Two components combine to form stair angles. The total rise is the height the stairs travel from the landing to the highest point they reach. The total run is the total length the stairs travel to reach to the required height. Accurately determining both the rise and run will ensure the stairs reach the required height at a safe angle. A common tool used to speed up the layout of a stair stringer will automatically set the correct stair angle.
Set the bottom of a four-foot level on the surface of the floor where the stairs will end. Lift the end of the level until it reads level. Pull a pencil along the bottom of the level to make a sharp line. Move the level. Align the bottom of the level with the pencil line. Adjust the level. Extend the pencil line. Continue the process until the pencil line reached the point the stairs will end.
Stretch a tape measure from the pencil line to the floor. Write down the dimension -- rise. Measure the distance from the landing the end of the floor where the top of the stairs will mount. Write down the dimension -- run.
Divide the rise by the ideal step height. For example, a rise of 78 inches divided by a step height of 7 inches for 11.14 steps in each stringer. Round up 11.14 to the nearest whole number of 12. Divide the 78-inch rise by 12 steps of each stringer for a step rise of 6 1/2 inches.
Set a framing square on top of a piece of 2-by-12-lumber. Align the 6 1/2 inch mark on the square's short leg with the top edge of the lumber. Align the tread width -- for example, 14 inches – with the top edge of the lumber. Drag a pencil along the outside of the framing square to draw a triangle that uses the top of the lumber as its hypotenuse. Move the square. Repeat the process until all 12 steps are marked on the 2-by-12-inch lumber.
Lift the stair stringer after all steps are marked. The stringer is sitting at the correct angle when the long pencil line -- step depth -- sits parallel to the ground.