Then doubleClick the Line tool. Now to measure the distance between two points all you need to do is hold down your left mouse click at the start point and then drag your mouse to the end point and then release your mouse button. Its dialog will have remembered the last-used values.
If you are on photoshop CS6 - Create a Marquee rectangle snapping from one point to the another.
The starting location X and Y. Use the miles km nautical miles yards switch to measure distances in km or in miles or nautical miles. Left mouse click on the first point on your picture and hold button down. The measure tool calculates the distance between any two points in the work area.