function calculateStairs() {
var totalRise = parseFloat(document.getElementById('totalRise').value);
var targetRiserHeight = parseFloat(document.getElementById('targetRiserHeight').value);
var treadDepth = parseFloat(document.getElementById('treadDepth').value);
if (isNaN(totalRise) || isNaN(targetRiserHeight) || isNaN(treadDepth) || totalRise <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Number of steps (risers) – round to nearest whole number
var numRisers = Math.round(totalRise / targetRiserHeight);
if (numRisers < 1) numRisers = 1;
// Actual height of each riser
var actualRiserHeight = totalRise / numRisers;
// Number of treads is usually risers minus 1
var numTreads = numRisers – 1;
// Total horizontal run
var totalRun = numTreads * treadDepth;
// Stringer length using Pythagorean theorem: a^2 + b^2 = c^2
var stringerLength = Math.sqrt(Math.pow(totalRise, 2) + Math.pow(totalRun, 2));
// Calculate angle in degrees
var angleRad = Math.atan(actualRiserHeight / treadDepth);
var angleDeg = angleRad * (180 / Math.PI);
// Display results
document.getElementById('resNumRisers').innerText = numRisers;
document.getElementById('resActualRiser').innerText = actualRiserHeight.toFixed(3) + '"';
document.getElementById('resNumTreads').innerText = numTreads;
document.getElementById('resTotalRun').innerText = totalRun.toFixed(2) + '"';
document.getElementById('resStringerLen').innerText = stringerLength.toFixed(2) + '"';
document.getElementById('resAngle').innerText = angleDeg.toFixed(2) + '°';
document.getElementById('stairResults').style.display = 'block';
}
Professional Stair Stringer Calculation Guide
Building a set of stairs for a deck, porch, or interior renovation is one of the most challenging tasks for DIYers and professionals alike. Precision is paramount; a discrepancy of even a quarter-inch can create a trip hazard. This Stair Stringer Calculator simplifies the geometry involved in laying out a stringer accurately.
Understanding Stair Terminology
Before you begin your layout, it is essential to understand the core components of stair construction:
Total Rise: The vertical distance measured from the finished lower floor to the finished upper floor. This must be exact.
Total Run: The horizontal distance the staircase will occupy from the start of the first step to the end of the last tread.
Riser Height: The vertical distance between each step. Building codes usually require this to be between 7 and 7.75 inches.
Tread Depth: The horizontal surface you step on. Standard codes usually require at least 10 inches.
Stringer: The structural member (usually a 2×12 board) that supports the treads and risers.
How the Math Works
Our calculator uses the Total Rise and your Target Riser Height to determine the exact geometry. The formula follows these steps:
Step Count: We divide the Total Rise by your target height (e.g., 7.5″) and round to the nearest whole number to get the total number of risers.
Exact Riser Height: We divide the Total Rise by that whole number of steps to ensure every single step is identical in height.
Horizontal Run: Since the top of the stringer usually mounts below the upper floor, there is typically one fewer tread than there are risers.
Stringer Length: Using the Pythagorean theorem (a² + b² = c²), where 'a' is Total Rise and 'b' is Total Run, we find the "bridge" length or the minimum board length needed for the stringer.
Standard Building Codes (The 7-11 Rule)
While local codes vary, the industry standard is often referred to as the "7-11 rule." This suggests a maximum riser height of 7 inches and a minimum tread depth of 11 inches. In many residential jurisdictions, a riser height up to 7 3/4 inches is permitted. Always check with your local building department before cutting lumber.
Real-World Example
Suppose you are building a deck and the distance from the ground to the top of the deck boards is 42 inches. Using a target riser height of 7 inches:
42 / 7 = 6 Risers.
Since we have 6 risers, we will have 5 treads.
If we choose a 10-inch tread depth, our Total Run is 50 inches (5 x 10).
The Stringer Length is the square root of (42² + 50²), which is approximately 65.3 inches. You would need a 6-foot (72″) 2×12 board to cut this stringer.
Pro Tips for Cutting Stringers
When marking your 2×12 board, use stair gauges on a framing square. These small brass clamps lock onto the square at your specific rise and run measurements, ensuring every step you mark is identical. Remember to "drop the stringer" by subtracting the thickness of your tread material from the very bottom riser height so the first step remains the same height as the others once the tread is installed.