The speed of light in a vacuum is exactly 299,792,458 meters per second, which is about 300,000 kilometers per second, or approximately 186,000 miles per second (671 million miles per hour)

. This value is a universal physical constant and is denoted by the symbol "c." It represents the maximum speed at which information, matter, or energy can travel through space

. The exact number 299,792,458 m/s is defined by international agreement because the meter itself is defined as the distance light travels in vacuum in 1/299,792,458 of a second. This means the speed of light is not measured but rather fixed by the definition of the meter

. The choice of this specific number was made to maintain consistency with earlier measurements and standards of length

. Light and all other forms of electromagnetic radiation travel at this speed in vacuum. However, light slows down when passing through materials like water or glass, but the universal constant speed in vacuum remains the same

. In summary:

• Speed of light in vacuum: 299,792,458 m/s (exact)
• Approximate equivalents: 300,000 km/s, 186,000 miles/s, 671 million mph
• Defined by the meter standard, not measured anew
• Maximum speed limit in the universe for matter and information

This speed is fundamental to physics and underpins many aspects of modern science and technology.