Map scale
The key.
The map scale is usually located in the legend box of a map.
To find out how far an inch on a map represents in actual distance, you would need to look for the map's scale. The scale of a map indicates the ratio between a unit of length on the map and the corresponding distance on the ground. Typically, the scale is provided in the map's legend or key. By using this scale, you can calculate the actual distance represented by an inch on the map.
If the scale is 1 centimeter to 1 kilometer, then the actual distance represented by the scale distance would be 1 kilometer for every 1 centimeter on the map. This means that if you measure a distance of, say, 5 centimeters on the map using this scale, the actual distance in real life would be 5 kilometers.
Maps are flat and Earth is round
The straight line distance would be 350 kilometers (3.5 x 100 km). Multiply the measured distance by the scale.
Sure thing, honey. If the map distance is 6 cm, the actual distance would be 63 km. If the map distance is 8.5 cm, the actual distance would be 89.25 km. Just multiply the map distance by 21 km and divide by 2 cm to get the actual distance. Easy peasy lemon squeezy!
The shortest 'theoretical' air distance from Washington DC, USA to Manila, Philippines is 8572 miles. Actual distance would depend on airport location and actual route of the flight.
You multiply by the scale factor. For example, if the scale is 1:100,000, you would multiply by 100,000.
if you really wanted to know the actual size you would go see it yourself you lazy people!
The straight line distance would be 350 killometers
I would assume that you would use cm or even mm if you wanted to be more accurate