Convert Femtometer to Kilometer
Convert femtometers to kilometers instantly. 1 femtometer = 1e-18 kilometer — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Kilometer to Femtometer converter for the reverse conversion.
Units explained
Femtometer
A femtometer is a metric unit of length equal to one quadrillionth of a meter (1×10⁻¹⁵ m). It is the standard unit for measuring sub-atomic dimensions, particularly the size of atomic nuclei. The femtometer is also known historically as the fermi.
The femtometer was defined using the SI prefix femto- (from Danish femten, meaning fifteen), adopted by the CGPM in 1964 to denote one quadrillionth. The synonymous name fermi was named after Italian physicist Enrico Fermi.
Femtometers are used to express the size of atomic nuclei (typically 1–10 fm in diameter), the range of strong nuclear forces, and characteristic dimensions in particle physics. The proton charge radius is approximately 0.84 fm.
SI prefix femto- adopted in 1964; the unit became standard in nuclear physics through the late 20th century. The name fermi (after Enrico Fermi) was used informally from the 1950s but officially superseded by femtometer.
Kilometer
A kilometer is a metric unit of length equal to one thousand meters. It is the standard unit for measuring road distances, geographic distances, and other large-scale measurements in metric countries.
The kilometer was defined alongside the meter in 1795 using the standard SI prefix kilo- (from Greek chilioi, "thousand"), denoting one thousand units.
Kilometers are used worldwide (except the United States and a few others) for road signage, geographic distance, athletic events, and scientific distances at planetary scale. Speed limits in most countries are given in km/h.
Adopted 1795 in France as part of the original metric system; the kilometer became the global standard for road and geographic distance through the 19th and 20th century metric adoption.
Femtometer to Kilometer conversion formula
The relationship between femtometers and kilometers:
To convert femtometers to kilometers, multiply the value in femtometers by 1e-18. To reverse, multiply kilometers by 1e+18.
How to use this converter
Type a value into the calculator. The result in kilometers updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Kilometer to Femtometer converter for the reverse direction.
Step-by-step: convert femtometers to kilometers
- Write down the value in femtometers (fm).
- Multiply that value by the factor 1e-18.
- The product is the equivalent value in kilometers (km).
- To reverse, multiply the kilometer value by 1e+18.
Worked examples
Example 1 — Convert 1 fm to km:
1 × 1e-18 = 1e-18 km
Example 2 — Convert 100 fm to km:
100 × 1e-18 = 1e-16 km
Real-world example — Wavelength to road distance
A trillion femtometers equals one kilometer — the kind of conversion that appears in physics problems spanning the electromagnetic spectrum across many orders of magnitude.
1e+12 fm × 1e-18 = 1e-6 km
Real-world example — Twelve orders of magnitude
A trillion femtometers maps to a single, recognizable distance in kilometers. This kind of conversion arises in cosmology and electromagnetic-spectrum exercises where atomic and astronomical scales sit side by side.
1e+12 fm × 1e-18 = 1e-6 km
Femtometer to Kilometer conversion table
Standard reference values for converting femtometers to kilometers:
| Femtometer [fm] | Kilometer [km] |
|---|---|
| 0.01 | 1e-20 |
| 0.1 | 1e-19 |
| 1 | 1e-18 |
| 2 | 2e-18 |
| 3 | 3e-18 |
| 4 | 4e-18 |
| 5 | 5e-18 |
| 10 | 1e-17 |
| 20 | 2e-17 |
| 30 | 3e-17 |
| 40 | 4e-17 |
| 50 | 5e-17 |
| 100 | 1e-16 |
| 500 | 5e-16 |
| 1000 | 1e-15 |
Frequently asked questions
How many kilometers is 1 femtometer?
How do I convert femtometers to kilometers?
How do I convert kilometers back to femtometers?
How many kilometers is 100 femtometers?
Popular length unit conversions
Convert Femtometer to other length units
Show all Femtometer conversions
Metric / SI (17 units)
Imperial / US Customary (4 units)
Sources & references
Conversion factor (1 fm = 1e-18 km) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Astronomical Union — System of Astronomical Constants
The IAU defines astronomical units including the AU (149597870700 m exactly) light-year and parsec used in astronomy and astrophysics.