Ordnance Survey maps with a scale of 1 cm:50 000 cm (which means 1 cm on the map is 50 000 cm on the ground or 1 cm is 500 m) have boxes called grid squares . The side of a full grid square measures 1 km.
Calculating a Regular-Shaped Area Use this method to measure any regular-shaped (rectangular) area:
Step 1: Count the number of grid squares across the base of the area you want to measure. Step 2: Count the number of squares going up the side of the area. Step 3: Multiply both numbers. This gives you your answer in square kilometres (sq km/km²).