Kilometer is 1000 meter units.
Kilometers to Femtometers Converter — km to fm
Convert Kilometer (km) to Femtometer (fm) using the exact conversion factor (1 kilometer = 1e+18 femtometer). See the formula, worked examples, conversion table, history, and common uses.
Kilometers to Femtometers converter
Converter
Length Converter
Convert SI, imperial, US customary, astronomical, scientific, and historical length units.
Context values
Some unit families need one extra value for a physically accurate result.
Femtometer is 1e-15 meter units.
Result = input x 1000 / 1e-15.
Length uses the meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Kilometers to Femtometers
Kilometer and Femtometer both measure length. Kilometer is most often used for road signage, speed limits, and vehicle odometers outside the us.
Formula
femtometers = kilometers × 1e+18
This factor comes from each unit's defined relationship to the category's base unit: 1 kilometer equals 1000 base units, and 1 femtometer equals 1e-15 base units, so dividing one by the other gives the direct kilometer-to-femtometer factor of 1e+18.
Simple example
1 km × 1e+18 = 1e+18 fm
1 kilometer = 1e+18 femtometers.
Real-world example
10 km × 1e+18 = 1e+19 fm
10 kilometers = 1e+19 femtometers.
Conversion table
| Kilometer (km) | Femtometer (fm) |
|---|---|
| 1 km | 1e+18 fm |
| 5 km | 5e+18 fm |
| 10 km | 1e+19 fm |
| 21.0975 km | 2.10975e+19 fm |
| 42.195 km | 4.2195e+19 fm |
| 100 km | 1e+20 fm |
Reverse conversion: Femtometer to Kilometer
1e+18 fm × 1e-18 = 1 km
1e+18 femtometers = 1 kilometers.
kilometers = femtometers × 1e-18
For a page dedicated to this direction, see Femtometer to Kilometer.
Understanding the Kilometer
Kilometer (km) measures distance. A kilometer is 1,000 meters — the meter is the SI base unit of length. It is classified as a SI derived (metric prefix) unit.
Where it's used: Standard road, distance, and mapping unit in nearly every country except the United States
Uses of the Kilometer today
- Road signage, speed limits, and vehicle odometers outside the US
- Geographic and mapping distances
- Endurance sports (marathons are measured in kilometers internationally, though the marathon distance itself is a fixed value in both systems)
History of the Kilometer
The meter was defined during the French Revolutionary metric reforms as one ten-millionth of the distance from the North Pole to the Equator along a meridian through Paris. 'Kilo-' is simply the SI prefix for 1,000, making the kilometer a direct decimal multiple.
The meter itself has been redefined several times as measurement precision improved — from a physical platinum-iridium bar (1889) to a krypton-86 wavelength (1960) to its current definition (1983) as the distance light travels in a fixed fraction of a second.
Historical origin. Kilometer was not the invention of a single named person; it developed through the process described above.
Understanding the Femtometer (fm)
Femtometer is 1e-15 meter units.
Kilometer — sources
- BIPM — The metre (SI base unit) — Current definition and history of the meter.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many femtometers are in 1 kilometer?
1 kilometer equals 1e+18 femtometers, using the exact defined conversion factor rather than an estimate.
How do I convert kilometer to femtometer?
Multiply the kilometer value by 1e+18. The converter above does this instantly to whatever precision you set.
How do I convert femtometer back to kilometer?
Use the reverse factor: 1 femtometer equals 1e-18 kilometers. You can also use the swap control in the converter above to flip the direction instantly.
What is a kilometer?
Kilometer (km) measures distance. A kilometer is 1,000 meters — the meter is the SI base unit of length. It is classified as a SI derived (metric prefix) unit.
What is a femtometer?
Femtometer (fm) is a unit of length.
Is the kilometer to femtometer conversion exact?
Yes. Both kilometer and femtometer are defined by fixed standards rather than physical artifacts, so the factor of 1e+18 used above is exact to as many digits as you choose to display.
Where is the kilometer still used today?
Standard road, distance, and mapping unit in nearly every country except the United States