Stair Calculator: Stringer, Rise & Run
Stairs are the one DIY project where the math is safety-critical: a quarter-inch error in riser height is enough to trip people every single time they use the stairs, and building codes exist for exactly this reason. This free stair calculator does the code math for you — enter your total rise, and it finds the number of steps, the exact riser height, the total run your staircase will occupy, and the stringer length to cut. Comfortable, legal stairs in one calculation.
Stair Stringer Calculator
How to Use This Stair Calculator
Measure the total rise: from the finished floor at the bottom to the finished floor at the top, in inches. For an 8-foot ceiling with standard joists, this is usually around 105 inches. Enter your preferred rise per step — 7 inches is the comfortable standard, and code allows up to 7.75 inches. Set your run (tread depth): 10.5 to 11 inches feels natural; 10 inches is the code minimum.
Click Calculate. The tool divides the total rise into whole steps, checks each one against the code maximum, and adjusts the step count if needed. It then shows the exact riser height, how many treads you will walk on, the horizontal distance the stair occupies, and the stringer length to cut.
The Stair Calculation Formula
First, number of risers = round(total rise ÷ desired rise). Then check the result: if total rise ÷ risers > 7.75 inches, add one more riser — code never allows a riser over 7.75 inches. The actual riser height is total rise ÷ risers, and there is always one more riser than tread: treads = risers − 1. The total run is treads × run, and the stringer length comes from the Pythagorean theorem: √(total run² + total rise²).
Worked example: a 96-inch total rise with a 7-inch desired rise gives 96 ÷ 7 = 13.7 — round to 14 risers. Check: 96 ÷ 14 = 6.86 inches, comfortably under the 7.75 limit. Treads: 14 − 1 = 13. Total run at 10.5 inches: 13 × 10.5 = 136.5 inches. Stringer: √(136.5² + 96²) = √27,848 ≈ 167 inches — about 14 feet. Cut three stringers (two outside, one center) from 2 × 12 boards.
Pro Tips for Building Safe Stairs
Consistency matters more than the numbers themselves. Code requires every riser in a flight to be within 3/8 inch of the others — a single tall or short step breaks the rhythm of walking, and that is how falls happen. Measure the finished floor heights, not the framing, or your first and last steps will be off by the flooring thickness.
Use 2 × 12 lumber for stringers so enough wood remains under each notch (at least 3.5 inches of uncut depth), and check your local code before you build — some areas require a landing over 12 feet of rise, most require handrails above three steps, and stairs are one of the most inspected items in any renovation. When in doubt, have a pro or the inspector check your layout before you cut.
Frequently Asked Questions
How many stairs do I need for an 8-foot ceiling?
A stairway with an 8-foot ceiling typically has about 105 inches of total rise floor-to-floor, which works out to 14 risers (13 treads) at a comfortable 7.5-inch rise. For a 96-inch total rise, 14 risers gives a 6.86-inch rise.
What is the standard stair rise and run?
Building codes (IRC) allow a maximum riser height of 7-3/4 inches and require a minimum tread depth of 10 inches. The comfortable sweet spot is a 7-inch rise with a 10.5- to 11-inch run.
How long should my stair stringer be?
Use the Pythagorean theorem: stringer length = √(total run² + total rise²). For a 96-inch rise with a 136.5-inch run, that is √(18,632 + 9,216) = about 167 inches. Buy 2x12 boards a few feet longer than the calculation for cutting allowance.
When do I need a landing in a staircase?
The IRC requires a landing when the total rise of a stairway exceeds 12 feet. Local codes can be stricter, and landings also need to be at least as deep as the stair is wide — check with your local building department.