function toggleInputs() {
var shape = document.getElementById("tankShape").value;
var labelLength = document.getElementById("labelLength");
var labelWidth = document.getElementById("labelWidth");
var inputWidth = document.getElementById("inputWidth");
if (shape === "cylindrical") {
labelLength.innerHTML = "Tank Height/Length";
labelWidth.innerHTML = "Tank Diameter";
document.getElementById("inputDepth").parentElement.style.display = "none";
} else {
labelLength.innerHTML = "Tank Length";
labelWidth.innerHTML = "Tank Width";
document.getElementById("inputDepth").parentElement.style.display = "block";
}
}
function calculateSepticVolume() {
var shape = document.getElementById("tankShape").value;
var units = document.getElementById("unitType").value;
var valLength = parseFloat(document.getElementById("inputLength").value);
var valWidth = parseFloat(document.getElementById("inputWidth").value);
var valDepth = parseFloat(document.getElementById("inputDepth").value);
if (isNaN(valLength) || isNaN(valWidth)) {
alert("Please enter valid dimensions.");
return;
}
var volumeCuFt = 0;
if (shape === "rectangular") {
if (isNaN(valDepth)) {
alert("Please enter the liquid depth.");
return;
}
if (units === "inches") {
volumeCuFt = (valLength * valWidth * valDepth) / 1728;
} else {
volumeCuFt = valLength * valWidth * valDepth;
}
} else {
// Cylindrical: Area = PI * r^2. Length here is the height of the cylinder.
var radius = valWidth / 2;
if (units === "inches") {
volumeCuFt = (Math.PI * Math.pow(radius, 2) * valLength) / 1728;
} else {
volumeCuFt = Math.PI * Math.pow(radius, 2) * valLength;
}
}
var gallons = volumeCuFt * 7.48052;
var liters = gallons * 3.78541;
document.getElementById("resGallons").innerHTML = Math.round(gallons).toLocaleString();
document.getElementById("resCubicFeet").innerHTML = volumeCuFt.toFixed(2);
document.getElementById("resLiters").innerHTML = Math.round(liters).toLocaleString();
var recommendation = "";
if (gallons = 750 && gallons = 1200 && gallons < 1700) {
recommendation = "Commonly required for 4-5 bedroom residential homes.";
} else {
recommendation = "Large capacity system suitable for 6+ bedrooms or commercial use.";
}
document.getElementById("recommendationBox").innerHTML = "
" + recommendation;
document.getElementById("septicResult").style.display = "block";
}
How to Calculate Septic Tank Volume
Understanding the capacity of your septic tank is critical for proper maintenance and avoiding costly overflows. This calculator helps you determine the total liquid volume based on the physical dimensions of the tank.
The Mathematical Formulas
The math changes depending on whether your tank is a standard rectangular box or a cylindrical vessel:
- Rectangular Tanks: Volume = Length × Width × Liquid Depth.
- Cylindrical Tanks: Volume = π × Radius² × Length (or Height).
Note: Always measure from the floor of the tank to the bottom of the outlet pipe to get the "Liquid Depth." The actual tank height is usually greater than the liquid capacity.
Standard Sizing Requirements
Most local building codes mandate minimum septic tank sizes based on the number of bedrooms in a home, rather than the number of current occupants. This accounts for potential future use.
| Number of Bedrooms |
Recommended Min. Capacity |
| 1 – 2 Bedrooms |
750 – 1,000 Gallons |
| 3 Bedrooms |
1,000 Gallons |
| 4 Bedrooms |
1,200 – 1,500 Gallons |
| 5+ Bedrooms |
1,500+ Gallons |
Practical Example
Imagine you have a rectangular concrete tank that is 8 feet long and 5 feet wide. If the water level sits at 4.5 feet deep, the calculation would be:
8ft (L) × 5ft (W) × 4.5ft (D) = 180 Cubic Feet
180 Cubic Feet × 7.48 (Gallons per cu ft) = 1,346.4 Gallons
In this scenario, you have a roughly 1,350-gallon tank, which is more than sufficient for a standard 3 or 4-bedroom home.
Why This Matters
If your tank is too small for your household size, wastewater does not spend enough time in the tank for solids to settle and grease to float (hydraulic retention time). This leads to solids entering your drain field, which can result in total system failure—a repair that often costs between $5,000 and $15,000.