What the stair stringer calculator estimates
Use this stair stringer calculator to check the geometry of a straight flight with the upper floor serving as the final tread. Enter total rise, chosen tread depth and a target riser height, all in inches. An optional zero-or-positive tread thickness supplies the bottom adjustment shown below the results.
The output is a layout reference. It does not generate a complete notch drawing, choose timber dimensions or confirm structural or building-code compliance.
How to use the inputs
Measure total rise between finished floors. Enter tread depth as the horizontal going of one step, excluding an added nosing projection. The target riser height controls how the calculator selects a whole number of equal risers; it is not a guaranteed final height.
For tread thickness, enter the actual material thickness relevant to the bottom step detail. The reported adjustment assumes that this thickness must be allowed for at the bottom. Confirm your finished-floor and upper-landing arrangement before using it to mark cuts.
If you already planned the rise and tread combination in the stair calculator, use those same inputs here. For a live side view, metric inputs or an available-run comparison, use the stair calculator first. This stringer tool uses inches and adds the bottom adjustment.
A worked example you can reproduce
Enter 119 inches of total rise, 10 inches of tread depth and a 7-inch target riser. This produces:
- 17 risers, each exactly 7 inches high.
- 16 treads below the upper floor.
- 160 inches of total horizontal run.
- A stair angle of about 35.0°.
- An inclined stringer layout length of about 195.30 inches, or 16 feet 3 5/16 inches.
With tread thickness entered as 1.5 inches, the bottom adjustment shown is 1.5 inches. This example's stair geometry also appears in the rise-only example on page 19 of the Johnson CALC-1500 manual.
Why the formula uses tread count
One repeated step segment has an inclined length of √(riser height² + tread depth²). Multiplying by the number of separate treads gives the repeated layout length. The angle is arctan(riser height ÷ tread depth).
The upper floor replaces the last tread, so a flight with 17 risers has 16 of these segments. Using the full floor-to-floor rise in a triangle with those 16 treads overstates the layout's angle and diagonal.
For a separate level railing at the landing, the baluster spacing calculator gives equal gaps and individual center marks. Its layout does not apply to the sloping stair rail.
Use Copy results or Print results to keep these inputs and the stated allowances with the layout.
Before selecting or cutting a board
Top and bottom termination details are not included in the inclined length. The tool does not establish notch depth, remaining section, bearing, fasteners, stringer spacing or supported load. Confirm those details, and the applicable requirements for the finished stairs, using the actual plans.
Our methods page explains calculation and rounding. To report an error, send the inputs and expected result.
Good questions. Straight answers.
Does this give the board length to buy or cut?
No. The result is the inclined length over the repeated tread layout. Top and bottom details, bearing and spare material must be considered separately.
Does it select the number or size of stringers?
No. Stringer count, spacing, member size, grade and connection details are outside this calculator's scope.
What does the tread-thickness value change?
It reports a typical bottom riser adjustment equal to the entered tread thickness. It does not change riser count, stair angle or the inclined layout length.