Stair Stringer Calculator

How to Use This Calculator

This stair stringer calculator is designed to give you the exact dimensions needed to cut stringers for a safe and code-compliant staircase. Begin by measuring the total vertical rise from the top of the finished floor below to the top of the finished floor above. Then measure the total horizontal run—the distance the stairs must cover from the wall or header to the point where the bottom tread will sit.

Enter these measurements in inches, along with your local building code limits. The calculator will compute the number of risers required, the individual riser height, tread depth (accounting for nosing overhang), the stair angle, and the approximate stringer length. If the computed tread depth is below your minimum code, you'll see an error message, prompting you to adjust your run or code parameters.

Formula and Methodology

The number of risers is determined by dividing the total rise by the maximum allowed riser height, rounding up to the nearest whole number. Then the actual riser height is found by dividing total rise by that number of risers. The available run for treads is the total run minus the nosing overhang (if any) because treads extend beyond the stringer. Tread depth is then calculated by dividing that adjusted run by the number of treads, which is one less than the number of risers.

The stair angle is derived from the arctangent of the riser height over the tread depth. The stringer length is approximated using the Pythagorean theorem over the total rise and total run, adding an allowance for stringer thickness. This ensures you have enough length to cut the notches properly.

Practical Examples

Suppose your total rise is 105 inches and total run is 120 inches, with a maximum riser of 7.75 inches and minimum tread of 10 inches. The calculator will estimate 14 risers (105 / 7.75 = 13.55 rounded up to 14), giving a riser height of 7.5 inches (105 / 14). After subtracting a 0.75 inch nosing, your available run is 119.25 inches. Dividing by 13 treads yields a tread depth of about 9.17 inches—which is less than 10, so you'd need to increase your run or allow a shallow nosing. This immediate feedback helps you adjust your design before cutting any wood.

Another example: If you have a total rise of 96 inches and a run of 100 inches, with a max riser of 7 inches, you get 14 risers (96/7=13.71) and a riser of ~6.86 inches. Tread depth after nosing would be (100-0.75)/13 ≈ 7.6 inches, which also likely fails. So often, you need to increase the run to meet code.

Tips and Best Practices

Always double-check your local building codes for both residential and commercial stairs. Codes specify not only the maximum riser and minimum tread depths but also the maximum variance between steps (typically 3/16 inch). When cutting stringers, use a framing square with stair gauges to ensure accuracy. Avoid making mistakes by measuring after the stringer is cut—do a dry fit before final installation.

Remember to consider the thickness of the tread material when marking the stringer, as that affects the effective rise. This calculator uses the total rise as instructed; it does not account for tread thickness. You may need to adjust your total rise measurement to account for finish materials. Finally, always use proper lumber that is at least 2x12 for stringers, and ensure they are securely attached to the structure.