A convenient and very easy to use calculator to obtain the spindle or baluster spacings between newel posts, or, any work which requires even spacings within a specified distance.

The app uses your mobile device to obtain the slope angle automatically. Place your mobile device along the incline of the stairs and register the displayed degrees by conveniently tapping the arrow button provided. This will transfer the value to the angle entry box, so no more awkward elevation measurements and spirit levels.
If you know the angle already, then you have the option to enter it manually.

Measure the diagonal distance between the newel posts along the slope of the stairs and not the horizontal distance.
When you have completed filling in the 4 entry boxes then tap on the "Calculate" button to show the results with a list of spindle positions and spacing widths adjusted for the slope.

You may need to tweak the regulation maximum spacing, defaulting at 98mm, to take into account turned spindles (balusters) with tapering profiles.

It should be noted that the underlying algorithm works out best fit within a millimeter or so. Under certain circumstances you are invited to adjust the maximum regulation width slightly and recalculate to perhaps reduce by one the number of spindles. Bear in mind that the legal maximum is 100mm in the UK. or usually 4 inches in the USA .