Convert ft to dm feet ↔ decimetres Free Online Secure
Type a value in feet 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 ft to dm (feet ↔ decimetres)
| ft | dm |
|---|---|
| 1 | 3.048 |
| 2 | 6.096 |
| 3 | 9.144 |
| 5 | 15.24 |
| 10 | 30.48 |
| 20 | 60.96 |
| 50 | 152.4 |
| 100 | 304.8 |
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 are aircraft altitudes given in feet?
Because international aviation standardised on feet decades ago and has never had a reason to change. Flight levels are altitudes in hundreds of feet, so level 350 is 35,000 feet. A foot is exactly 0.3048 metres, so converting is exact; a few countries, such as China and Russia, have used metres instead, which is why pilots must know both.
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.
Eight-foot sheets, metric saws: when you need to convert ft to dm
Plywood in the United States comes in sheets four feet by eight. A cabinetmaker in Rotterdam, reading the same project in a shared online forum, thinks in decimetres and wants to know how big that really is. The answer is 12.192 dm by 24.384 dm, and getting from one number to the other is exactly what it means to convert ft to dm. This page performs the calculation as you type, so you can move between an American tape measure and a metric drawing without a calculator of your own.
The foot is a unit with a long and slightly untidy history, while the decimetre is a clean tenth of a metre. The link between them is a single exact figure, 3.048, and that makes the conversion simple in principle but awkward in the head. Below you will find the rule, worked examples, a short profile of each unit, a comparison, a tour of the situations where the conversion appears, and the usual traps.
How to convert ft to dm: multiply by 3.048
The exact factor
Since 1959 the international foot has been exactly 0.3048 metres. A decimetre is 0.1 metres, so one foot is 3.048 decimetres, a figure with no rounding. To convert ft to dm, multiply the number of feet by 3.048. Two feet are 6.096 dm, five feet are 15.24 dm, ten feet are 30.48 dm and a hundred feet are 304.8 dm. Going the other way, divide by 3.048.
Examples to copy
A standard door, 7 ft tall, is 21.336 dm. A basketball rim at 10 ft is 30.48 dm above the floor. A room 12 ft wide measures 36.576 dm. A fence panel of 6 ft is 18.288 dm. If you round to one decimal, which suits most carpentry, those become 21.3, 30.5, 36.6 and 18.3 dm.
The quick mental estimate
Multiply the feet by three, then add one and a half per cent. For 10 ft: 30 plus 0.45 gives 30.45, within a hair of the exact 30.48. The trick works because three decimetres are almost exactly one foot, and the real value is just 1.6 per cent more.
Feet and inches
If your measurement is in feet and inches, turn the inches into a fraction of a foot first: 5 ft 9 in is 5 plus 9 divided by 12, which is 5.75 ft, or 17.526 dm. Only then multiply by 3.048.
The foot
From body to standard
Feet were once literal. Different towns and trades had their own foot, from about 25 to more than 35 centimetres. The English foot we know today was settled gradually, and in 1959 the English-speaking nations agreed on the exact value of 30.48 centimetres, which is why one foot is exactly three point zero four eight decimetres.
Daily use
Feet survive in the United States for building, flooring, human height and for room sizes. In aviation they are the world standard for altitude: a cruising airliner at 36,000 ft is flying at 109,728 dm, a number nobody quotes, which is a good reason to use a calculator for the other direction.
The decimetre
A tenth of a metre
The decimetre is exactly one tenth of a metre, close to the width of a hand. It is part of the original French metric scheme and is the origin of the litre: a cube of one decimetre per side holds a litre of water. In the Netherlands and Belgium, children learn it at school and craftspeople still say it aloud.
Where you find it
Outside the classroom the decimetre appears in square and cubic form, dm squared and dm cubed, in floor-area tables for very small rooms, and in plans that use a half-metre module broken into five decimetres.
Foot and decimetre side by side
One foot is a little over three decimetres, so a length in decimetres is about three times larger than the same length in feet. A 20 ft container is 60.96 dm long; a 40 ft container is 121.92 dm. Because the factor is close to three, estimating in either direction is easy once you remember the one and a half per cent correction.
The decimetre is a metric unit built for tens, the foot an imperial unit built for twelves. This is why a foot divides naturally into halves, thirds and quarters of inches, whereas a decimetre divides into tens of centimetres. Pick the unit your audience uses, and state the other in brackets if you work with international readers.
Where you will need it
Importing furniture plans
A woodworking blog in Texas lists a table at 6 ft by 3 ft by 2.5 ft. In decimetres that is 18.288 by 9.144 by 7.62, which a Dutch or Belgian reader can compare with the width of the room in a flat. Doing the “convert ft to dm” calculation step for the whole set before you shop prevents ordering a table that will not pass the door.
Sports courts and fields
A basketball rim at 10 ft is 30.48 dm, a volleyball net at 7 ft 11 5/8 in for men is 24.3 dm, a tennis net at 3 ft is 9.144 dm at the posts. These figures are the same everywhere, but the way they are written depends on the country, so a coach moving between countries benefits from converting once and pinning the results.
Shipping containers and vehicles
Containers are described as 20 ft and 40 ft worldwide, with an inside width of about 7.7 ft or 23.5 dm. A pallet stack of 4 ft is 12.192 dm. When you plan how many pallets fit, the decimetre figures make the arithmetic with metric dimensions straightforward.
Photography and film
Film and video lenses are often marked with a focus scale in feet. A subject at 6 ft is 18.288 dm away, about an arm and a half. Photographers who also read European manuals convert once and keep a card in the camera bag.
The height of people
Feet and inches for the body
In English-speaking countries height is given as feet and inches: 5 ft 6 in is 5.5 ft, 16.764 dm; 6 ft is 18.288 dm; 6 ft 3 in is 6.25 ft, 19.05 dm. To convert ft to dm for a person height, work in two steps, inches to feet and feet to decimetres, and you will not make the common slip of treating the inches as hundredths.
Children and growth charts
Paediatric charts in the United States show length in inches and feet, while European charts show centimetres. A baby 1.7 ft long is 5.18 dm, or 51.8 cm, close to the average at birth.
A little history of the foot
Roman and Saxon feet
The Roman foot was about 29.6 cm, 2.96 dm, divided into twelve unciae, which is where the inch comes from. The Saxons used a longer foot. By the Middle Ages, every region had its variant, and merchants needed tables to convert.
The 1959 agreement
When engineering and science became international, the differences became too costly. In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed that the yard would be exactly 0.9144 metres, which fixed the foot at 0.3048 metres. Ever since, the answer to every “convert ft to dm” calculation question is exact.
Spreadsheet recipe
Formula
In a spreadsheet, if feet are in column A, the formula in column B is =A2*3.048. Format the cell with one or two decimals. For feet and inches, use =(A2+B2/12)*3.048 with inches in column B.
Documenting your conversion
Put the factor in a labelled cell so that colleagues can check it. That is much clearer than hiding 3.048 inside a formula.
Teaching the conversion
A classroom activity
Ask students to measure their own height in decimetres and in feet. A tape in both units reveals the ratio directly. Plot ten heights on a graph with feet on one axis and decimetres on the other and the points fall on a straight line with slope 3.048.
Why the exercise is useful
Students see that a conversion factor is the slope of a line and that unit changes do not change the object, only its description.
Ladders, ceilings and stairs
Safe working heights
Ladders are labelled by their length in feet. A 6 ft stepladder is 18.288 dm, and because you should never stand on the top two steps, the safe working height is lower than the label suggests. A 24 ft extension ladder is 73.152 dm, and it should overlap by 3 ft, or 9.144 dm, when extended.
Ceiling height
American homes have ceilings of 8 ft, which is 24.384 dm, and newer ones 9 ft, 27.432 dm. European ceilings of 2.5 m are 25 dm, close to the older American standard. Converting the figure shows that the two are nearly the same.
Pools, ponds and aquariums
Depth markings
US pools mark depth in feet: 3 ft in the shallow end, 12 ft for diving, 9.144 dm and 36.576 dm respectively. If you find a pool plan with those figures and want to price a liner in metric units, the “convert ft to dm” calculation step is the first thing to do.
Tanks
An aquarium listed as 4 ft long is 12.192 dm. Because 1 dm cubed holds a litre, a tank 12.192 dm by 4.5 dm by 5 dm (4 ft by 1.5 ft by 1.64 ft) holds around 274 litres before the substrate and decorations are taken out.
Gardens and fences
Fence panels
Panels are sold 6 ft high and 8 ft wide, which is 18.288 by 24.384 dm. Posts are set every 8 ft, or 24.384 dm. Over a boundary of 100 ft, 304.8 dm, you will need 12.5 panels, so thirteen with a cut one. The same maths works in any unit once you convert.
Raised beds
Raised beds are often built 4 ft wide, so you can reach the centre from both sides. That is 12.192 dm, about right for an arm length of 6 dm each way. Lengths of 8 ft, or 24.384 dm, use standard boards with no waste.
Cables, hoses and ropes
Standard lengths
Extension cords come in 25 ft, 50 ft and 100 ft: 76.2, 152.4 and 304.8 dm. Garden hoses in 50 ft are 152.4 dm. If a European label asks for a hose at least 15 dm, any of those pass the test, but you need to convert ft to dm to be sure.
Climbing and sailing
Climbing ropes are 60 m, or 600 dm, which is 196.85 ft. A sailor's dock line of 25 ft is 76.2 dm. Rope lengths are rounded generously, so a conversion error of a few per cent rarely matters, but a mistake of a factor of ten always does.
Aviation and the strange case of altitude
Why pilots use feet
Altitude in aviation is given in feet almost everywhere, except in a few countries that use metres. A cruising level of 35,000 ft is 106,680 dm, or 10,668 m. Because the international standard is feet, air traffic controllers in metric countries speak feet on the radio.
Why you would convert
You rarely convert altitude to decimetres, but the exercise shows how large the numbers get when a long distance is expressed in a small unit. Use the decimetre for small and medium lengths and leave kilometres for the sky.
Reading tape measures
Feet, inches and fractions
An American tape marks feet every 12 inches, with fractions down to 1/16 inch. To translate a reading such as 3 ft 4 1/2 in, convert to feet first (3 plus 4.5/12, equals 3.375 ft) and then multiply by 3.048 to get 10.287 dm.
Checking your tape
Most tapes are dual marked with centimetres. If yours is, compare the two scales and the conversion agrees: 1 ft lines up with 30.5 cm, or 3.05 dm. A dual tape is the best way to build a feel for the equivalence.
Common mistakes with decimals
Feet are not decimal
5.6 ft is five and six tenths of a foot, which is 5 ft 7.2 in, not 5 ft 6 in. People often type feet and inches as a decimal, such as 5.6, meaning 5 ft 6 in, and the conversion then comes out wrong. If your source writes 5.6 and means 5 ft 6 in, convert it first to 5.5 ft.
A foot is not 3 dm
Using 3 as the factor for a “convert ft to dm” calculation is a decent estimate but produces a 1.6 per cent error. Over 30 ft that is 1.4 dm, enough to leave a gap in a fitted shelf.
Rounding for the job
How many decimals to keep
A decimetre is about 10 cm, so a single decimal is worth 1 cm and two decimals 1 mm. For rough planning, keep one decimal. For fitting work, keep two. For machining, convert to millimetres instead.
Rounding up for materials
If your “convert ft to dm” calculation result is 24.384 dm of board, buy 25 dm or the next stock length. The saw kerf, usually 3 mm, eats about 0.03 dm for every cut.
More values for convert ft 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 ft = 0.3048 dm
- 0.5 ft = 1.524 dm
- 3 ft = 9.144 dm
- 4 ft = 12.192 dm
- 7 ft = 21.336 dm
- 15 ft = 45.72 dm
- 20 ft = 60.96 dm
- 75 ft = 228.6 dm
- 200 ft = 609.6 dm
- 500 ft = 1,524 dm
- 2,000 ft = 6,096 dm
- 10,000 ft = 30,480 dm
Mistakes when you convert ft to dm
Dividing instead of multiplying
Feet are the larger unit, so the number of decimetres is larger than the number of feet. If 10 ft gives 3.28, you divided instead of multiplying.
Mixing feet and inches in one number
Convert the inches first. 5 ft 9 in is 5.75 ft, not 5.9 ft.
Forgetting the unit
Write dm or ft after each number. A bare 30.48 could be centimetres, decimetres or feet, depending on who wrote it.
Quick answers
How many decimetres in a foot?
Exactly 3.048.
How do I convert ft to dm?
Multiply the feet by 3.048.
What is 10 ft in dm?
30.48 dm.
Is the conversion exact?
Yes, since 1959 the foot is exactly 0.3048 metres.
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 ft to in, Convert ft to yd, Convert in to dm, Convert m to dm, Convert dm to in, 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 ft.
In short, to convert ft to dm multiply by 3.048, sanity-check by tripling and adding a little, and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.