Slow (up to 10 min/inch)
Moderate (11-20 min/inch)
Moderately Fast (21-30 min/inch)
Fast (31-40 min/inch)
Very Fast (41-50 min/inch)
Conventional (Gravel/Chamber Trench)
Mound System
Drip Distribution
Aerobic Treatment Unit (ATU)
Your estimated septic system size will be displayed here.
Understanding Septic System Sizing
A properly sized septic system is crucial for protecting public health and the environment by effectively treating household wastewater. An undersized system can fail prematurely, leading to sewage backups and groundwater contamination, while an oversized system can be unnecessarily expensive.
Key Factors in Septic System Sizing
The size of a septic system is primarily determined by the amount of wastewater generated by a household and the soil's ability to absorb and treat that wastewater. Our calculator considers the following key factors:
Number of Bedrooms: This is the most common proxy for estimating the potential number of occupants in a home. Building codes and health departments often use the number of bedrooms to determine the minimum required treatment capacity, as it represents the maximum foreseeable occupancy.
Estimated Daily Wastewater Flow: While the number of bedrooms is a good starting point, the actual daily wastewater flow is a more direct measure. The provided value of 75 gallons per person per day is a widely used assumption for typical residential use, but actual usage can vary significantly based on habits, appliance efficiency, and the number of people actually living in the home.
Soil Percolation Rate (Perk Rate): This measures how quickly water drains through the soil. A slower percolation rate (higher minutes per inch) means the soil is less permeable and requires a larger absorption area (drainfield) to prevent waterlogging and system failure. A faster rate means the soil can handle more liquid in a smaller area.
Septic System Type: Different system designs have varying treatment efficiencies and dispersal capabilities. For instance:
Conventional Systems: These are the most common and rely on gravity to distribute effluent into a gravel-filled trench or chambers. They require ample space and suitable soil.
Mound Systems: Used when the water table is high or the soil is very slow-draining, these systems elevate the drainfield above the natural soil surface.
Drip Distribution Systems: These use a network of small-diameter tubing to distribute effluent over a large area at a shallow depth, offering efficient dispersal.
Aerobic Treatment Units (ATU): These advanced systems introduce oxygen to the wastewater, significantly improving treatment before it reaches the drainfield, making them suitable for challenging site conditions.
How the Calculator Works (Simplified Logic)
The calculation involves estimating the total daily wastewater flow and then determining the required drainfield size based on the soil's percolation rate and the chosen system type. A common approach is as follows:
Calculate Maximum Daily Flow:
Maximum Daily Flow (gallons) = Number of Bedrooms * Daily Wastewater Flow per Person
For example, with 3 bedrooms and 75 gallons/person/day:
3 bedrooms * 75 gallons/bedroom = 225 gallons/day
Determine Required Drainfield Area: The required area is inversely proportional to the soil's percolation rate. Faster percolating soils need smaller areas. Different system types also have different loading rates (gallons per square foot).
Required Drainfield Area (sq ft) = Maximum Daily Flow (gallons) / Loading Rate (gallons/sq ft/day)
The Loading Rate is derived from the soil percolation rate and the system type. For instance:
A Moderate percolation rate (20 min/inch) might have a standard loading rate of 0.5 gallons/sq ft/day for a conventional system.
A Fast rate (40 min/inch) might allow a higher loading rate like 0.8 gallons/sq ft/day.
Mound or Drip systems might have different loading rates, often allowing for slightly higher flows in less ideal soils due to their design.
ATUs significantly improve effluent quality, often allowing for smaller drainfields or use in areas where other systems wouldn't be permitted.
Our calculator uses simplified industry standards for loading rates based on the selected inputs.
Disclaimer
This calculator provides an *estimate* only. Local health department regulations, site-specific soil testing (like a formal percolation test), topography, and the presence of other environmental factors (like wells or surface water) are critical for determining the final septic system design and size. Always consult with a qualified septic system designer or installer and your local health authority before making any decisions.
function calculateSepticSize() {
var bedrooms = parseFloat(document.getElementById("bedrooms").value);
var dailyWastewater = parseFloat(document.getElementById("dailyWastewater").value);
var soilPercolation = parseFloat(document.getElementById("soilPercolation").value);
var systemType = document.getElementById("systemType").value;
var resultElement = document.getElementById("result");
// Clear previous results and error messages
resultElement.innerHTML = "";
// Input validation
if (isNaN(bedrooms) || bedrooms <= 0) {
resultElement.innerHTML = "Please enter a valid number of bedrooms (at least 1).";
return;
}
if (isNaN(dailyWastewater) || dailyWastewater <= 0) {
resultElement.innerHTML = "Please enter a valid daily wastewater flow (at least 1 gallon/person/day).";
return;
}
if (isNaN(soilPercolation) || soilPercolation <= 0) {
resultElement.innerHTML = "Please select a valid soil percolation rate.";
return;
}
// — Calculation Logic —
// 1. Calculate Maximum Daily Wastewater Flow (based on bedrooms)
// This is a common regulatory starting point.
var maxDailyFlow = bedrooms * dailyWastewater; // gallons/day
// 2. Determine Soil Loading Rate (gallons per square foot per day) based on percolation and system type
// These are simplified values. Actual rates are more complex and depend on specific soil horizons and local codes.
var loadingRate;
var loadingRateUnits = "gallons/sq ft/day";
switch (systemType) {
case "conventional":
if (soilPercolation <= 10) { // Slow
loadingRate = 0.2;
} else if (soilPercolation <= 20) { // Moderate
loadingRate = 0.4;
} else if (soilPercolation <= 30) { // Moderately Fast
loadingRate = 0.6;
} else if (soilPercolation <= 40) { // Fast
loadingRate = 0.8;
} else { // Very Fast
loadingRate = 1.0;
}
break;
case "mound":
// Mound systems have specific design requirements. Loading rates can be similar or slightly higher than conventional in some cases, but area calculation is more complex.
// For simplification, we'll use a slightly more permissive rate assuming the mound design addresses limitations.
if (soilPercolation <= 10) { // Slow
loadingRate = 0.3;
} else if (soilPercolation <= 20) { // Moderate
loadingRate = 0.5;
} else if (soilPercolation <= 30) { // Moderately Fast
loadingRate = 0.7;
} else { // Fast/Very Fast (less critical for mound design benefits)
loadingRate = 0.9;
}
break;
case "drip":
// Drip systems often allow for higher loading rates due to fine distribution and shallow depth.
if (soilPercolation <= 10) { // Slow
loadingRate = 0.5;
} else if (soilPercolation <= 20) { // Moderate
loadingRate = 0.7;
} else { // Moderately Fast and faster
loadingRate = 1.0;
}
break;
case "aerobic":
// ATUs produce high-quality effluent, allowing for much smaller drainfields or alternative dispersal methods.
// The drainfield size calculation for ATUs can be significantly different and often based on effluent quality standards rather than simple perk rates.
// For simplification, we'll use a high loading rate, but emphasize that ATU design is specialized.
loadingRate = 1.5; // High loading rate for ATU effluent
break;
default:
resultElement.innerHTML = "Invalid system type selected.";
return;
}
// 3. Calculate Required Drainfield Area
var requiredArea = maxDailyFlow / loadingRate; // sq ft
// Round up to nearest practical unit or standard size if needed, but for now, just display calculated area.
var roundedArea = Math.ceil(requiredArea);
// — Display Results —
resultElement.innerHTML = "Estimated Daily Flow: " + maxDailyFlow.toFixed(0) + " gallons";
resultElement.innerHTML += "Required Drainfield Size: " + roundedArea.toFixed(0) + " sq ft";
resultElement.innerHTML += "(Based on " + loadingRate.toFixed(1) + " " + loadingRateUnits + " for " + systemType + " system)";
// Add specific notes based on system type if desired (optional enhancement)
if (systemType === "aerobic") {
resultElement.innerHTML += "Note: ATU systems require specific maintenance and permits. Drainfield size may vary significantly based on local regulations and effluent testing.";
} else if (systemType === "mound") {
resultElement.innerHTML += "Note: Mound systems require specialized design and construction.";
}
}