Wood Stair Stringer Calculator: Rise, Run & Layout

This calculator works out the number of treads, the rise and run of each step, and the stringer length needed to build a straight wood stair. You enter the total rise (floor-to-floor height), the total run (horizontal distance available), and the tread thickness, and it returns the key layout numbers you need before cutting a single stringer.

Formula

The core formulas used here are the standard stair geometry relationships:

• Number of steps (n) = ceil(Total rise / Target rise). The target rise is typically 7.5 inches (190 mm), a common comfortable step height. The ceiling function rounds up to the nearest whole number.
• Actual rise per step = Total rise / n
• Actual run per step = Total run / n
• Stringer length = sqrt(Total run² + Total rise²) — this is the hypotenuse of the right triangle formed by the total rise and total run.

Variables are measured in inches (or any consistent unit). The stringer length is the straight-line distance along the angled board, not the total board length needed (you’ll add extra for the top and bottom connections).

Worked Example

Using the default values: total rise = 105 in, total run = 120 in, tread thickness = 1.5 in.

1. Number of steps: ceil(105 / 7.5) = ceil(14) = 14 steps
2. Rise per step: 105 / 14 = 7.50 in
3. Run per step: 120 / 14 = 8.57 in
4. Stringer length: sqrt(120² + 105²) = sqrt(14400 + 11025) = sqrt(25425) = 159.45 in

The calculator rounds the run to 8.57 in and the stringer to 159.45 in. Note that the stringer length is the hypotenuse; the actual board will need extra length at both ends for the connections.

How to Use the Results

The rise per step and run per step are the two numbers you’ll mark on the framing square to lay out the stringer. The number of steps tells you how many treads and risers to cut. The stringer length gives you the minimum board length, but you should always buy a board longer than this — typically 12 inches longer — to allow for the top and bottom bearing points.

For a typical residential stair, the rise per step should be between 7 and 8 inches, and the run (tread depth) should be at least 10 inches when you include the nosing. If your calculated run is less than 10 inches, you may need to adjust the total run or add a nosing overhang.

Common Mistakes and Misconceptions

• Mixing units: Always use the same unit (inches, feet, cm) for all inputs. A common error is entering feet for rise and inches for run.
• Forgetting tread thickness: The tread thickness affects the actual rise when the stringer is cut. The calculator here does not adjust for tread thickness in the rise formula; you’ll subtract the tread thickness from the top riser height when cutting. Many calculators include this as an optional adjustment.
• Rounding too early: Rounding the rise per step to 7.5 before calculating the run can lead to cumulative errors. Keep full precision until the final answer.
• Not accounting for the bottom riser: The bottom riser is often shorter than the others because the floor is the last tread. Adjust the bottom riser height accordingly.

Choosing the Right Stringer Material

Stringers are typically cut from 2×12 lumber (nominal 2 inches by 12 inches, actual 1.5 x 11.25 inches). The width is important because the stringer must be wide enough to support the treads and risers. For wider stairs (more than 36 inches), you may need three stringers instead of two. Pressure-treated lumber is used for exterior stairs; interior stairs use kiln-dried lumber to minimize warping.

The board must be free of large knots and cracks, especially in the areas where you’ll cut the notches. A straight, dry board is essential for accurate layout.

Building Code Considerations

Most building codes follow the International Residential Code (IRC) for residential stairs. The maximum rise is 7.75 inches, and the minimum run is 10 inches (including nosing). The minimum stair width is 36 inches. The maximum variation between the tallest and shortest riser is 3/8 inch. Always check your local code, as some areas have stricter requirements.

For commercial stairs, the rules are different: maximum rise is 7 inches, minimum run is 11 inches, and handrails are required on both sides. These numbers are not universal, so verify with your local authority.

Alternatives to a Straight Stringer

This calculator handles straight stairs only. If your stair has a landing, a winder, or a curve, the geometry is more complex. For L-shaped or U-shaped stairs, you’ll need to calculate each flight separately. For winders, the run at the narrow end must meet code minimums, which requires a different layout method.

Another alternative is to use pre-cut metal stringers or adjustable stringer brackets, which can simplify the build but may limit your design flexibility. These are often used for decks and exterior stairs.

Frequently Asked Questions

What is the ideal rise and run for a stair? The ideal rise is about 7 inches, and the ideal run is about 11 inches. This combination gives a comfortable step that matches the natural stride of most adults. The IRC allows up to 7.75 inches rise and a minimum of 10 inches run.

How do I measure total rise accurately? Measure from the finished floor below to the finished floor above. If you’re building a deck, measure from the ground (or deck surface) to the top of the decking. Always measure in several places and use the maximum, because floors are rarely perfectly level.

Can I use this calculator for outdoor stairs? Yes, but you’ll need to adjust for the thickness of the treads and the fact that outdoor treads are often deeper (11-12 inches) for comfort. Also, use pressure-treated lumber and consider that the ground may settle, so a concrete pad or adjustable post bases are recommended.

What does ‘tread thickness’ do in the formula? In this calculator, tread thickness is not used in the main formulas, but it’s shown as an input because it affects the actual cut of the stringer. When you cut the notches, the top riser is typically shortened by the tread thickness so that the total rise remains correct. Some calculators incorporate this automatically; here you can adjust the top riser manually.

Why is my stringer length longer than the board I bought? The stringer length is the hypotenuse of the triangle, but the actual board must extend beyond the top and bottom points to rest on the floor and the upper landing. You need to add at least 6 inches at each end, so buy a board that is at least 12 inches longer than the calculated stringer length.

Can I cut stringers without a calculator? Yes, you can use a framing square with stair gauges to lay out the steps directly. However, you still need to calculate the number of steps and the rise and run. A calculator helps avoid arithmetic errors and speeds up the process, especially when you need to adjust for code compliance.

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in t | Total rise | in | 105 | 0
in r | Total run | in | 120 | 0
in th | Tread thickness | in | 1.5 | 0
out rise | Rise per step | in | t / ceil(t / 7.5) | 2
out run | Run per step | in | r / ceil(t / 7.5) | 2
out steps | Number of steps | - | ceil(t / 7.5) | 0
out stringer | Stringer length | in | sqrt((r)^2 + (t)^2) | 2