Convert mm to µm millimetres ↔ micrometres Free Online Secure

From the workshop to the microscope: how to convert mm to µm

A millimetre is about the thickness of a coin; a micrometre is a thousandth of that, smaller than a speck of dust and about the size of a bacterium. The two units meet wherever precision work begins: a sheet of foil of 0.05 mm is 50 micrometres, a paper of 0.1 mm is 100 micrometres, a machined surface that must be flat to 0.002 mm is flat to 2 micrometres. Whenever a figure in millimetres has to be read in micrometres, you need to convert mm to µm, and this page does it as you type.

The rule is a multiplication by one thousand, which moves the decimal point three places to the right. It is exact and easy, but a missing zero is a thousand-fold error in a field where such errors are costly. The sections below give the rule with examples, portraits of both units, a comparison, a tour of the places where the conversion matters and the mistakes to avoid.

How to convert mm to µm: multiply by 1,000

The rule

One millimetre is 1,000 micrometres. To convert mm to µm, multiply the number of millimetres by 1,000 or move the decimal point three places to the right. 1 mm is 1,000 µm, 0.5 mm is 500 µm, 0.05 mm is 50 µm, 0.002 mm is 2 µm and 12 mm is 12,000 µm. In the other direction, divide by 1,000.

Examples

A human hair of 0.07 mm is 70 µm. A sheet of paper of 0.1 mm is 100 µm. Aluminium foil of 0.016 mm is 16 µm. A cling film of 0.01 mm is 10 µm. A tolerance of 0.005 mm is 5 µm. The digits stay the same and the decimal point moves.

Mental method

For a mental convert mm to µm, read the decimals in groups of three: 0.075 mm is 75 µm, 1.250 mm is 1,250 µm, 0.008 mm is 8 µm. The three digits after the decimal point are the micrometres, and the whole millimetres become thousands.

Symbols

The symbol for micrometre is µm, with the Greek letter mu. If you cannot type it, use um in plain text, and say micrometre in speech. The old name micron is still met in industry.

The millimetre

A thousandth of a metre

The millimetre is the standard unit of mechanical drawings. Dimensions are given in whole millimetres and tolerances in fractions of one: 0.1 mm is a typical general tolerance, 0.01 mm a fine one.

Limits

Below a tenth of a millimetre, decimals pile up and the micrometre becomes the natural unit. A surface roughness of 0.0008 mm is hard to read; 0.8 µm is simple.

The micrometre

A millionth of a metre

The micrometre is a millionth of a metre or a thousandth of a millimetre. The prefix micro comes from the Greek for small. It is the unit of cells, bacteria, fibres, coatings and the finest machining.

Everyday scale

A red blood cell is 7 µm, a bacterium 1 to 5 µm, a human hair 70 µm, a grain of fine flour 50 µm, a dust particle 10 to 100 µm. Anything smaller than about 40 µm is invisible to the naked eye.

Millimetre and micrometre compared

A millimetre is a thousand micrometres. If a micrometre were a millimetre, a millimetre would be a metre. The ratio is the same as that between a metre and a millimetre, so the step feels familiar once you have worked with metres and millimetres.

Use millimetres for parts you can hold, micrometres for coatings, particles and tolerances. Where both appear, as in a drawing with a thickness of 0.8 mm and a surface finish of 1.6 µm, the unit tells the reader which scale matters.

Where you will need it

Machining and tolerances

A shaft of 25.000 mm with a tolerance of plus 0, minus 0.013 mm is, in micrometres, 25,000 µm with a tolerance of 13 µm. Fits between shafts and holes are specified in micrometres: a clearance of 20 to 45 µm is a running fit. When a drawing is in millimetres and a machine controller in micrometres, you convert mm to µm for every dimension.

Surface finish

Surface roughness is quoted as Ra in micrometres: 3.2 µm for a rough turned surface, 0.8 µm for a ground one, 0.1 µm for a lapped one. A drawing may say 0.0008 mm; the finish is easier to compare in micrometres.

Foils and films

Kitchen foil is 10 to 16 µm. A food bag is 25 µm. A bin liner is 20 to 40 µm. A banknote paper is 100 µm. Packaging specifications list these in micrometres, while the roll width and length are in millimetres and metres.

Cameras and sensors

A camera sensor pixel of 0.004 mm is 4 µm. A phone pixel is 1 µm. A full frame sensor of 36 mm has about 9,000 pixels of 4 µm across. These small numbers explain why lens quality matters more with small pixels.

Filtration

Pore sizes

A coffee filter has pores of 20 µm. A water filter may be rated at 5 µm, 1 µm or 0.2 µm. A HEPA filter catches 99.97 per cent of particles of 0.3 µm. A surgical mask stops particles around 3 µm. Filter specifications use micrometres because particles are micrometres in size.

Choosing

A pre-filter of 50 µm, 0.05 mm, protects a fine filter from sand. Each stage should be about a tenth of the previous in pore size.

Typing the symbol

How to write µm

On Windows, hold Alt and type 0181. On a Mac, press Option and m. In HTML, use the entity µ. In plain text, write um or micrometre. Beware of the Greek letter mu, which looks the same but has a different code; some systems treat them differently.

Consistency

A document that mixes µm and um looks sloppy. Choose one and stay with it. If the document will be searched, use micrometre in full at least once.

A worked example

The thickness of a film

A plastic film is listed as 0.045 mm thick. To convert mm to µm, multiply by 1,000: 45 µm. A customer wants 40 to 50 µm. The film is in the middle of the range. Another supplier gives 0.038 mm, 38 µm, which is out of range by 2 µm, 5 per cent. The decision follows from the converted numbers.

Check

Divide 45 by 1,000 and return to 0.045. If you get 0.45 or 0.0045, you moved the decimal point the wrong number of places.

Final checks

Round trip

Divide the result by 1,000. If you return to the millimetre figure, the conversion is right. If the result is 10 or 100 times off, count the places again.

Notes

Write both figures in your notes. A reader can then see how you moved from one unit to the other.

Dust, smoke and air quality

Particle sizes

Air-quality reports speak of PM10 and PM2.5, particles of up to 10 µm and 2.5 µm. In millimetres these are 0.01 and 0.0025 mm, numbers that nobody wants to read. Pollen is 15 to 100 µm, mould spores 3 to 40 µm, cigarette smoke 0.1 to 1 µm. If a filter specification gives a pore size in millimetres, you must convert mm to µm to see which of these it will stop.

Comparing with a hair

A hair of 70 µm is about 28 times wider than a PM2.5 particle and 7 times wider than a PM10. The comparison explains why such particles stay in the air for hours and pass through the nose into the lungs.

Batteries, foils and electrodes

Thin layers in energy storage

A lithium-ion cell has copper foil of 8 to 12 µm and aluminium foil of 12 to 16 µm as current collectors, with active layers of 50 to 100 µm on each side and a separator of 12 to 25 µm. The whole stack of one layer is around 200 µm, 0.2 mm. A pouch cell of 5 mm contains 25 such layers. Manufacturers describe electrodes in micrometres and the finished cell in millimetres.

Why the difference matters

An error of 10 µm in a foil is 10 per cent of its thickness, and over 25 layers adds 0.25 mm to the cell. That is why battery engineers measure in micrometres even though the product is measured in millimetres.

Printing, paper and ink

Dot sizes and paper

An inkjet droplet lands as a dot of 20 to 50 µm. A laser printer toner particle is 5 to 8 µm. A sheet of newsprint is 70 µm, copier paper 100 µm, glossy photo paper 250 µm, card 300 to 400 µm. Printers specify paper thickness in micrometres or in grams per square metre; the micrometre figure tells you whether a sheet will go through the feed.

Stack height

A ream of 500 sheets of 100 µm paper is 50 mm thick. Divide the thickness of a stack in millimetres by the number of sheets and multiply by 1,000 to get micrometres per sheet. A stack of 10 mm for 100 sheets is 100 µm per sheet.

Keeping a conversion log

A habit for precise work

When you work with micrometres, keep a small log: the original figure in millimetres, the converted figure, the date, the instrument and who did it. Months later, when a part is out of tolerance, you can check whether the number was entered correctly or whether the instrument was out of calibration. A log of ten lines per day costs a minute and has saved many batches of parts.

Paint, plating and coatings

Thickness of layers

A car's paint has a total thickness of 100 to 150 µm: primer 25, base 15, clear coat 40 to 50. A chrome plating is 0.25 µm. Galvanised steel has 7 to 85 µm of zinc. A thickness gauge reads micrometres; drawings may list 0.1 mm for the whole system.

Checking a coating

A coating of 0.08 mm on a specification is 80 µm. If the gauge reads 65 µm, it is thin by 15 µm, 19 per cent. Convert both to the same unit before you decide.

Textiles and fibres

Fibre diameters

Wool fibres are 15 to 40 µm. Cotton is 12 to 20 µm. Silk is 10 µm. Microfibre is less than 10 µm, which gives it its name. A synthetic thread of 0.03 mm is 30 µm. Fine merino wool is 17 µm, and wool grades by diameter in micrometres.

Why it matters

Fibres under 20 µm feel soft; above 30 µm they prickle. A label that says 0.019 mm is far less readable than 19 µm.

Biology and medicine

Cells and organisms

Human cells are 10 to 30 µm across, 0.01 to 0.03 mm. An egg cell is 120 µm. A pollen grain is 20 to 100 µm. A mite is 300 µm, 0.3 mm. Microscope slides show scale bars in micrometres, while the stage movement is in millimetres.

Microscopy

A graticule in the eyepiece has 100 divisions that equal 1 mm at 10 times magnification, so each division is 10 µm. At 40 times, 2.5 µm. Calibrating means converting millimetres on a stage micrometer to micrometres per division.

3D printing and microfabrication

Layer height and resolution

A desktop printer prints layers of 0.1 to 0.3 mm, 100 to 300 µm. A resin printer reaches 25 to 50 µm. A positioning accuracy of 0.01 mm is 10 µm. Print settings use millimetres, while resin printers quote micrometres.

Printed electronics

Inkjet-printed lines are 20 to 100 µm wide. A track on a circuit board is 100 to 200 µm, 0.1 to 0.2 mm. Fine-pitch components have 0.4 mm pitch, 400 µm.

Spreadsheet formulas

Formula

To convert mm to µm for a column, put millimetres in column A and enter =A2*1000 in column B. Format without decimals unless the source has more than three. To reverse, divide by 1000.

Check values

Enter 1: 1,000. Enter 0.05: 50. Enter 0.0025: 2.5. If the result is 100 for 0.05, the sheet used the wrong factor.

Rounding and significant figures

Match the precision

A thickness measured as 0.12 mm has two significant figures; in micrometres, 120 µm, which is still two. Writing 120.0 µm suggests four. Keep 1.2e2 if you want to be explicit.

Instrument limits

A digital caliper reads 0.01 mm, 10 µm. A micrometer screw gauge reads 0.001 mm, 1 µm. Do not report more digits than the instrument gives.

Teaching

Sheets of paper

To show the “convert mm to µm” calculation step, measure the thickness of a stack of 100 sheets of paper with a ruler, say 10 mm. One sheet is 0.1 mm, 100 µm. Students then compare with a hair of 70 µm and a sheet of cling film of 10 µm.

The ladder

Draw the ladder metre, millimetre, micrometre, nanometre. Each step is a thousand. Moving from millimetre to micrometre is one step down: multiply by a thousand.

History of the micron

The old name

The micrometre was called the micron from 1879 until 1967, when the name was officially dropped in favour of micrometre to avoid confusion with the instrument of the same name. In industry, micron survives; the symbol µ alone was also used.

The micrometer gauge

A micrometer is also the name of a measuring screw, invented in the 17th century for telescopes. A modern micrometer reads to 1 µm. The confusing name is why the unit is spelled micrometre in English outside the US.

More values for convert mm to µm

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 mm = 100 µm
• 0.5 mm = 500 µm
• 3 mm = 3,000 µm
• 4 mm = 4,000 µm
• 7 mm = 7,000 µm
• 15 mm = 15,000 µm
• 20 mm = 20,000 µm
• 75 mm = 75,000 µm
• 200 mm = 200,000 µm
• 500 mm = 500,000 µm
• 2,000 mm = 2,000,000 µm
• 10,000 mm = 10,000,000 µm

Mistakes when you convert mm to µm

Moving the point two places

That gives hundredths, not micrometres. For micrometres, three places.

Moving the wrong way

Micrometres are smaller, so the number grows. If 0.05 mm becomes 0.00005, you moved left.

Confusing with nanometres

A nanometre is a thousandth of a micrometre. Check the prefix: micro, not nano.

Quick answers

How many micrometres in a millimetre?

One thousand.

How do I convert mm to µm?

Multiply by 1,000.

What is 0.05 mm in micrometres?

50 µm.

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 mm to cm, Convert mm to dm, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L, Convert K to °C, Convert ft² to acres, Convert acres to ha. The reverse of this page is Convert µm to mm.

In short, to convert mm to µm multiply by a thousand, move the decimal point three places to the right and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.