Calculate minimum required manhole diameter based on pipe size and configuration.
0° – Straight Run
Up to 45°
Up to 90°
Recommended Specifications
Min. Inside Diameter
–
Structure Type
–
Wall Thickness (Est)
–
Total Volume (Cu. Ft)
–
How to Size a Precast Manhole
Sizing a manhole correctly is critical for the structural integrity of wastewater and stormwater systems. The internal diameter must be large enough to accommodate the incoming and outgoing pipes while leaving enough concrete "wall" between openings to prevent the structure from collapsing or leaking.
Key Factors in Manhole Selection
Maximum Pipe Diameter: The larger the pipe, the larger the manhole circumference needed to maintain structural ligaments between holes.
Angle of Deflection: When pipes enter at 90-degree angles, they require significantly more horizontal space than a straight-through run.
Number of Connections: Multiple inlets into a single structure increase the risk of "honeycombing" or structural failure if the manhole is too small.
Depth: Deep manholes (usually over 12 feet) may require thicker walls and larger base slabs to handle soil and hydrostatic pressure.
Standard Sizing Reference Table
Max Pipe Diameter
Recommended MH Diameter
Standard Wall Thickness
Up to 18″
48″ (4 ft)
5″
21″ to 30″
60″ (5 ft)
6″
36″ to 42″
72″ (6 ft)
7″
48″ +
84″ – 120″ (7-10 ft)
8″ +
Engineering Rule of Thumb
A common engineering guideline for sizing manholes with multiple entries is the "Ligament Rule." There should be at least 6 to 12 inches of concrete wall remaining between any two pipe openings (measured on the inside circumference) to ensure the structure remains sound.
function calculateManholeSize() {
var pipeDia = parseFloat(document.getElementById('largestPipe').value);
var pipeCount = parseInt(document.getElementById('pipeCount').value);
var angle = parseInt(document.getElementById('maxAngle').value);
var depth = parseFloat(document.getElementById('structureDepth').value);
if (isNaN(pipeDia) || pipeDia <= 0) {
alert("Please enter a valid pipe diameter.");
return;
}
var minDia = 48; // Base standard
var wallThickness = 5;
var type = "Standard Circular";
var warning = "";
// Sizing Logic based on Largest Pipe
if (pipeDia <= 18) {
minDia = 48;
wallThickness = 5;
} else if (pipeDia <= 30) {
minDia = 60;
wallThickness = 6;
} else if (pipeDia <= 42) {
minDia = 72;
wallThickness = 7;
} else if (pipeDia <= 54) {
minDia = 84;
wallThickness = 8;
} else if (pipeDia = 4 && minDia 24 && minDia 0) {
volume = Math.PI * Math.pow(radiusFt, 2) * depth;
}
// Display results
document.getElementById('mhResultBox').style.display = 'block';
document.getElementById('resDiameter').innerText = minDia + " Inches";
document.getElementById('resType').innerText = type;
document.getElementById('resWall').innerText = wallThickness + " Inches";
document.getElementById('resVolume').innerText = volume > 0 ? volume.toFixed(2) + " ft³" : "N/A";
document.getElementById('resWarning').innerText = warning;
// Scroll to results
document.getElementById('mhResultBox').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}