Definition of Square kilometre

• 1,000,000 m²
• 100 ha (hectare)

Conversely:

• 1 m² = 0.000 001 (10^-6) km²
• 1 hectare = 0.01 (10^-2) km²
• 1 square mile = 2.589 988 km²
• 1 acre = 0.004 047 km², 40.47 ha or 4047 m²

Note: "km²" means (km)², square kilometre or kilometre squared, and not k(m²), kilo–square metre. For example, 3 km² is equal to 3 * (1,000 m)² = 3,000,000 m², not 3,000 m².