.
Estink Estink, 25–50 mm

Testsieger

Estink Estink, 25–50 mm
7.5/10about £22.58
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Check zero before you trust it

On a 0–25 mm tool, clean the faces and close them on the ratchet. The two zeros should agree. The spanner can shift the sleeve if they do not. That is a session check, not a UKAS certificate. Many reviews of the £14.95 Proster say it arrived needing that wrench. Some buyers then saw about 0.01 mm repeat. It is a stop only when the adjuster runs out and the lines still disagree. Above 25 mm you cannot close the faces and call it zero. A 25–50 mm frame needs a 25 mm rod. A 50–75 mm frame needs a 50 mm rod. Close on the rod with the ratchet. The Estink listing names no rod. The Ejoyous names a wrench and no 50 mm standard. Do not zero either in fresh air. When the spanner cannot reach a true zero, the limit is micrometer calibration in the UK.

When the number is not usable

Stop after a drop, when the faces will not meet, or when the spanner cannot zero a 0–25 mm tool. One Ejoyous review reads a known offset instead of adjusting it. That can be a private hobby note. It is not a size to pass on. A slipping lock, mentioned once on the Proster and once on the TOPWAY, means the figure can change in your hand. Also stop for a bore, a thread or a groove these faces cannot sit on. The steps avoid a handling mistake. They do not make a £10 tool match an error the maker never printed. If the number accepts safety-critical or paid inspection work, hand it to someone who can state an uncertainty.

Common questions

Where do I hold it?
Hold the frame, not a fist around the spindle. Your hand is warm, and a long dwell moves the frame. Support the part so it cannot rock, wipe both faces, then close. If you must walk to the light, lock only after the ratchet has stopped the spindle. Do not carry the force in your wrist while you squint at the lines.
Thimble or ratchet?
Use the thimble to get close, then the ratchet until it clicks. The ratchet is the force limit. The thimble used as a clamp bends the frame. If the tool has no ratchet, stop at a light repeatable contact and do not invent a click. The lock comes after that contact. It holds the spindle. It does not decide how hard the faces were pressed.
What is the half-millimetre line?
One turn of a metric thimble is 0.5 mm. The sleeve shows whole millimetres and a half-millimetre line on the second half of each millimetre. If that line is showing, add 0.5 mm before the thimble. Forget it and 5.78 mm becomes 5.28 mm. That miss is larger than any debate about the last 0.01 mm line.
How do the sleeve and thimble add?
Read the sleeve, including the half millimetre, then add the thimble. The teaching sum is 5, plus 0.5 because the half line shows, plus 0.28, which is 5.78 mm. That figure explains the scale. It is not a part measured for this page. Write the unit. Read square-on so parallax does not donate the next line.
How do I check zero on a 0–25 mm tool?
Clean the faces, close on the ratchet, and see whether both zeros meet. The spanner usually moves the sleeve if they do not. Many Proster buyers describe that on arrival. The check is for this session. If the spanner reaches the end of its travel and zero is still out, stop. Do not record the size.
What changes above 25 mm?
You zero on a setting rod of the lower end. A 25–50 mm tool needs a 25 mm rod. A 50–75 mm tool needs a 50 mm rod. Close on it with the ratchet and read zero there. The Estink and the Ejoyous do not name that rod. Fresh air is not a 25 mm bar, however far you wind the thimble.

Related pages

Micrometer vs vernier caliper

Outside micrometer

Best outside micrometer: which listing you should keep

Outside micrometer buying guide: range before brand

Micrometer calibration in the UK: home zero or UKAS

Sources

Public UK sources first, then the amazon.co.uk listings compared on this page. Shelf prices and scores were taken from the stored listings on 20 October 2026. This is not a laboratory test.

  1. BSI, BS EN ISO 3611:2023 (retrieved 20 October 2026)
  2. ISO 3611:2023 (retrieved 20 October 2026)
  3. UKAS LAB 24 (retrieved 20 October 2026)
  4. NPL mgpg79 (retrieved 20 October 2026)
  5. NPL mgpg40 (retrieved 20 October 2026)
  6. Mitutoyo UK, 193-111 (retrieved 20 October 2026)
  7. amazon.co.uk Outside Micrometres bestsellers (retrieved 20 October 2026)
  8. Consumer Rights Act 2015 (retrieved 20 October 2026)