Stairs are made up of tread (or "going"), which is the horizontal part of the stair you step on, and risers, which are the verticals holding up the tread. Laws regulate the height of risers, and require risers to be of uniform height.
The height of stair risers should be 220 mm (8 11/16 inches) or less. Risers must be of uniform height, with a variation of 9 mm (3/8 inch) or less. Open risers are permitted if they are 10 cm (4 inches) high or less.
Calculating rise and run
To calculate the rise, or height, of a stairway, measure the height from one floor level to the next. Divide that total by 22 cm (8 11/16 inches) to see how many stairs you will need. Multiply the number of stairs by the expected width of each tread or step to determine the run, or the length of the stairway.
The building regulations in the UK are known as document K. These may change from time to time, so check the latest version available on the UK government website (see Resources).