How to Calculate How Many Treads & Stairs to Another Floor

Plan stairs according to who will be using them. Older people and those with limited mobility often prefer a shorter rise, which will make the stair extend into the room more. However, if the rise is too short, the necessary extra length of the tread, or stair step, may make it awkward to navigate to the next step. A total run and rise length should be close to 17.5 inches.

Establish the distance in inches between the base floor and the upper floor.

Divide that distance by how high you want each stair to be, which is the riser height. For instance, if the height between floors is 80 inches and you want an easily climbed step of 6 inches, then you need 13.3 steps to get to the higher floor.

Round the steps to the next higher whole number to give 14 steps total. Always count the base floor and the top floor as a step.

Divide the number of steps into the distance between the floors. In our example, this would be 14 steps divided into 80 inches, or 5.71 inches. Then subtract the height of the stair step from this figure to get the distance from one stair to the other. For example, if the stair itself is 1 inch thick the distance would be 4.71 inches.

Determine the depth of the tread by subtracting the rise -- or in this example, 5.71 – from 17.5. The run, or tread depth, is 11.78 inches. This provides an angle of descent right between 29 and 30 percent, which is shallow and barely on the edge of acceptability for code. Changing to 13 steps instead of 14 will improve the angle of descent and give a distance of 6.15 inches for the rise, which is very close to the desired 6 inches.