Calculating Your Leach Field Size
The size of your leach field depends largely on how much wastewater your home produces each day — which is influenced by both the size of your home and the number of people living in it. A larger household naturally uses more water, meaning it needs a bigger drain field.
For instance, a single person living in a four-bedroom house will likely need a smaller leach field than a family of five living in the same-sized home, simply because daily water usage is lower.
The good news is, you won’t need to handle these calculations yourself. A licensed septic designer or engineer will evaluate your specific situation to determine the appropriate field size. They’ll take into account key factors like how well your soil drains and the percolation rate — both of which are essential to making sure your system works properly and complies with local regulations.
To figure out how large your leach field needs to be, you’ll first need to understand your soil. The type of soil on your property—and how it behaves throughout the year—has a big impact on how well your septic system will function. One critical factor is the seasonal groundwater level. It must sit at least four feet below the bottom of your leach field trenches to ensure proper drainage and avoid contamination.
As mentioned earlier, evaluating soil quality and percolation rates is a job best handled by a professional. They have the tools and training to assess whether your property meets the necessary requirements.
That said, if you already know your soil percolation rate and you’re confident that the soil is suitable and the groundwater conditions are within acceptable limits, you can calculate the leach field size yourself. Just use our leach field size calculator to instantly estimate the size of your leach field.
How the Calculator Works
The leach field size is based on a simple formula:
Required Area (sq ft) = Flow Rate (gallons/day) ÷ Application Rate (gallons/day/sq ft)
To get your flow rate, you’ll start by estimating how much wastewater your home generates. This depends on when your house was built and the type of plumbing fixtures you have.
- If your home was built before 1979, estimate 150 gallons per day per bedroom.
- If your home has standard low-flow fixtures (toilets flushing up to 3.5 gallons, faucets and showerheads using up to 3 gallons per minute), use 130 gallons per day per bedroom.
- If you have ultra-efficient plumbing (toilets using just 1 gallon per flush), the estimate drops to 90 gallons per day per bedroom.
Multiply the gallons per day by the number of bedrooms to get your total flow rate.
Next, the application rate depends on your soil’s percolation rate — that is, how quickly water drains through it. This value (in gallons per day per square foot) is typically pulled from a percolation rate table, which we’ve referenced at the bottom of our calculator, to match your soil’s absorption speed.
Once you have both values, simply plug them into the formula — and the calculator will do the rest, giving you an estimate of how many square feet your leach field should be.
Example
Imagine you’re purchasing a 4-bedroom home built back in 1975 in a quiet neighborhood outside Maple Grove. During the inspection, it’s discovered that the existing leach field has failed, and you’ll need to install a new absorption field as part of the septic system upgrade.
You bring in a licensed soil specialist, and after conducting a percolation test, they confirm that the soil and groundwater conditions are acceptable. The test results show a percolation rate of 24 minutes per inch. How big should your leach field be?
Answer
Since the home was constructed before 1979, the estimated daily wastewater flow is calculated using the older standard of 150 gallons per bedroom. With 4 bedrooms, that’s:
4 × 150 = 600 gallons per day
Based on a 24-minute percolation rate, the application rate from the state guidelines is 0.6 gallons per day per square foot. To find the total trench area needed, you divide the total daily flow by the application rate:
600 ÷ 0.6 = 1,000 square feet
So, for this house, you’ll need 1,000 square feet of absorption trench area to meet septic system requirements.