# How to Estimate Stairs

Estimating stairs is really not as tricky as you may suppose. Just remember that stairs go from floor to floor, not floor to ceiling. It is important that each stair rise exactly the same height as the others. In addition, when you reach the top, the final stair should sit flush with the floor on the upper level. Major discrepancies between stair riser heights could lead to trips and falls. It is also important to comply with codes for riser height, where applicable. Comfortable risers usually fall between 5 inches and 7.75 inches.

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Measure the exact height where your stairs will go, remembering to begin at the surface of the lower floor to the surface of the upper floor, not merely to the ceiling. If the floor uses 2-by-8-inch lumber for the joists, there is actually 7.25 inches of additional height above the typical 8-foot wall. Add to that the depth of subfloor and regular flooring for around 1.25 inches or so additional height.

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Convert the total height measurement to inches with your calculator. For this example, 96 + 7.25 + 1.25 = 104.5 inches.

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Measure your floor from the spot where you want the stairs to begin at the bottom step to the base of the wall at the place where the top step will meet the upper floor, then convert the number to inches. For example, if you have 12.5 feet of horizontal distance to work with, that converts to 150 inches.

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Divide the floor measurement – in inches -- by the width of the tread boards you plan to use. For example, if you have 1-by-10-inch boards, which are actually 9.25 inches wide, divide 150 by 9.25 to get 16.21 steps. Because you can't have a partial step, call it 16 steps.

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Check to see if that number of steps comes out to a good height for each riser by dividing the overall height measurement by the estimated number of stairs. In this case, divide 104.5 by 16 to get 6.53 inches. Because 7 inches is generally considered ideal, it might be better to divide by 15 stairs to get closer and have one less step to build. Doing the math, you see that 104.5 divided by 15 equals 6.966. Using standard builder's measurements, they are 6 31/32 inches each. because that is just a hair short of 7 inches, it is very close to perfect.

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Double-check the measurements and calculations for accuracy. In this example, the estimated number of steps needed is 15, each with a 10-inch run and a 6 31/32 riser.
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