Convert Kilometer to Femtometer
Convert kilometers to femtometers instantly. 1 kilometer = 1e+18 femtometer — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Femtometer to Kilometer converter for the reverse conversion.
Units explained
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
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 The name fermi (after Enrico Fermi) was used informally from the 1950s but officially superseded by femtometer.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 to Femtometer conversion formula
The relationship between kilometers and femtometers:
To convert kilometers to femtometers, multiply the value in kilometers by 1e+18. To reverse, multiply femtometers by 1e-18.
How to use this converter
Type a value into the calculator. The result in femtometers updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Femtometer to Kilometer converter for the reverse direction.
Step-by-step: convert kilometers to femtometers
- Write down the value in kilometers (km).
- Multiply that value by the factor 1e+18.
- The product is the equivalent value in femtometers (fm).
- To reverse, multiply the femtometer value by 1e-18.
Worked examples
Example 1 — Convert 1 km to fm:
1 × 1e+18 = 1e+18 fm
Example 2 — Convert 100 km to fm:
100 × 1e+18 = 1e+20 fm
Real-world example — Kilometres to wavelengths
One kilometer equals one trillion femtometers — a conversion physics teachers use to convey the gulf between everyday geographic and atomic scales.
1 km × 1e+18 = 1e+18 fm
Real-world example — Geographic to wavelength scale
One kilometer equals one trillion femtometers — illustrating the 12-order-of-magnitude span between geographic distance and atomic-feature scales.
1 km × 1e+18 = 1e+18 fm
Kilometer to Femtometer conversion table
Standard reference values for converting kilometers to femtometers:
| Kilometer [km] | Femtometer [fm] |
|---|---|
| 0.01 | 1e+16 |
| 0.1 | 1e+17 |
| 1 | 1e+18 |
| 2 | 2e+18 |
| 3 | 3e+18 |
| 4 | 4e+18 |
| 5 | 5e+18 |
| 10 | 1e+19 |
| 20 | 2e+19 |
| 30 | 3e+19 |
| 40 | 4e+19 |
| 50 | 5e+19 |
| 100 | 1e+20 |
| 500 | 5e+20 |
| 1000 | 1e+21 |
Frequently asked questions
How many femtometers is 1 kilometer?
How do I convert kilometers to femtometers?
How do I convert femtometers back to kilometers?
How many femtometers is 100 kilometers?
Popular length unit conversions
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Metric / SI (17 units)
Imperial / US Customary (27 units)
Nautical (1 units)
Astronomical (9 units)
Atomic / Physics (6 units)
Typographic (3 units)
Sources & references
Conversion factor (1 km = 1e+18 fm) 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 Hydrographic Organization — Resolution on the Nautical Mile
International authority that standardised the nautical mile at exactly 1852 m in 1929 — the value adopted worldwide for sea and air navigation.