Stringer Length Calculator: How to Measure & Use It

This calculator works out the total length of stringer material needed for a set of stairs, based on the total rise, total run, and the number of steps you plan to build. You enter those three numbers and get back the stringer length in the same unit you used, plus the individual rise and run per step. The result is the minimum length of a straight board (like a 2×12) you would need to cut your stair stringers from.

Formula

The stringer is the diagonal support that runs along the side of a staircase. If you imagine the stairs as a right triangle, the total rise is the vertical side, the total run is the horizontal side, and the stringer is the hypotenuse. The formula is the Pythagorean theorem:

Stringer length = √(total rise² + total run²)

• Total rise – vertical distance from the finished floor at the bottom to the finished floor at the top (in inches, feet, cm, etc.)
• Total run – horizontal distance the stairs cover, from the face of the bottom step to the face of the top step (same unit as rise)
• Stringer length – the straight-line length along the diagonal board, before you cut any notches

This gives the minimum board length. You will usually add a few inches for overhang or adjustments, and you must always check local building codes for required tread depth and riser height.

Worked Example

Using the default values from the calculator above: total rise = 36 in, total run = 48 in, number of steps = 6.

1. Square the rise: 36² = 1296
2. Square the run: 48² = 2304
3. Add them: 1296 + 2304 = 3600
4. Take the square root: √3600 = 60

So the stringer length is 60 inches (5 feet). The rise per step is 36 ÷ 6 = 6 inches, and the run per step is 48 ÷ 6 = 8 inches. That matches the calculator output.

When to Use This Calculator

Use this tool when you are planning a new set of stairs – for a deck, porch, interior step, or shed. It is most useful in the planning stage, before you buy lumber, because it tells you the shortest board you can buy. It also helps you check whether a given rise and run combination will meet typical building guidelines (like a 7-inch max riser and 10-inch min tread, though codes vary).

You might also use it to compare different step counts. For example, if you have a 36-inch rise, using 5 steps gives a 7.2-inch riser (close to the common 7-inch max), while 6 steps gives a 6-inch riser – more comfortable but requiring more horizontal run.

How to Choose Your Inputs

Before you enter numbers, you need to know your actual measurements:

• Measure the total rise from the finished surface below to the finished surface above. If you are building on a concrete pad, measure from the top of the pad. If the upper floor has carpet or tile, measure to that finished height, not the subfloor.
• Decide the number of steps – this is often dictated by code (max riser height). Divide total rise by your desired riser height (e.g., 7 inches) and round up to get the number of steps.
• Calculate the total run = number of steps × tread depth (the horizontal part you step on). For example, 6 steps with a 10-inch tread gives 60 inches of run. Some codes require a nosing overhang, which adds a little length.

If you only know the rise and a desired riser height, you can compute steps = rise / riser_height, then run = steps × tread_depth. The calculator assumes you already have those three numbers.

Common Mistakes and Misconceptions

The biggest mistake is confusing total run with the horizontal length of the stairway opening. The total run is measured from the face of the first riser to the face of the last riser – not including any landing or the thickness of the upper floor. Another frequent error is forgetting to account for the thickness of the tread material when cutting notches; the actual stringer cutout needs to be adjusted so the finished tread depth meets code.

A common misconception is that the stringer length equals the sum of the rise and run. It does not – it is the hypotenuse, which is always shorter than the sum of the two legs. For a 36-inch rise and 48-inch run, the sum is 84 inches, but the stringer is only 60 inches.

Also, this calculator gives the straight-line length of the board. When you cut notches (the triangular cutouts that hold the treads and risers), you lose material at each notch. The actual board you need is usually the stringer length plus a few inches for the top and bottom connections, and you may need to buy a longer board to have enough material for the notches.

Limits of the Formula

This calculation assumes a perfectly straight, single-flight stair with no landings, no winders, and no curves. It also assumes the stringer runs at a constant angle, which is true for a straight stair. If your stair has a landing in the middle, you need to calculate each flight separately and add the lengths. If you have a curved or spiral stair, the geometry is far more complex and this simple triangle does not apply.

The formula also ignores the thickness of the stringer itself and any structural requirements like the minimum board size (often 2×12) and the need for support at mid-span. Always consult local building codes and a structural engineer for anything beyond a simple residential step.

Safety and Building Codes

Stair building is one of the most safety-critical parts of construction. A poorly built stair can cause falls. Even if your stringer length is correct, you must follow code for:

• Riser height – typically between 4 and 7.75 inches, with all risers in a flight within 3/8 inch of each other.
• Tread depth – usually at least 10 inches, measured from nosing to nosing.
• Handrails – required for stairs with more than a few steps, with specific height and graspability rules.
• Headroom – usually at least 6 feet 8 inches from the nosing line to the ceiling.

This calculator only gives you the diagonal length. It does not tell you if your rise and run meet code. Always verify with your local building department before cutting lumber.

Alternatives and Related Tools

If you are designing a full stair, you might also want to calculate the number of steps from rise and desired riser height, or the total run from step count and tread depth. Some calculators combine all of that. This one is focused on the stringer length, so you can use it alongside a stair calculator that checks code compliance.

For a quick estimate without a calculator, you can use the 3-4-5 rule: if the rise is 3 units and the run is 4 units, the stringer is 5 units. For other ratios, use the formula above. A tape measure and a framing square are the traditional tools, but the calculator is faster and reduces arithmetic errors.

Frequently Asked Questions

Do I need to add extra length to the stringer for the notches?

Yes. The calculated length is the straight diagonal from the bottom of the first riser to the top of the last riser. The actual board needs to be longer because you cut triangular notches out of the top edge, and you also need material to attach to the top floor and bottom landing. A common practice is to add 6 to 12 inches, depending on the size of the notches and the connection details.

Can I use this for a deck stair with a landing?

No, not directly. A landing breaks the stair into two flights. You would calculate each flight separately – each with its own total rise and total run – and then add the two stringer lengths. The landing itself adds no diagonal length, but you need to account for the horizontal depth of the landing in your overall layout.

What is the difference between stringer length and stair stringer board length?

Stringer length is the diagonal distance along the slope. The board length you buy is the physical length of the 2×12 or other lumber, which must be at least the stringer length plus extra for the top and bottom connections and for the notches. In practice, you often buy a board that is a foot or more longer than the calculated stringer length.

Does the number of steps affect the stringer length?

Only indirectly. The number of steps affects the total run (if you keep the same tread depth) and the rise per step. The stringer length depends on the total rise and total run, not directly on the step count. However, if you change the number of steps, you change the run (for a given tread depth), which changes the stringer length.

Can I use metric units with this calculator?

Yes. The formula works with any unit as long as you use the same unit for rise and run. If you enter centimeters for both, the stringer length comes out in centimeters. Just be consistent – do not mix inches and feet without converting.

Is the stringer length the same as the length of the 2×12 I need?

No. The 2×12 is the board you cut the stringer from. The board must be longer than the stringer length because you need material for the top and bottom connections and for the triangular cutouts. For example, a 60-inch stringer might come from a 72-inch (6-foot) board, depending on the notch size and connection method.

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in rise | Total rise | in | 36 | 0
in run | Total run | in | 48 | 0
in steps | Number of steps | - | 6 | 1
out len | Stringer length | in | sqrt(rise^2 + run^2) | 2
out rise_step | Rise per step | in | rise / steps | 2
out run_step | Run per step | in | run / steps | 2