How do I calculate the number of risers and tread depth?

To find the number of risers, divide the total floor-to-floor height (Total Rise) by a target riser height (e.g., 7 inches or 180 mm) and round to the nearest whole number. The exact riser height is then the Total Rise divided by that number. The number of treads is typically one less than the number of risers. An ideal tread depth can be found using ergonomic rules like the Blondel formula (2 x Riser Height + Tread Depth ≈ 25 inches).

What is the ideal riser height and tread depth?

For comfort and safety, most building codes specify a maximum riser height of around 7.75 inches (197 mm) and a minimum tread depth of 10 inches (254 mm). An ideal riser is often considered to be around 7 inches. The relationship between the two is crucial; a comfortable stair follows a rule where two risers plus one tread equals approximately 24 to 25 inches.

How do I compute stringer length?

The stair stringer length is calculated using the Pythagorean theorem. It is the hypotenuse of a right triangle formed by the Total Rise (height) and the Total Run (horizontal distance) of the staircase. The formula is: Stringer Length = √( (Total Rise)² + (Total Run)² ).

What headroom is required for stairs?

Most building codes require a minimum headroom of 6 feet 8 inches (203 cm) measured vertically from the sloped plane of the stair nosings to the ceiling or any obstruction above. Insufficient headroom is a serious safety hazard.

How do I convert between metric and imperial units?

This calculator handles unit conversions automatically. The key conversion factors are: 1 inch = 25.4 millimeters, and 1 foot = 304.8 millimeters. You can enter your measurements in either system and the results will be displayed in both for convenience.

Are these calculations code-compliant?

This calculator provides designs based on common building standards and ergonomic rules for comfort. However, it is NOT a substitute for local building codes, which can vary significantly by region. ALWAYS consult your local building authority and a licensed professional to ensure your project is fully compliant with all applicable codes before construction.

Stair Calculator — Rise, Run, Tread, Riser Count & Layout

🪜 Stair Calculator

Recommended Stair Layout

How Stairs Are Measured — Rise, Run & Slope

Designing a safe and comfortable staircase begins with understanding its core components. The two most critical measurements are the **rise** and the **run**. The **Total Rise** is the vertical distance from the surface of the lower floor to the surface of the upper floor. This is the single most important measurement for any stair calculation. From this, we can determine the individual step height, known as the **Riser Height**. The **Total Run** is the total horizontal distance the staircase will occupy. This is determined by the depth of each step, known as the **Tread Depth**, multiplied by the number of treads. The relationship between the total rise and total run defines the staircase's **Slope** or angle, a key factor in its safety and ease of use.

Building Code & Comfort Guidelines

Stair construction is strictly regulated by building codes to ensure safety. While local codes vary, they generally follow common principles for comfort and safety. A widely accepted guideline in many residential settings (like the International Residential Code - IRC) is:

Maximum Riser Height: Typically around 7.75 inches (197 mm). Taller steps can be difficult and dangerous to climb.
Minimum Tread Depth: Usually 10 inches (254 mm). This provides adequate space for foot placement.
Minimum Headroom: A clear vertical height of at least 6 feet 8 inches (203 cm) must be maintained above the nosing of all treads.

Beyond code, comfort is governed by ergonomic principles like the **Blondel Formula**, which states that **(2 x Riser Height) + Tread Depth** should equal approximately 24 to 25 inches (61-63 cm). This calculator uses these rules to suggest layouts that are not only compliant but also comfortable to use.

How This Stair Calculator Works (Formulas Explained)

This tool automates complex stair geometry calculations. It takes your Total Rise and finds the most suitable and compliant stair layouts. Here’s how it works:

Determine Riser Count: It first divides your Total Rise by a standard comfortable riser height (e.g., 7 inches) to get an approximate number of risers. It then finds the nearest whole numbers (e.g., if the result is 14.5, it will test solutions for 14, 15, and 16 risers).
Calculate Exact Riser Height: For each whole number, it calculates the precise riser height: $Riser\ Height = Total\ Rise / Number\ of\ Risers$. It checks if this height is within code limits.
Recommend Tread Depth: Using the Blondel Formula, it recommends a comfortable tread depth for each valid riser height.
Calculate Total Run & Stringer Length: With the number of treads (typically one less than risers) and tread depth, it calculates the Total Run. The stringer length is then found using the Pythagorean theorem: $Stringer\ Length = \sqrt{Total\ Rise^2 + Total\ Run^2}$.

The calculator presents the best options, allowing you to choose the layout that best fits your space and preferences.

Spiral & Winder Stairs — Special Considerations

Spiral and winder stairs are space-saving solutions but come with their own set of rules. The primary concern is the tread width. Since the treads are wedge-shaped, they are very narrow near the center post. Building codes specify a minimum tread depth at the **walking line**, which is typically measured 12 inches out from the narrow end of the tread. This ensures that a user has sufficient foot support. Due to their complexity, the design and construction of spiral or winder stairs should always be done in strict consultation with local building codes and often requires a professional fabricator.

Frequently Asked Questions

Why is the number of treads usually one less than the number of risers? Because the top "tread" is actually the landing or the floor of the upper level. The staircase itself consists of a series of steps leading up to that final surface.
What is a stair stringer? The stringer (or stringer board) is the structural backbone of the staircase. It's the long, angled board into which the risers and treads are fixed. Its length and the accuracy of its cuts are critical to the entire structure.
What is the recommended angle for a staircase? For a standard staircase, an angle between 30 and 37 degrees is generally considered safe and comfortable. Anything steeper can feel like a ladder, and anything shallower takes up an excessive amount of space.

Disclaimer: This tool provides design estimates for planning purposes ONLY. All dimensions and layouts must be verified by a licensed architect or builder. You are responsible for ensuring your construction complies with all local building codes and safety regulations before beginning any work.