Stair Calculator

According to the International Residential Code (IRC), the maximum riser height is 7.75 inches (19.6 cm) and the minimum tread run is 10 inches (25.4 cm). For optimal comfort, many contractors target a 7-inch riser and 11-inch run.

A flush mount attaches the stair stringer level with the upper deck floor, meaning the deck floor serves as the last step. A suspended (or standard) mount attaches the stringer one step down, so you step down from the deck onto the first tread.

It is a time-tested comfort formula stating that the sum of one riser and one tread run should equal between 17 and 18 inches. If this sum is too low, the steps feel cramped; if it is too high, it breaks natural stride length.

Standard residential stairs should have an angle of incline between 30 and 38 degrees. Anything steeper than 40 degrees feels like climbing a ladder, while angles below 25 degrees behave more like a ramp.

Use the Pythagorean theorem: square the total rise and total run, add them together, and take the square root. Always buy lumber that is at least 1 or 2 feet longer than this calculation to account for the angular cuts at the top and bottom.