There is no need to measure the length of the stairs, simply follow the method below for a simple staircase.
Rectangular rooms are easy. Simply measure the width and length of the room, always remembering to measure into doorways, alcoves and recesses.
Sketch out a plan like the one on the right and simply fill in the dimensions.
Remember to measure over the top step and into doorways and recesses. Mark the position of your stairs with an X as shown in the example to the right.
The same as a standard room or landing only this time mark the stairs with an X and the front door with a D as shown in the example.