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how fast is c ( All Images Videos News Maps Shopping Settings All regions Safe search: off Any time Speed of light - Wikipedia whttps:en.wikipedia.orgwikiSpeedoflight The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299 792 458 metres p second (approximately 300 000 kms, or 186 000 mis).