What Is a Decimeter?
A decimeter (symbol dm) is a metric unit of length equal to one tenth of a meter, exactly 0.1 m. It equals 10 centimeters or 100 millimeters, and it is just under 4 inches: one decimeter is 3.937 in. In the hand it is roughly the width of an adult’s palm.
The decimeter is one of the original units of the 1795 French metric system, which was designed so that lengths, areas and volumes would connect by powers of ten. It is the unit that ties length to capacity: a cube one decimeter on each side holds exactly one liter. The General Conference on Weights and Measures confirmed that equality in 1964, when it redefined the liter as exactly one cubic decimeter.
The decimeter is legal and correct SI, but uncommon in daily life. It is taught in schools in parts of Europe and Asia, used occasionally in technical writing, and appears in radio, where UHF frequencies between 300 MHz and 3 GHz are called decimetric waves because their wavelengths range from 10 cm to 1 m.
The Netherlands gave the decimeter a more familiar name. When the Dutch adopted the metric system in 1820, they kept traditional words for the new units: the meter was the el, the decimeter the palm, the centimeter the duim (thumb) and the millimeter the streep. The kilometer became the mijl, the Dutch word for mile, so for several decades a Dutch “mile” was exactly 1 km, or 0.621371 statute miles. The old names were dropped in 1870.
What Is a Mile?
The mile measures 16,093.44 decimeters. Unlike the decimeter, it was not designed around tens. It descends from the Roman mille passus, a thousand paces, and took its English length of 5,280 feet from a 1593 statute that defined it as 8 furlongs.
That non-decimal history is why converting between the two needs a factor. Inside the metric system every step is a power of ten, so decimeters to kilometers is simply a division by 10,000; the mile breaks that pattern with its 1,760 yards and 5,280 feet. Since the 1959 International Yard and Pound Agreement, the mile has been exactly 1,609.344 m, so the factor is exact even if it is not a round number.
Metric Prefixes Explained: Deci-
Deci- comes from the Latin decimus, “tenth”, and means one tenth (10⁻¹). Together with centi-, deca- and hecto-, it is one of the four SI prefixes that are not a power of 1,000, which is why engineers rarely use it. The NIST guide to SI prefixes lists it alongside the familiar milli- and kilo-.
Outside length, deci- is more common than you might expect. The decibel (dB), used for sound levels, is a tenth of a bel, a unit named after Alexander Graham Bell. In medicine, the deciliter (dL) is the standard volume for many US lab results, such as blood glucose in milligrams per deciliter. And the liter itself is defined as a cubic decimeter, so every bottle of water is a decimeter cube in disguise.
For length, deci- sits between centi- and the meter:
- 1 centimeter = 0.1 dm = 0.00000621371 mi
- 1 decimeter = 0.0000621371 mi
- 1 meter = 10 dm = 0.000621371 mi
Decimeters suit objects that are too big for centimeters to feel natural but too small for meters. The table shows some of them, and how small they still are when converted to miles.
| Length | Decimeters (dm) | Miles (mi) |
|---|---|---|
| Side of a 1-liter cube | 1 | 0.0000621371 |
| Smartphone (approx.) | 1.5 | 0.0000932057 |
| A4 paper (long side) | 2.97 | 0.000184547 |
| One foot | 3.048 | 0.000189394 |
| One meter | 10 | 0.000621371 |
| Olympic swimming pool | 500 | 0.0310686 |
| One kilometer | 10,000 | 0.621371 |
The decimeter is also useful for checking volume conversions in your head. A box 3 dm × 2 dm × 2 dm holds 12 dm³, which is 12 liters, with no conversion factor needed. The same box measured as 30 × 20 × 20 cm gives 12,000 cm³, the same volume with three more zeros.
How Many Decimeters in a Mile?
There are exactly 16,093.44 decimeters in a mile, so to convert miles to decimeters you multiply by that number. A tenth of a mile is 1,609.344 dm, and a 5K race of 50,000 dm is 3.11 miles.