How to Calculate How Many Stairs You Need

Using a tape measure, determine the total vertical distance that the stairs need to cover. This is the 'total rise'. Ensure the measurement is taken from the finished surface of the lower floor to the finished surface of the upper floor. For accuracy, use a level if needed to ensure a true vertical measurement.

The ideal riser height is typically between 7 and 7.75 inches. Divide the total rise by a target number of risers to find the exact riser height. For example, if the total rise is 100 inches, aiming for a riser height of around 7 inches would mean 100 / 7 = ~14.28 risers. You'll need to round this to a whole number.

Divide the total rise (from Step 1) by your chosen ideal riser height (from Step 2). Round the result to the nearest whole number. This will be your number of risers. For instance, if your total rise is 100 inches and you aim for a 7-inch riser height, 100 / 7 = 14.28, which rounds to 14 risers.

Divide the total rise by the number of risers calculated in Step 3. This will give you the actual height for each riser. For example, if the total rise is 100 inches and you have 14 risers, the actual riser height is 100 / 14 = 7.14 inches.

The number of treads is always one less than the number of risers. This is because the upper floor serves as the final 'tread'. Using the number of risers from Step 3, subtract one to find the number of treads. If you have 14 risers, you will need 13 treads.

Multiply the number of treads (from Step 5) by the desired tread depth. A common tread depth is 10 to 11 inches, but this can vary. Ensure the tread depth is consistent and meets local code requirements. For example, if you have 13 treads and a desired tread depth of 10 inches, the total run is 13 * 10 = 130 inches.

Divide the total run (calculated in Step 6) by the number of treads (from Step 5). This gives you the actual tread depth. For example, if you calculated a total run of 130 inches for 13 treads, the actual tread depth is 130 / 13 = 10 inches.