Convert X-unit to Fermi
Convert x-units to fermis instantly. 1 x-unit = 100.21 fermi — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Fermi to X-unit converter for the reverse conversion.
Units explained
X-unit
The X-unit (or siegbahn) is a historical X-ray crystallography unit of approximately 1.0021×10⁻¹³ meters. It was used before X-ray wavelengths could be measured directly in meters.
Introduced by Manne Siegbahn in 1925 as a self-consistent unit for X-ray crystallography. The 'X-unit' was defined to make the X-ray wavelength of the molybdenum K-alpha line a round number.
X-units appear in X-ray crystallography literature from 1925 through the 1960s. Modern crystallography uses meters or angstroms; X-units are mostly of historical interest now.
Defined by Manne Siegbahn in 1925; superseded by direct SI measurement of X-ray wavelengths after the 1960s; retained in literature for historical comparison.
Fermi
A fermi is a unit of length equal to one femtometer (1×10⁻¹⁵ m). It is named after physicist Enrico Fermi and is widely used in nuclear physics as a synonym for the femtometer.
The fermi was introduced informally in mid-20th-century nuclear physics literature. Although the BIPM has formally standardised the SI name 'femtometer', the fermi remains in widespread informal use.
Fermis are used to express nuclear sizes, hadron radii, and characteristic length scales in particle physics. Functionally identical to the femtometer.
Named after Enrico Fermi; in informal use from the 1950s; officially equivalent to the SI femtometer.
X-unit to Fermi conversion formula
The relationship between x-units and fermis:
To convert x-units to fermis, multiply the value in x-units by 100.21. To reverse, multiply fermis by 0.009979044.
How to use this converter
Type a value into the calculator. The result in fermis updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Fermi to X-unit converter for the reverse direction.
Step-by-step: convert x-units to fermis
- Write down the value in x-units (X).
- Multiply that value by the factor 100.21.
- The product is the equivalent value in fermis (F).
- To reverse, multiply the fermi value by 0.009979044.
Worked examples
Example 1 — Convert 1 X to F:
1 × 100.21 = 100.21 F
Example 2 — Convert 100 X to F:
100 × 100.21 = 10021 F
Real-world example — Molecular dimensions
The diameter of small molecular structures (around 2 x-units) is often converted into related sub-micron units when comparing measurements across different microscopy techniques or imaging modalities.
2 X × 100.21 = 200.42 F
Real-world example — Wavelengths across the spectrum
Optical and atomic-scale phenomena are routinely cross-converted between sub-micron units. A photon of wavelength 800 x-units can be re-expressed in fermis for direct comparison with another instrument's calibration data sheet.
800 X × 100.21 = 80168 F
X-unit to Fermi conversion table
Standard reference values for converting x-units to fermis:
| X-unit [X] | Fermi [F] |
|---|---|
| 0.01 | 1.0021 |
| 0.1 | 10.021 |
| 1 | 100.21 |
| 2 | 200.42 |
| 3 | 300.63 |
| 4 | 400.84 |
| 5 | 501.05 |
| 10 | 1002.1 |
| 20 | 2004.2 |
| 30 | 3006.3 |
| 40 | 4008.4 |
| 50 | 5010.5 |
| 100 | 10021 |
| 500 | 50105 |
| 1000 | 100210 |
Frequently asked questions
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Metric / SI (4 units)
Imperial / US Customary (4 units)
Sources & references
Conversion factor (1 X = 100.21 F) 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.