Convert mm to dm millimetres ↔ decimetres Free Online Secure
Type a value in millimetres and read it in decimetres 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 mm to dm (millimetres ↔ decimetres)
| mm | dm |
|---|---|
| 1 | 0.01 |
| 2 | 0.02 |
| 3 | 0.03 |
| 5 | 0.05 |
| 10 | 0.1 |
| 20 | 0.2 |
| 50 | 0.5 |
| 100 | 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
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.
Does anyone still use decimetres?
Rarely on its own: a decimetre is ten centimetres, and most people jump straight from centimetres to metres. It survives in school lessons and in one important place, the litre, which is exactly one cubic decimetre. That is why a litre of liquid fits in a cube ten centimetres on each side.
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.
A hundred small marks make a hand span: how to convert mm to dm
There are exactly one hundred millimetres in a decimetre, so a length of 100 mm is the width of a hand and 1,000 mm is ten hand spans. The conversion between the two is rare in everyday life, because people jump straight from millimetres to centimetres or metres, but it appears in school exercises, in Dutch and Flemish plans, and in anything that links length to volume, since the litre is a cubic decimetre. When you read a workshop drawing in millimetres and a plan in decimetres, you have to convert mm to dm, and this page does it as you type.
The rule is a division by one hundred, which moves the decimal point two places to the left. The sections below give the rule with examples, short portraits of both units, a comparison, the situations in which the conversion appears and the mistakes to avoid. A special section covers the link between millimetres, decimetres and litres, which is the most useful application.
How to convert mm to dm: divide by 100
The rule
One decimetre is 100 millimetres. To convert mm to dm, divide the number of millimetres by 100 or move the decimal point two places to the left. 100 mm is 1 dm, 250 mm is 2.5 dm, 45 mm is 0.45 dm, 1,200 mm is 12 dm and 7 mm is 0.07 dm. In the other direction, multiply by 100.
Examples
A plank 1,800 mm long is 18 dm. A pipe of 320 mm is 3.2 dm. A phone 150 mm tall is 1.5 dm. A sheet of paper 297 mm long is 2.97 dm, and 210 mm wide is 2.1 dm. A screw of 60 mm is 0.6 dm.
The mental method
For a mental convert mm to dm, think in two steps: first divide by ten to get centimetres, then by ten again to get decimetres. 450 mm becomes 45 cm and then 4.5 dm. Each step is easy, and the two together are a division by one hundred. Or count the hundreds: 450 mm is four hundreds and a half, so 4.5 dm.
Leading zeros
For lengths below a hundred millimetres, the result starts with 0 and a decimal: 30 mm is 0.3 dm and 8 mm is 0.08 dm. Do not drop the zero: 0.08 is not 0.8.
The millimetre
A thousandth of a metre
The millimetre is a thousandth of a metre, the unit for small parts, machine drawings and rainfall. A pencil lead is half a millimetre, a coin two. In workshops everything is in millimetres, because dimensions come out as whole numbers.
The ratio to larger units
Ten millimetres make a centimetre, a hundred a decimetre, a thousand a metre. Moving along that ladder is the basis of the metric system.
The decimetre
A tenth of a metre
The decimetre is a tenth of a metre, ten centimetres, a hundred millimetres. The prefix deci is from Latin decimus, tenth. A cube with an edge of one decimetre holds exactly one litre of water, which then weighs one kilogram.
A unit for learning
The decimetre is useful in education because it sits in the middle of the ladder and links length, area and volume neatly. It is also in use in craft trades in the Netherlands and Belgium.
Millimetre and decimetre compared
A decimetre is a hundred millimetres. A number in millimetres is a hundred times larger than the same length in decimetres. Use millimetres for small parts, decimetres for the size of a hand or for lengths that feed into litres.
If you give a length in both units, write each with its symbol and avoid calculations that mix them. For example, 250 mm (2.5 dm) is a clear way to report a length.
Where you will need it
Litres from millimetres
A box measures 300 by 200 by 150 mm. In decimetres, 3 by 2 by 1.5, which gives 9 cubic decimetres, 9 litres. This is the most useful reason to convert mm to dm: the volume in litres comes straight out of the decimetre sides. Doing the same in millimetres gives 9,000,000 cubic millimetres, which is harder to read.
Paper sizes
An A4 sheet is 210 by 297 mm, 2.1 by 2.97 dm. An A3 sheet is 297 by 420 mm, 2.97 by 4.2 dm. An A0 sheet of 841 by 1,189 mm is 8.41 by 11.89 dm, close to a square metre. The decimetre figures show how close the series is to a metre.
Drawing and workshop
A board of 1,200 mm by 400 mm is 12 by 4 dm. A shelf of 800 mm is 8 dm. Teachers ask students to read a technical drawing in millimetres and rewrite it in decimetres to practise the shift of two places.
Packaging
Shipping boxes are measured in millimetres but their capacity is described in litres. A box of 600 by 400 by 400 mm is 6 by 4 by 4 dm, 96 litres. The conversion decides how many boxes fit on a pallet and how much each holds.
Cubes, litres and kilograms
The link to water
A cube of 100 mm per side has a volume of 1,000,000 cubic millimetres, 1,000 cubic centimetres, 1 cubic decimetre, one litre. Filled with water it weighs one kilogram. So a tank of 500 by 300 by 200 mm, 5 by 3 by 2 dm, holds 30 litres and 30 kilograms of water.
Why it works
The metric system was designed so that length, volume and mass are linked. When you convert mm to dm for the sides of a box, the product is the volume in litres without another conversion.
Dutch and Flemish practice
A unit in daily speech
In the Netherlands and Belgium, craftspeople and teachers still use decimeters. A Dutch plan may give a wall as 42 dm, which is 4,200 mm. Knitting patterns speak of stitches per decimeter. Visitors reading such documents need the conversion.
Reading a plan
If a plan has no unit, look at the other numbers. A door of 8 is probably 8 dm, 800 mm; a ceiling of 25 is 25 dm, 2,500 mm. The context settles which unit is meant.
Science labs
Concentrations and volumes
A lab flask of 100 mm height is 1 dm. A beaker of 50 mm diameter has an area of 19.6 square centimetres, 0.196 dm squared. A solution of 0.1 moles per dm3 is 0.1 moles per litre. Textbooks use dm when they want to link length to volume.
Graph scales
Graph paper is ruled in millimetres, with heavier lines every 10 mm and the heaviest every 100 mm, one decimetre. A student plotting data uses the heavy lines as a scale.
A worked example
The volume of a fish tank
A tank measures 900 by 450 by 450 mm. To convert mm to dm for each side: 9 by 4.5 by 4.5 dm. The volume is 182.25 litres. Subtract 10 per cent for substrate and decorations and you get 164 litres of water, which weighs 164 kg. The stand must carry about 190 kg including glass.
Checking
In millimetres, the volume is 182,250,000 cubic millimetres. Divide by 1,000,000 to get litres: 182.25. Both routes agree, but the decimetre route is easier.
Final checks
Round trip
Multiply your answer in decimetres by 100. If you return to the millimetres you started with, the conversion is right. For volumes, multiply the litres by 1,000,000 to return to cubic millimetres.
Notes
Write both the millimetre and decimetre figures in your notes. A later reader can then follow the steps.
Pallets, crates and shipping
Counting boxes
A euro pallet is 1,200 by 800 mm, 12 by 8 dm. A box of 400 by 300 by 200 mm, 4 by 3 by 2 dm, fills 24 litres. Eight boxes fit on one layer, 12 by 8 dm divided by 4 by 3 dm, so four layers hold 32 boxes and 768 litres. A shipper who reads the box in millimetres and the pallet in decimetres must convert mm to dm for one of them before counting.
Limits
Pallet height is limited to about 1,800 mm, 18 dm, in many warehouses. Four layers of 200 mm boxes make 800 mm, 8 dm, leaving a margin. A crate of 500 mm height may be stacked at most three high.
Fuel tanks, bottles and jugs
Capacity from dimensions
A rectangular fuel tank of 600 by 400 by 300 mm is 6 by 4 by 3 dm, 72 litres. A cylindrical bottle of 80 mm diameter and 300 mm height is a cylinder of 0.8 by 3 dm, 1.5 litres. A jug of 150 mm diameter and 250 mm height holds 4.4 litres. Makers print the capacity on the label; the decimetre route lets you check it.
Why check
A label may overstate the capacity by counting to the brim. A real fill level is 5 to 10 per cent lower, so a jug that measures 4.4 litres may hold 4 litres.
Area: square decimetres
Surfaces in dm squared
A tile of 300 by 300 mm is 3 by 3 dm, 9 square decimetres. A panel of 600 by 800 mm is 6 by 8 dm, 48 square decimetres, 0.48 square metres. Remember that the area factor is 10,000: 1 dm squared is 10,000 mm squared.
Common error
People convert an area of 48,000 mm squared to decimetres squared by dividing by 100, getting 480. The correct factor is 10,000: 4.8 dm squared. Convert the sides first and multiply afterwards.
3D printing volumes
From millimetres to litres
A print of 120 by 80 by 60 mm is 1.2 by 0.8 by 0.6 dm, 0.576 litres, a bounding volume. A printed object fills a fraction of this, perhaps 20 per cent, so the plastic is about 0.115 litres. At a density of 1.25 g per cubic centimetre, 144 g of filament.
Print time
Print time scales with volume. A bigger box of 200 by 200 by 200 mm is 8 litres, nearly fourteen times the volume of the first, and takes much longer to print. The decimetre figures show the proportion at a glance.
Kitchen and household
Containers
A food container of 200 by 150 by 60 mm is 2 by 1.5 by 0.6 dm, 1.8 litres. A casserole dish of 300 by 200 by 70 mm holds 4.2 litres. A bucket of 250 mm diameter and 300 mm height is about 14.7 litres. Cooks think in litres, makers in millimetres.
Cupboards
A cupboard 600 mm wide, 500 deep and 800 high is 6 by 5 by 8 dm, 240 litres of space. A set of ten like it holds 2,400 litres, 2.4 cubic metres.
Gardens and soil
Planters
A planter 800 mm long, 250 wide and 200 deep is 8 by 2.5 by 2 dm, 40 litres of soil. A bag of compost holds 50 litres. A larger trough of 1,200 by 400 by 300 mm is 144 litres, three bags.
Pots
Pots are described by diameter in millimetres: 200 mm, 2 dm, holds about 4 litres. A 300 mm pot holds 12 litres. The relation is not exactly cubic, but the trend is.
Spreadsheet formulas
Formula
To convert mm to dm for a column, put millimetres in column A and enter =A2/100 in column B. Format with one or two decimals. For volumes, enter =(A2/100)*(B2/100)*(C2/100) to get litres from three sides in millimetres. For areas, divide each side by 100 first.
Check values
Enter 100: result 1. Enter 45: 0.45. Enter 1000: 10. If you get 0.1 for 1000, you divided by 10,000.
Rounding
Match the precision
A length of 253 mm measured to the nearest millimetre is 2.53 dm. Writing 2.530 dm would claim a tenth of a millimetre. A length known only to the nearest centimetre, 250 mm, is 2.5 dm.
Volumes
When you multiply three rounded sides, the error grows. Sides of 3.0, 2.0 and 1.5 dm each rounded from measurements to 0.05 dm give a volume uncertain by about 0.5 litre, 6 per cent. Keep the original millimetre figures for the final calculation.
Teaching the metric ladder
The cube demonstration
To show the “convert mm to dm” calculation step, build a cube of 100 mm per side from card. Fill it with rice and pour the rice into a litre jug: it fills the litre. Students see that 100 mm by 100 mm by 100 mm is exactly one litre. The demonstration links the conversion of length to volume.
Counting
Ask students to measure the cube in millimetres, centimetres and decimetres and write 100, 10 and 1. Then ask for the volume in cubic millimetres, cubic centimetres and cubic decimetres: 1,000,000, 1,000 and 1.
History of the litre
From the decimetre to the litre
In 1795 the French defined the litre as one cubic decimetre. After a revision in 1901 it was for a time defined as the volume of one kilogram of water at its greatest density, equal to 1.000028 cubic decimetres. Since 1964 the litre is again exactly one cubic decimetre.
The decimetre today
The unit decimetre is hardly ever used alone, but it lives on in dm3 and dm2, and in the litre. For chemists, moles per dm3 is a standard unit of concentration.
More values for convert mm to dm
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 = 0.001 dm
- 0.5 mm = 0.005 dm
- 3 mm = 0.03 dm
- 4 mm = 0.04 dm
- 7 mm = 0.07 dm
- 15 mm = 0.15 dm
- 20 mm = 0.2 dm
- 75 mm = 0.75 dm
- 200 mm = 2 dm
- 500 mm = 5 dm
- 2,000 mm = 20 dm
- 10,000 mm = 100 dm
Mistakes when you convert mm to dm
Moving one place
That gives centimetres. For decimetres, two places.
Moving three places
That gives metres. Check the unit before you move the point.
Wrong factor for areas
Areas use 10,000 and volumes 1,000,000 between millimetres and decimetres. Convert the sides first.
Quick answers
How many millimetres in a decimetre?
One hundred.
How do I convert mm to dm?
Divide by 100.
What is 450 mm in decimetres?
4.5 dm.
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 m, Convert mm to cm, Convert in to dm, Convert m to dm, Convert dm to ft, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L. The reverse of this page is Convert dm to mm.
In short, to convert mm to dm divide by a hundred, move the decimal point two 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.