Calcylator
Millimetres to Micrometres

Millimetres to micrometres:
multiply by 1,000

Convert mm to µm for coatings, tolerances and thin materials, with a chart and the slips that lead to errors of a thousand times.

Calcylator Editorial Team

Updated · 5 min read

How many micrometres are in a millimetre?

One millimetre is exactly 1,000 micrometres. The prefix micro means one millionth and milli means one thousandth, so a micrometre is one thousandth of a millimetre, written 0.001 mm.

The unit symbol is µm. In engineering and manufacturing the micrometre is still often called a micron, and the two names mean the same length.

A micrometre is a very small length, one millionth of a metre, and it is the natural unit for things you can barely see by eye. Engineers move between millimetres and micrometres all the time because a part is drawn to the nearest tenth of a millimetre, while the finish, coating or clearance on it is measured in micrometres. Instruments such as micrometre screw gauges and dial indicators often show both scales, so being fluent in the factor of 1,000 saves time and avoids costly rework. It is worth practising the shift of three decimal places until it is automatic, since the same step connects metres, millimetres and micrometres.

Formula and worked examples

Millimetres to micrometres =µm = mm × 1,000
µm:
length in micrometres
mm:
length in millimetres
1,000:
micrometres in one millimetre (exact)
To go back, divide: mm = µm ÷ 1,000.

  • Coating or sheet

    0.35 mm

  • Multiply

    0.35 × 1,000

Length in µm

350 µm

The decimal point moves three places to the right.

The reverse is just as easy. A 45 µm film is 45 ÷ 1,000 = 0.045 mm. Moving the decimal point three places is all the work there is, with no rounding.

  1. Write the length in millimetres.
  2. Move the decimal point three places to the right, or multiply by 1,000.
  3. Add the µm unit.
  4. Sanity check: the micrometre figure should be 1,000 times bigger.

Chart: millimetres to micrometres

Millimetres to micrometres
mmµm
1 mm1000 µm
2 mm2000 µm
5 mm5000 µm
10 mm10000 µm
25 mm25000 µm
50 mm50000 µm
100 mm100000 µm
250 mm250000 µm
500 mm500000 µm
1000 mm1000000 µm

Where mm to µm conversions show up

  • Paint, plating and anodising specifications, where coating thickness is given in micrometres but the part is drawn in millimetres.
  • Surface finish and flatness tolerances on machined parts.
  • Filter pore sizes, and the thickness of foil, film and paper. Ordinary printer paper is about 0.1 mm, or roughly 100 µm.
  • Lab microscopy, where a 0.5 mm field of view is 500 µm.

Quick sanity checks for millimetres to micrometres

Picture a 1.2 mm steel sheet. It is 1,200 µm thick, so a 60 µm coating is 5 percent of the total. Working in one unit avoids mistakes when you compare the two.

Tolerances are a good test. A drawing that asks for 0.02 mm accuracy is asking for 20 µm, a figure that inspection tools often show directly.

A micrometre is also one thousandth of a millimetre, so a 100 µm gap is a tenth of a millimetre.

Common mistakes when converting mm to µm

  • Being off by a factor of 1,000. If the micrometre number is smaller than the millimetre number, you divided.
  • Mixing up the prefixes. A nanometre is a thousandth of a micrometre, so do not confuse nm with µm.
  • Typing the symbol as m or u. The micro sign µ matters: 5 mm is 5,000 µm but 5 mm is not 5,000 m.
  • Treating micron and millimetre as similar names. They are three orders of magnitude apart.

Common questions

How many micrometres are in 1 mm?

There are exactly 1,000 micrometres in 1 millimetre. Equivalently, 1 µm is 0.001 mm. Micrometres are also called microns, so 1 mm is the same as 1,000 microns, and 0.5 mm is 500 µm.

How do I convert mm to micrometres?

Multiply the millimetres by 1,000. For example, 2.5 mm is 2,500 µm. In practice you simply shift the decimal point three places to the right, and the answer is always a thousand times larger than the mm figure.

How do I convert micrometres to mm?

Divide the micrometres by 1,000, which moves the decimal point three places to the left. A 250 µm film is 0.25 mm thick. The millimetre figure is always a thousand times smaller than the µm figure.

Is a micron the same as a micrometre?

Yes. Micron is the common name for the micrometre, symbol µm, which is one millionth of a metre or 0.001 mm. Both terms describe the same length, so 10 microns equals 10 µm and 0.01 mm.

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