How to Calculate Stair Dimensions
Divide the total rise by the desired riser height and round up to the nearest whole number to get the riser count. Then divide the total rise by the riser count to get the actual (equalized) riser height. Multiply the riser count minus one (top riser is the landing) by the tread depth to get the total horizontal run. For a deck that is 56 inches high with a desired 7-inch riser: 56/7 = 8 risers. Actual riser: 56/8 = 7.0 inches exactly. Total run: 7 treads x 11 inches = 77 inches (6.4 feet). Stringer length: sqrt(56^2 + 77^2) = 95.2 inches (7.9 feet).
The stringer is the diagonal support that holds the treads and risers. It must be cut from a board long enough to span the total diagonal distance. Always buy the next length up from your calculated stringer length to account for the plumb cut at the top and the level cut at the bottom. For a 95-inch stringer, you need at least a 10-foot 2x12. For very long runs, a mid-span landing breaks the staircase into two shorter, more manageable sections. For the deck or platform the stairs connect to, our fence calculator can help with railing material estimates.
Stair Building Codes and Requirements
The IRC governs residential stair construction in most US jurisdictions. Key requirements: maximum riser height is 7.75 inches (7 inches in some local codes), minimum tread depth is 10 inches, maximum variation between risers is 3/8 inch, minimum stair width is 36 inches, handrails are required on at least one side for stairs with 4 or more risers, handrail height must be 34-38 inches measured from the tread nosing, and handrail grip size must be 1.25-2 inches in cross section. Guard rails (on open-sided stairs) must be at least 36 inches high with balusters spaced no more than 4 inches apart.
For outdoor deck stairs, additional requirements apply: stringers must be supported at the base on a concrete footing or pad, not resting on bare soil. Open risers (no vertical board between treads) are permitted on deck stairs but the opening must not allow a 4-inch sphere to pass through. Stair illumination is required at the top and bottom of every exterior stairway. For basement stairs, the headroom minimum is 6 feet 8 inches measured vertically from the tread nosing to the ceiling above. Always check your local building department for amendments to the IRC — many jurisdictions have stricter requirements.
How to Build Stair Stringers
Start with a 2x12 board at least 2 feet longer than your calculated stringer length. Place a framing square at the top corner with the riser measurement on the tongue (short side) and the tread measurement on the blade (long side). Mark along both edges of the square. Slide the square down to the next mark and repeat for each step. Cut along the marks with a circular saw, finishing the corners with a handsaw or jigsaw. Cut the bottom of the stringer to remove one riser height (so the first step sits at ground level) and cut the top to remove one tread depth (so it meets the landing).
Test-fit the first stringer before cutting the remaining ones. Place it in position and check that each tread is level and each riser is plumb. Adjust if necessary and use the corrected stringer as a template for the others. For deck stairs up to 36 inches wide, use three stringers (one at each end and one in the middle). For wider stairs, add stringers every 16-18 inches on center. Attach stringers to the deck with a stringer hanger bracket or a 2x12 cleat nailed to the rim joist. At the base, attach to a concrete pad using angle brackets and concrete anchors. For the concrete base, use our concrete slab calculator to estimate the small pad.
Concrete and Stone Stairs
Concrete stairs follow the same dimensional rules but require formwork instead of stringers. Build a sloped form from 2x lumber that matches the stair profile. The form includes the side walls (riser forms) and the bottom (soil is typically used as the base form). Pour concrete into the form in one continuous pour, working from the bottom up. Rod and tamp each step to eliminate voids. Float the treads smooth and round the nosing edge with an edging tool. Remove forms after 24-48 hours and cure for 7 days before use.
Stone stairs use large natural stone slabs (flagstone, bluestone, or granite) as individual treads, each resting on a compacted gravel base. Each stone must be level front-to-back with a slight forward pitch (1/8 inch per foot) for water drainage. The rise between stones must be consistent. Stone stairs are the most expensive option ($50-150 per tread for materials) but also the most durable and aesthetically appealing. For landscape stairs in a garden or hillside, precast concrete step treads ($15-40 each) are a practical middle ground between poured concrete and natural stone. Coordinate with a gravel calculator for the base material under stone steps.
Common Stair Building Mistakes
Unequal riser heights are the most dangerous and most common mistake. Even a 1/4 inch difference between adjacent risers causes people to stumble. The IRC allows a maximum 3/8 inch variation across all risers in a flight, but the best practice is zero variation. Calculate carefully and measure each cut. The second mistake is cutting stringer notches too deep. The remaining uncut portion of the stringer (the throat) must be at least 5 inches deep for a 2x12 stringer. Deeper cuts weaken the stringer and can cause it to fail under load.
Not providing adequate support at the base of the stringers causes the stairs to sink over time, especially on soft or wet soil. Always pour a concrete pad or footing at the base of exterior stairs. The pad should extend at least 6 inches beyond the stringer in all directions. Not attaching stringers properly at the top is another critical error. Use manufactured stringer hangers or bolt a 2x12 cleat to the rim joist and nail through the stringer into the cleat. Nailing through the rim joist into the end grain of the stringer provides virtually no holding strength. For the complete deck project, our tile calculator helps if you are tiling the stair treads.
For related calculations, try the gravel driveway to estimate materials for your project.