SI Unit of Length

SI unit of length = Metre (m)

Definition (current, since 1983): 1 metre = the distance light travels in vacuum in 1/299,792,458 seconds. (This fixes the speed of light at exactly c = 299,792,458 m/s)

Symbol: m Base quantity: Length Dimension: [L]

Historical definitions: • 1791: 1/10,000,000 of the distance from equator to North Pole. • 1889–1960: length of international prototype metre bar (platinum-iridium). • 1960–1983: 1,650,763.73 wavelengths of krypton-86 radiation. • 1983–present: speed-of-light definition (current).

Prefix → Symbol → Value Tera → T → 10¹² m Giga → G → 10⁹ m Mega → M → 10⁶ m Kilo → k → 10³ m (1 km = 1000 m) Hecto → h → 10² m Deca → da → 10 m Metre → m → 1 m Deci → d → 10⁻¹ m Centi → c → 10⁻² m (1 cm = 0.01 m) Milli → m → 10⁻³ m (1 mm = 0.001 m) Micro → μ → 10⁻⁶ m (1 μm = 10⁻⁶ m) Nano → n → 10⁻⁹ m (1 nm = 10⁻⁹ m) Pico → p → 10⁻¹² m Femto → f → 10⁻¹⁵ m

Special units: • Ångström (Å) = 10⁻¹⁰ m (used for atomic/molecular sizes) • Astronomical Unit (AU) = 1.496 × 10¹¹ m (Earth-Sun distance) • Light year (ly) = 9.461 × 10¹⁵ m (astronomical distances) • Parsec (pc) = 3.086 × 10¹⁶ m

Scale of lengths in the universe: Object → Approximate size Proton radius → 10⁻¹⁵ m (1 fm) Atom radius → 10⁻¹⁰ m (1 Å) Virus → 10⁻⁷ m Bacterium → 10⁻⁶ m (1 μm) Height of human → ~1.7 m Height of Everest → 8,849 m Earth's radius → 6.4 × 10⁶ m Earth–Moon distance → 3.84 × 10⁸ m Earth–Sun distance → 1.5 × 10¹¹ m (1 AU) Nearest star (Proxima Centauri) → 4 × 10¹⁶ m Observable universe → ~4 × 10²⁶ m

Dimensional formula of length: [L¹] or [M⁰L¹T⁰]