Ejector Pit Sizing Calculator
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Calculated Requirements
Total Fixture Units (DFU):
0
Est. Peak Inflow Rate:
0 GPM
Rec. Min Pump Capacity:
0 GPM
Active Sump Volume Needed:
0 Gallons
Tether Length / Drawdown:
0 Inches
Total Minimum Pit Depth:
0 Inches
*Total depth includes 10″ bottom buffer for pump intake and 6″ top buffer for alarm lag.
Understanding Ejector Pit Sizing
Sizing a sewage ejector pit correctly is critical to preventing plumbing backups and extending the lifespan of your sewage pump. Unlike standard sump pumps for groundwater, ejector pumps handle solid waste and require specific calculations based on the volume of wastewater entering the system (measured in Drainage Fixture Units or DFU) and the physical dimensions of the basin.
1. Drainage Fixture Units (DFU)
Plumbing codes assign a "load" value to every fixture in your home. This value, known as a Drainage Fixture Unit (DFU), helps estimate the peak volume of water that might enter the pit at once.
- Toilet: 6 DFU (High volume, solids handling required)
- Washing Machine: 3 DFU
- Shower/Bathtub: 2 DFU
- Bathroom Sink: 1 DFU
Our calculator sums these values to estimate the peak inflow rate in Gallons Per Minute (GPM). For residential sewage ejectors, even a single toilet usually necessitates a pump capable of handling 2″ solids and a minimum flow rate of 30-40 GPM.
2. The Importance of Cycle Time
One of the most common causes of pump failure is "short cycling." This occurs when the pit is too small or the tether (float switch range) is too short, causing the pump to turn on and off rapidly. Ejector pump motors rely on the fluid to cool them and need a minimum run time (typically 60 seconds) to dissipate heat effectively.
Sizing Rule: The volume of water between the "Turn On" and "Turn Off" float levels (the Active Volume) must be sufficient to keep the pump running for at least 60 seconds given its discharge rate.
3. Calculating Basin Depth
Once you know the required active volume (in gallons) to achieve a 60-second runtime, you must determine how much vertical space that volume occupies in your pit. This is determined by the diameter of the pit.
For example, in a standard 18-inch diameter pit, 1 gallon of water takes up approximately 0.9 inches of depth. In a wider 24-inch pit, 1 gallon only takes up about 0.5 inches. Wider pits allow for shorter vertical draws, which can be beneficial in basements with limited digging depth.
4. Total Depth Requirements
The total depth of your ejector pit isn't just the active water volume. You must account for:
- Bottom Buffer (10-12 inches): Space for the pump body and to prevent the pump from sucking in sediment or running dry.
- Active Volume (Drawdown): The height calculated based on pump runtime.
- Top Buffer/Lag (6-8 inches): Space above the "On" float for an alarm float and the inlet pipe connection.
function calculatePit() {
// 1. Get Inputs
var toilets = parseFloat(document.getElementById('toilets').value) || 0;
var showers = parseFloat(document.getElementById('showers').value) || 0;
var sinks = parseFloat(document.getElementById('sinks').value) || 0;
var laundry = parseFloat(document.getElementById('laundry').value) || 0;
var diameter = parseFloat(document.getElementById('pitDiameter').value);
var cycleTime = parseFloat(document.getElementById('cycleTime').value) || 60;
// 2. Calculate DFU (Drainage Fixture Units)
// Values: Toilet=6, Shower=2, Sink=1, Laundry=3
var totalDFU = (toilets * 6) + (showers * 2) + (sinks * 1) + (laundry * 3);
// 3. Estimate Peak Inflow GPM
// Rough plumbing estimation:
// If 10 DFU, usually calculated via Hunter's Curve.
// For residential ejectors, if there is a toilet, the pump is usually sized
// for solids handling which generally starts at 40-60 GPM capability.
// However, we need Inflow to determine capacity.
// Simplified Logic: Base 10 GPM + (DFU * 0.8).
// If Toilets > 0, Min Inflow consideration is higher for simultaneous flush.
var estInflowGPM = 0;
if (totalDFU > 0) {
estInflowGPM = 10 + (totalDFU * 0.5);
}
// 4. Recommend Pump Capacity
// Pump should discharge faster than inflow to prevent overflow,
// but typically residential pumps are 4/10 HP or 1/2 HP delivering ~60-80 GPM at low head.
// We will assume a required Pump Capacity of at least 1.5x Inflow or Minimum 30 GPM for sewage.
var recPumpGPM = Math.max(estInflowGPM * 1.2, (toilets > 0 ? 30 : 15));
// 5. Calculate Required Active Volume (Gallons)
// Volume = Pump GPM * (Run Time Minutes)
// This ensures the pump runs for the full cycle time to empty the pit.
var requiredVolume = recPumpGPM * (cycleTime / 60);
// 6. Calculate Geometry (Cylinder)
// V (gallons) to V (cubic inches). 1 Gallon = 231 cubic inches.
var volumeCuIn = requiredVolume * 231;
// Area = pi * r^2
var radius = diameter / 2;
var area = Math.PI * Math.pow(radius, 2);
// Height (Drawdown) = Volume / Area
var drawdownHeight = volumeCuIn / area;
// 7. Calculate Total Depth
// Bottom buffer (pump height/sediment) ~ 10 inches
// Top buffer (inlet lag/alarm) ~ 6 inches
var bottomBuffer = 10;
var topBuffer = 6;
var totalDepth = drawdownHeight + bottomBuffer + topBuffer;
// 8. Display Results
document.getElementById('resDFU').innerText = totalDFU.toFixed(1);
document.getElementById('resGPM').innerText = Math.round(estInflowGPM) + " GPM";
document.getElementById('resPumpCap').innerText = Math.round(recPumpGPM) + " GPM";
document.getElementById('resVol').innerText = requiredVolume.toFixed(1) + " gal";
document.getElementById('resDrawdown').innerText = drawdownHeight.toFixed(1) + '"';
document.getElementById('resTotalDepth').innerText = Math.ceil(totalDepth) + '"';
}
// Initialize on load
calculatePit();