Here is a spreadsheet that I have been using for awhile and have finally made it available for use.
You will need to know the following information to properly fill it in:
The Percolation Rate or “T time” of the soil. That is the amount of time it takes for water to drop 1 cm in a 4″ hole. Sandy soil will have a T time of 0.5 – 15, sandy silt will have a little higher T time like 25-35, and clay will have a very high T time of greater then 50 (it takes at least 50 min for water to drop 1 cm in a 4″ wide hole).
You will also need to know the number of bedrooms and what fixtures you have in your house or are planning to install in the future and the total square footage (excluding the basement).
Once you type all the info in it will give you an idea of how large your system will need to be to meet the minimum code requirements.
It is to be used as a guide only and we do not take responsibility for any mistakes or errors.
Consult a local installer or local governing authority to determine if additional requirements need to be adhered to in your area.