Speed of light in vacuum
c denotes the speed of light in vacuum. Its numerical value is fixed exactly in SI units.
Key knowledge
c = 299 792 458 m s⁻¹, exactly by the SI definition.
Reference values
c299792458m s⁻¹
Exact value · SI definitionValues and equations
c = 299 792 458 m s⁻¹
- c
- c denotes the speed of light in vacuum. m s⁻¹
Conditions and scope
- Applies to light in vacuum; it is not the speed in a material medium.
Exact definitions do not remove uncertainty from practical measurements.
Sources · 5
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Bureau International des Poids et Mesures (BIPM)3 sources
- SI defining constantsBureau International des Poids et Mesures (BIPM)Reference guide
- The International System of UnitsBureau International des Poids et Mesures (BIPM)Reference guide
- The International System of Units, 9th edition, version 4.01 (2026)Bureau International des Poids et Mesures (BIPM)Reference guide
- Meet the ConstantsNational Institute of Standards and Technology (NIST)Reference guide
- 国際単位系(SI)第9版 · Japanese translation, V3.01National Metrology Institute of Japan, AISTReference guide · PDF
Evidence and review
Exact SI value, symbol, unit and defining scope.
2026-10-09 · English value and scope checked against BIPM; value compared with NIST. This review covers the definition only.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Agreement within this scope
The vacuum speed of light has exact SI value 299792458 m/s.
Exactness is definitional and does not imply zero uncertainty in practical measurements.
National Metrology Institute of Japan, AIST
- 国際単位系(SI)第9版 · Japanese translation, V3.01 JapanJapaneseShared standard
2.2 Table1; 2.3.1 Metre, printed p15
- 国際単位系(SI)第9版 · Japanese translation, V3.01
Entry details
Permanent link to this entry ↗- Record identity
- si-speed-of-light
- Revision
- 1
- Structured reference record
- SI