It really is very easy to ‘measure-up’ for your straight Stair lift.
All you need is a good quality metal tape measure.
Firstly, take a general overview of your stairway layout and decide which side of the staircase the Stair lift will be mounted on, when viewed from the bottom. Be aware that the Stair lift rail will overhang the top landing by 8" and meet the floor at the foot of the stairs 14" beyond the ‘nose’ of the first step.
Take note of any fixtures, such as window sills, hand rails, central heating radiators, doorways etc., that intrude into the stairway area - or could be blocked by the presence of the Stair lift. Decide, which side of the stairway will be most appropriate for the Stair lift, requiring the least amount of disruption - or obstruction. In most cases, it’s a fairly obvious choice.