Fortress Stair Calculator: Steps & Rise

The Fortress stair calculator works out the number of steps, riser height, and tread depth for a set of stairs, based on the total rise (vertical height) and total run (horizontal length) you enter. You input the rise and run in inches or centimeters, choose a target riser height if you want, and the calculator returns the number of steps, the exact riser height, and the tread depth. This page explains the geometry behind the calculation, how to use it for a Fortress brand stair stringer or any straight staircase, and what to watch out for.

Formula

The core formula is simple: number of steps = total rise ÷ target riser height, rounded to the nearest whole number. Then the actual riser height is total rise ÷ number of steps, and the tread depth is total run ÷ (number of steps − 1).

• Total rise (R) – vertical distance from finished floor to finished floor, in inches (or cm).
• Total run (L) – horizontal distance the stairs cover, from the face of the first step to the face of the last step, in inches (or cm).
• Target riser height (T) – your desired riser height, typically 7 inches (18 cm) for residential stairs.
• Number of steps (N) – the count of individual risers.
• Tread depth (D) – the horizontal depth of each step, not including any nosing overhang.

The angle of the staircase is atan(rise / run), expressed in degrees. That angle tells you how steep the stairs are.

Worked Example

Using the defaults from the calculator: total rise = 105 in, total run = 120 in, target riser = 7 in.

1. Number of steps = 105 ÷ 7 = 15.0 → round to 15 steps.
2. Actual riser height = 105 ÷ 15 = 7.00 in.
3. Tread depth = 120 ÷ (15 − 1) = 120 ÷ 14 = 8.57 in.
4. Stair angle = atan(105 / 120) = atan(0.875) ≈ 41.2°.

The calculator shows 15 steps, a 7.00-inch riser, an 8.57-inch tread, and a 41.2° angle. That matches the formula output.

Why Fortress Stairs Need a Specific Calculation

Fortress is a brand of aluminum and steel stair stringers and railing systems. Their pre-engineered stringers are often sold in fixed lengths with a set number of steps, so you don’t cut them yourself. That means you need to know exactly how many steps your height requires before you order. The calculator above gives you that number, but you must also check the manufacturer’s catalog for the specific stringer lengths and step counts they offer. For example, if your rise calls for 15 steps, you need a stringer that has 15 risers, not 14 or 16.

Fortress stringers typically have a fixed tread depth (often around 10 or 11 inches) and a fixed riser height (often 7 inches). If your total rise doesn’t divide evenly by 7, you may need to adjust the bottom step or use a different stringer length. The calculator helps you see that mismatch before you buy.

How to Measure Total Rise and Run

Accurate measurement is the most important part of any stair calculation. Here’s how to do it:

• Total rise: Measure vertically from the finished floor at the bottom to the finished floor at the top. If you have a landing, measure to the top of the landing surface.
• Total run: Measure horizontally from the face of the bottom step (where the tread begins) to the face of the top step (where the last tread ends). For a straight run, this is the horizontal distance between the two floor edges.
• Use a level and a tape measure. For a tall staircase, use a straight board and a level to transfer the height down.
• Always measure in the same unit (inches or centimeters). Mixing feet and inches is a common error.

Common Mistakes and How to Avoid Them

People often make these errors when calculating stairs:

• Forgetting the last tread: The number of treads is one less than the number of steps. If you have 15 risers, you have 14 treads (the top floor is not a tread).
• Using the wrong rise: The rise must be from finished floor to finished floor, not from the bottom of the stringer to the top. If you have thick flooring or a landing, include that thickness.
• Rounding too early: Round the number of steps first, then compute the riser height. If you round the riser height first, you’ll get a wrong step count.
• Ignoring building codes: Many areas have maximum riser height (often 7.75 in) and minimum tread depth (often 10 in). The calculator gives you the numbers, but you must check local codes.

Limitations of the Calculator

This calculator assumes a straight, single-flight staircase with uniform risers and treads. It does not account for:

• Landings or turns – those require separate calculations.
• Nosing overhang – tread depth in the calculator is the horizontal depth, not including the nosing (the protruding edge).
• Stringer thickness – if you use a pre-cut stringer, the actual tread depth may be different due to the stringer’s design.
• Headroom clearance – you need to verify that the stairs don’t hit the ceiling above.

For a Fortress system, always cross-check the calculator’s step count with the manufacturer’s stringer options. The calculator gives you the theoretical number, but the actual product may come in fixed step counts.

Frequently Asked Questions

What is a good riser height for residential stairs?

Most building codes allow riser heights between 4 and 7.75 inches (10–20 cm), with 7 inches (18 cm) being a common target for comfort. The calculator uses 7 inches as the default, but you can adjust it to match your local code or personal preference.

How do I calculate the number of steps for a Fortress stringer?

Measure the total rise from finished floor to finished floor. Divide that by the riser height of the Fortress stringer you plan to use (often 7 inches). Round to the nearest whole number. That’s your step count. Then check that the stringer you order has that many steps.

What if my rise doesn’t divide evenly by 7?

That’s normal. You can adjust the target riser height slightly (e.g., 7.1 inches) to make the division come out even. The calculator shows the actual riser height after rounding, so you can see the exact value. If the actual riser is within code limits, it’s fine.

Why is the tread depth one less than the number of steps?

Because the top floor serves as the last “tread.� If you have 15 risers, you have 15 steps up, but only 14 actual treads (the 15th step is the upper floor). The run is divided by steps minus 1 to get the tread depth.

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

No, this calculator is for a single straight flight. If you have a landing, you need to split the total rise into two separate flights and calculate each one independently. The landing itself adds a tread and a riser, so it’s more complex.

What is the maximum stair angle for safety?

Typical building codes limit stair angle to about 45 degrees, but most comfortable stairs are between 30 and 40 degrees. The calculator gives you the angle, so you can check if your stairs are too steep. If the angle is above 45°, you likely need a longer run.

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in rise | Total rise | in | 105 | 0
in run | Total run | in | 120 | 0
in target | Target riser height | in | 7 | 0
out steps | Number of steps | - | round(rise / target) | 0
out riser | Actual riser height | in | rise / steps | 2
out tread | Tread depth | in | run / (steps - 1) | 2
out angle | Stair angle | deg | atand(rise / run) | 1