Your post office is at a distance of 1.25 km from your house.
Then, the distance in meters is 1250 m.
A kilometre is a unit of length equal to one thousand meters in the metric system.
In the metric system, a meter is a unit of length that is the base unit of length of the International Unit System (SI).
Based on meters, kilograms and seconds, the meter is used as the base unit of length in the SI and other M.K.S. systems to help derive certain units of measurement, such as the newton, for force.