Based on a straight staircase. Just measure the tread (depth) and riser (height) of one step and multiply that by how many stairs you have, this is then your answer. For example - Tread = 22.5cm, Riser = 22.5cm (45cm's) muliplied by 13 steps (45cm's x 13) = 585cm's (5.85m)
If your staircase is more involved and consists of winders and sub landings then the process is not much different. You just need to measure the tread and riser of each winder and each platform and then add that measurement to the already measured straight steps.
We recommend adding a little on just incase you make a mistake or at least to give you a margin for error