Convert µm to mm micrometres ↔ millimetres Free Online Secure
Type a value in micrometres and read it in millimetres at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert µm to mm (micrometres ↔ millimetres)
| µm | mm |
|---|---|
| 1 | 0.001 |
| 2 | 0.002 |
| 3 | 0.003 |
| 5 | 0.005 |
| 10 | 0.01 |
| 20 | 0.02 |
| 50 | 0.05 |
| 100 | 0.1 |
Where these units come from
The metre was first defined in 1793 as a ten-millionth of the distance from the equator to the North Pole — a definition nobody could check without surveying a quarter of the planet. It has been redefined four times since, each time to something steadier: a platinum bar, then a prototype kept in Paris, then a wavelength of krypton light, and since 1983 the distance light travels in vacuum in 1/299,792,458 of a second. The inch stopped being a separate thing in 1959, when six countries agreed to define it as exactly 25.4 mm. That agreement is why every conversion on this page between a metric and an imperial length is exact rather than measured: both sides now descend from the same definition.
Where you meet them
Millimetres are the working unit of engineering drawings and of anything made in a workshop; centimetres belong to tailoring, furniture and school rulers; metres to rooms, fabric and running tracks. Feet and inches survive in aviation altitudes worldwide, in American and British construction, and in the diagonal of every screen you have ever bought. Nautical miles are not a quaint survival — one is a minute of latitude, which is why ships and aircraft still navigate in them.
Questions people ask
What is a micrometre, and is it the same as a micron?
A micrometre is a millionth of a metre, and micron is simply its older name. The symbol is µm, with the Greek letter mu, which is why plain-text forms often write um. A red blood cell is about 7 micrometres across and most bacteria are between 1 and 10, so it is the natural unit for anything seen through a microscope.
Why do engineering drawings use millimetres?
Because a millimetre is small enough that nearly every dimension of a manufactured part is a whole number or a short decimal, which leaves no room for the arithmetic mistakes that fractions invite. Drawings in millimetres state the unit once and then drop it, and tolerances are written the same way, in hundredths or thousandths of a millimetre.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
From the scale of cells to the scale of a ruler: how to convert µm to mm
The micrometre belongs to the microscope, the millimetre to the ruler, and the two meet whenever someone tries to compare the very small with something they can hold. A datasheet says a semiconductor die is 3,000 micrometres across; a ruler would say 3 mm. A report says microplastics up to 5,000 micrometres; a reader wants to know that is 5 mm, the size of a grain of rice. To read such figures, you must convert µm to mm, and this page does it as you type.
The rule is a division by one thousand, which moves the decimal point three places to the left. It is exact and simple, but because the numbers in micrometres are so often large, such as 25,400 or 150,000, it is easy to miscount the places. The sections below give the rule with examples, portraits of both units, a comparison, the places where the conversion matters and the mistakes to avoid.
How to convert µm to mm: divide by 1,000
The rule
One millimetre is 1,000 micrometres. To convert µm to mm, divide the number of micrometres by 1,000 or move the decimal point three places to the left. 1,000 µm is 1 mm, 2,500 µm is 2.5 mm, 70 µm is 0.07 mm, 5 µm is 0.005 mm and 150,000 µm is 150 mm. In the other direction, multiply by 1,000.
Examples
A hair of 70 µm is 0.07 mm. A grain of sand of 500 µm is 0.5 mm. A die of 3,000 µm is 3 mm. A particle of 5,000 µm is 5 mm. A chip package of 12,000 µm is 12 mm. A bacterium of 2 µm is 0.002 mm.
Mental method
For a mental convert µm to mm, read the last three digits as the decimals: 3,250 µm is 3 mm and 250 thousandths, so 3.25 mm. For a number below 1,000, the answer starts with 0.: 850 µm is 0.85 mm, 85 µm is 0.085 mm, 8.5 µm is 0.0085 mm.
Large figures
When a datasheet gives a long number such as 25,400, mark the thousands: 25 thousand and 400, which is 25.4 mm, an inch. The comma in the original is your guide to the decimal point.
The micrometre
A millionth of a metre
The micrometre is one millionth of a metre. The prefix micro comes from the Greek for small. Cells, fibres, filters and coatings are measured in it. The symbol is µm; the older name is micron.
Sizes in micrometres
A virus is 0.1 µm, a bacterium 2 µm, a red cell 7 µm, a hair 70 µm, a grain of flour 50 µm. The naked eye can see down to about 40 µm.
The millimetre
A thousandth of a metre
The millimetre is a thousandth of a metre. It is the standard unit for mechanical drawings, hardware and rainfall. A pencil lead is 0.5 mm, a coin 2 mm, a credit card 0.76 mm.
Why convert
Most people have an intuitive feel for the millimetre, because rulers show it. By converting from micrometres, you give an abstract figure a size you can imagine.
Micrometre and millimetre compared
A millimetre is a thousand micrometres. A number in micrometres is a thousand times larger than the same length in millimetres. Use micrometres for objects smaller than a tenth of a millimetre, millimetres for the rest.
If a document uses both, do not let them alternate in one table. A column of sizes should share one unit, with the other in a second column if necessary.
Where you will need it
Semiconductors
A silicon die for a simple microcontroller is 2,000 by 2,000 µm, 2 by 2 mm. A larger processor die is 10,000 by 12,000 µm, 10 by 12 mm. Bond wires are 25 µm, 0.025 mm. Solder balls on a package are 400 µm, 0.4 mm. The datasheet gives micrometres for details, millimetres for the package, and you convert µm to mm to compare.
Microplastics and pollution
Microplastics are defined as plastic particles up to 5,000 µm, 5 mm. Fibres from clothes are 10 to 50 µm. Fragments in the sea are 100 to 5,000 µm. A report that says particles of 300 µm are common is describing particles of 0.3 mm, specks you could just see.
LEDs and displays
A micro-LED measures 50 µm, 0.05 mm. A pixel on a phone is 50 to 60 µm in pitch. A large pixel on a video wall is 2,500 µm, 2.5 mm. These numbers explain why close-up views look smooth on a phone but dotted on a billboard.
Textiles and paper
A paper fibre is 20 µm wide and 2,000 µm long, 0.02 by 2 mm. A cotton fibre is 15 µm wide and 25,000 µm long, 25 mm. The length-to-width ratio of 1,700 explains why cotton makes strong thread.
Surface roughness and flatness
Reading a finish
A surface roughness of 1.6 µm is 0.0016 mm. A flatness tolerance of 25 µm over 100 mm, 0.025 mm over 100 mm, means the surface may deviate by one part in 4,000. A sheet of glass flat to 5 µm over 100 mm is a quarter as uneven.
Why both units
The overall size of a part is in millimetres, the tolerance in micrometres. Drawings put the tolerance in the same unit as the size to avoid confusion, but a roughness symbol often has micrometres.
Language and symbols
Symbols
The symbol µm uses the Greek letter mu. In plain text, write um. Do not confuse micrometre, the unit, with micrometer, the gauge, a spelling used in American English for both.
Words
Micro, from Greek mikros, small. Milli, from Latin mille, thousand. The millimetre is thus a thousandth of a metre and the micrometre a millionth, a thousand times smaller.
A worked example
Is a particle visible?
A report says airborne particles averaged 35 µm. To convert µm to mm, divide by 1,000: 0.035 mm. The naked eye resolves about 0.04 mm on a white background, so 35 µm particles are at the limit of visibility, appearing as dust motes in a sunbeam. A particle of 2.5 µm, 0.0025 mm, is invisible, though it can be breathed in.
Check
Multiply 0.035 by 1,000 and return to 35. If you get 3.5 or 350, you have moved the decimal point one place too few or too many.
Final checks
Round trip
Multiply your answer by 1,000. If you return to the micrometres you started with, the conversion is right. If you are out by 10 or 100, count the places again.
Notes
Write both figures in your notes. A later reader can then check how you moved from one unit to the other.
Sand, silt and soil
Grain sizes
Soil scientists classify particles by size: clay below 2 µm, silt from 2 to 63 µm, fine sand 63 to 200 µm, medium sand 200 to 630 µm, coarse sand 630 to 2,000 µm. In millimetres these boundaries are 0.002, 0.063, 0.2, 0.63 and 2 mm. A geologist reading a paper in micrometres and a field guide in millimetres must convert µm to mm to place a sample.
Why the scale matters
Water drains through sand but not through clay. A soil of 70 per cent particles under 63 µm, 0.063 mm, holds water for days. A soil with grains of 500 µm, 0.5 mm, drains in hours. The conversion tells a gardener whether a sample is silty or sandy.
Pharmacy, powders and crystals
Particle and tablet sizes
Micronised drugs are ground to particles of 1 to 10 µm so that they dissolve quickly or reach the lungs. A tablet is 6,000 to 12,000 µm, 6 to 12 mm, across. A coating on the tablet is 20 to 100 µm thick. An inhaler delivers particles of 1 to 5 µm, 0.001 to 0.005 mm.
Crystals
Table salt crystals are 300 to 500 µm, 0.3 to 0.5 mm. Sugar is 500 to 1,000 µm. A snowflake is 1,000 to 5,000 µm, 1 to 5 mm. Chemists examine crystals under a microscope with a scale in micrometres.
Dust from space
Micrometeorites
Each day, tonnes of tiny meteorite particles reach Earth. Most are 50 to 500 µm, 0.05 to 0.5 mm, the size of a grain of sand. Researchers collect them from roofs, polar ice and deep-sea sediments. A particle of 200 µm is 0.2 mm, just visible to the naked eye as a speck.
Interplanetary dust
Dust grains in the solar system are 0.1 to 100 µm. Comet tails shine because of particles of 1 µm, 0.001 mm. Space missions that capture such dust use aerogel with a density low enough to stop the grains gently.
Gemstones, watches and fine jewellery
Tiny settings
A melee diamond is 1,000 to 2,000 µm, 1 to 2 mm. A claw that holds it is 300 µm thick, 0.3 mm. A watch bearing jewel is 1,000 to 3,000 µm across, with a hole of 100 µm, 0.1 mm. A hair spring is 50 µm, 0.05 mm, thick. Jewellers and watchmakers speak in tenths of a millimetre; a catalogue from a supplier of tools may give micrometres.
Comparing
When two sources use different units, a “convert µm to mm” calculation brings the figures together. A bezel groove of 0.15 mm and a ring wire of 160 µm differ by 10 µm, which is why the ring would not seat.
Quality control on the factory floor
Gauges and tolerances
An inspector uses a gauge that reads in micrometres to check a part drawn in millimetres. A bore of 12.000 mm with a tolerance of plus 18 µm, plus 0.018 mm, must measure between 12.000 and 12.018 mm. The gauge shows 12,011 µm. Converting gives 12.011 mm: within tolerance. Recording both units in the inspection report avoids disputes with customers and auditors.
Printing and imaging
Print resolution
A printer at 600 dots per inch has a dot pitch of 42.3 µm, 0.0423 mm. At 1,200 dpi, 21 µm. A photographic grain can be 5 to 20 µm. A press plate engraving has lines of 50 µm. All of these figures are easier in micrometres.
Camera pixels
A full frame sensor of 36 by 24 mm with 6,000 by 4,000 pixels has a pixel pitch of 6 µm. A phone sensor of 6 by 4.5 mm with 4,000 pixels across has 1.5 µm. Compare the pixel sizes after you convert µm to mm.
Hair, skin and the human body
Tiny parts
A hair is 70 µm, 0.07 mm. The surface skin layer is 20 µm. A capillary is 8 µm. A nerve fibre is 1 to 20 µm. The retina has cones 2 µm across, which sets the limit of vision. The sizes help doctors and cosmetic scientists understand what a treatment can reach.
Sensing
A fingertip can tell a surface with features of a few micrometres from a perfectly smooth one, which is why a roughness of 3 µm, 0.003 mm, can be felt by hand.
Fibre optics and lasers
Light in glass
A single-mode fibre has a core of 9 µm and a cladding of 125 µm, 0.125 mm. A multimode fibre has a core of 50 or 62.5 µm. A coated fibre is 250 µm, 0.25 mm. A cable of 3,000 µm, 3 mm, holds one or two fibres. Telecom manuals use micrometres for the fibre and millimetres for the cable.
Splicing
A splicer aligns fibres to within 1 µm. A cleaver cuts at an angle below 1 degree. The tolerance on each core is smaller than 0.001 mm.
Dust, food and powders
Particle sizes
Icing sugar is 10 to 100 µm, fine flour 50 to 100 µm, table salt 300 to 500 µm, coarse sugar 500 to 1,000 µm. Ground coffee for espresso is 200 to 300 µm, for filter 500 to 800 µm, 0.5 to 0.8 mm. A grinder setting changes by tens of micrometres.
Why it matters
A fine espresso grind at 250 µm, 0.25 mm, brews in 25 seconds. At 400 µm, 0.4 mm, it runs too fast. Baristas adjust their grinders by fractions of a millimetre.
Spreadsheet formulas
Formula
To convert µm to mm for a column, put micrometres in column A and enter =A2/1000 in column B. Format with three decimals. To reverse, multiply by 1000.
Check values
Enter 1000: result 1. Enter 70: 0.07. Enter 25400: 25.4. If you see 0.7 for 70, the sheet divided by 100.
Rounding
Match the precision
A thickness of 87 µm given to two figures is 0.087 mm, not 0.0870. If the original is 87.3 µm, the result is 0.0873 mm. Keep the digits of the source.
Large numbers
For a figure of 12,345 µm, the result is 12.345 mm. If the original has three significant figures, 12,300 µm, the answer is 12.3 mm.
Teaching
With a hair
To teach the “convert µm to mm” calculation, tape a hair next to a ruler and look at it with a magnifying glass. Ask how many hairs would fit in a millimetre. Answer: about 14. Then compute the width of a hair in micrometres: 1,000 divided by 14 is 71 µm.
Estimation
Ask students to guess the size of an object in micrometres, then convert to millimetres and check with a ruler. The error is usually large; the point is to get a feel for the scale.
History of the micrometre
The micron and the microscope
The unit became important with the microscope in the 17th century, when Leeuwenhoek measured cells with grains of sand. The name micron was adopted in 1879 and replaced by micrometre in 1967. The convenience of a unit between the millimetre and the nanometre explains its survival.
Industrial use
In the 20th century the micrometre became the standard for coatings, filters and precision parts. Semiconductor makers measured features in micrometres until the 1990s, when nanometres took over.
More values for convert µm to mm
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 µm = 0.0001 mm
- 0.5 µm = 0.0005 mm
- 3 µm = 0.003 mm
- 4 µm = 0.004 mm
- 7 µm = 0.007 mm
- 15 µm = 0.015 mm
- 20 µm = 0.02 mm
- 75 µm = 0.075 mm
- 200 µm = 0.2 mm
- 500 µm = 0.5 mm
- 2,000 µm = 2 mm
- 10,000 µm = 10 mm
Mistakes when you convert µm to mm
Moving two places
That gives hundredths, not millimetres. For millimetres, three places.
Moving the wrong way
The millimetre is larger, so the number gets smaller. If 70 µm becomes 70,000, you multiplied.
Confusing with nanometres
A nanometre is a thousandth of a micrometre. Check the prefix.
Quick answers
How many millimetres in a micrometre?
0.001.
How do I convert µm to mm?
Divide by 1,000.
What is 70 µm in millimetres?
0.07 mm.
Is the conversion exact?
Yes, exact.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with lengths, these conversions are close at hand: Convert in to mm, Convert m to mm, Convert mm to in, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L, Convert m/s to knots, Convert cups to pt. The reverse of this page is Convert mm to µm.
In short, to convert µm to mm divide by a thousand, move the decimal point three places to the left and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.