πͺ Stair Stringer Calculator
Steps, risers, runs, stringer length, and code checks.
Number of Steps
How to Use This Calculator
Measure the total rise from the finished floor below to the finished floor above, in inches. Enter your desired riser height (7 inches is the comfortable target), and pick a tread run. The calculator works out the whole number of steps, the exact riser height each step needs, the total run of the staircase, the stringer board length, and the stair angle, and it flags IRC code violations automatically.
Measure total rise in inches from the finished surface below to the finished surface above. Include flooring thickness on both levels.
Enter your desired riser height, defaulting to 7 inches. The calculator rounds to a whole number of steps and adjusts the actual riser to fit.
Pick a tread run. Deeper treads are more comfortable but make the stairs longer; 11 inches is the sweet spot for most homes.
Check the code flags. If the riser exceeds 7.75 inches or the tread is under 10 inches, adjust your inputs until the flags clear.
The Formulas
Total run uses steps minus 1 because the top step is the upper floor itself: there is one fewer tread than there are risers. This is the classic mistake in stair layout, and getting it wrong shifts the whole staircase by a full tread. The stringer length is the hypotenuse of the rise/run triangle, but buy lumber longer than this number to allow for the plumb and level cuts at each end.
The stair angle comes from the same triangle. A 7-inch rise and 11-inch run gives atan(7/11) = 32.5 degrees, right in the comfortable zone. Anything over 37 degrees will feel steep to most people.
Worked Example
Total rise of 42 inches, desired riser 7 inches, tread run 11 inches. Steps = round(42/7) = 6 steps. Actual riser = 42/6 = 7 inches exactly. Total run = (6 - 1) x 11 = 55 inches. Stringer length = sqrt(42^2 + 55^2) = sqrt(1764 + 3025) = sqrt(4789) = 69.2 inches, so buy an 8-foot (96-inch) 2x12. Stair angle = atan(42/55) = 37.4 degrees, just inside the comfortable range. Riser 7 in is under the 7.75 in cap and tread 11 in clears the 10 in minimum, so this layout passes code.
Tips
Cut stringers from straight, knot-free 2x12 lumber, and use a framing square with stair gauges to mark every step identically; the IRC allows only 3/8 inch of variation between risers. Support long stringers mid-span to prevent bounce, and always attach the stringers to the deck or floor framing with proper hangers or ledgers, not just nails. For outdoor stairs, use pressure-treated lumber rated for ground contact.
Estimates for planning only. Stair codes vary by jurisdiction; verify against your local building code and get a permit where required.
Frequently Asked Questions
How do you calculate the number of steps for stairs?
Divide the total rise (floor to floor, in inches) by your target riser height, usually 7 inches, and round to the nearest whole number. That gives the step count. Then divide the total rise by the step count to get the exact riser height. For example, a 42-inch total rise divided by 7 gives exactly 6 steps at 7 inches each.
What is the maximum riser height allowed by code?
The International Residential Code (IRC) caps riser height at 7.75 inches and requires a minimum tread depth of 10 inches. Risers in a single flight must also be uniform within 3/8 inch of each other. This calculator flags any result that breaks these limits so you can adjust before you cut.
How long should stair treads be?
The IRC minimum tread depth is 10 inches, and most comfortable stairs use 10 to 11 inches of run per step. Deeper treads feel safer and are easier on knees, but they make the staircase longer: each extra inch of run adds an inch times the number of steps to the total run. Nosings can add up to 1.25 inches of effective depth.
How do you find the stringer length?
Treat the total rise and total run as the two legs of a right triangle and use the Pythagorean theorem: stringer length = sqrt(total rise^2 + total run^2). Add extra length for the top and bottom cuts and buy lumber longer than the calculated length. A 2x12 board is the standard stringer material.
What is a comfortable stair angle?
Comfortable residential stairs land between 30 and 37 degrees. The classic 7-inch rise with an 11-inch run gives 32.5 degrees. Steeper than 40 degrees feels ladder-like and is common only in ships ladders and attic access. If your angle comes out steep, you may need more run, which means a longer staircase footprint.