How Big Of A Septic Tank Do I Need?

Septic systems are the most common type of sewage treatment for people living off of municipal or communal sewage systems.  The treatment of sewage is necessary even for people going “off grid”.  Most, and probably all jurisdictions in North America have some requirements for sewage treatment.  Treating sewage is also significantly better for the environment as exposure to untreated waste water is a common way to spread disease in humans and other animals.  Septic systems break down the organic components in sewage and provide water that is safe to be released into a form of biological treatment.  This is usually soil, in the form of a drain field.  I frequently get asked how large a septic tank is needed for someone installing or upgrading their waste management system. How large a tank needs to be ultimately depends on how much water will be put through it.

Predicting how much water will enter your septic tank will can be simple, or it can be very difficult but it always starts with your water use. To estimate your water usage there are some things you will need to know.
How many people are in your household? How many people are usually in your house and on your system?  This includes visitors which only visit once a year. How much water are you currently using? If you have a water bill now you can see it easily.  The water you use day to day becomes the waste water you have to deal with later.  The age of people in your household will play a factor.  Even if you are good at conserving water, children will waste a lot more water and they require more water in the form of bathing and laundry.  Both of those traits will increase the demand on your septic system when many kids are around.  Larger septic tanks are required for people not used to conserving water, when choosing your tank size, try to remember, most people are horrible at conserving water.

Ok, here are some guidelines for determining the size of the tank required.  The smallest tank size allowed in some jurisdictions is one thousand gallons.  A one thousand gallon tank can handle around 600 gallons of sewage per day.  In terms of percentages, a septic tank should he 40% larger than the flow of sewage into it, or the sewage flow should not be greater than 60% of the tank capacity.

What if you do not know how much water people are using or how much waste water you are creating?  You can approximate demand through the size of your house.  The larger the square footage the larger the septic tank required.  The number of bedrooms is another way to determine the size of tank required, by indirectly measuring how many residents will be living there.  More bedrooms means the builder anticipated a certain number of residents.  Room counting isn’t the ideal method.  Some rooms have multiple occupants,  the master bedroom usually has two people sleeping in it, and many children’s rooms contain two or more children.

Like I stated before, the best method is to determine and measure your water use.  Below is a table of common water usages and the resulting amounts the average person in North America will use in a day.  (The data came from http://inspectapedia.com/septic/wateruse.htm)

Table 1. Home and Outdoor Living Water Requirements
Use Flow Rate
GPM
Total Use – U.S. Gals.
Adult or child - 50-100/day
Baby - 100/day
Automatic washer 5 30-50/load
Dishwasher 2 7-15/load
Garbage Disposer 3 4-6/day
Kitchen sink (a) 3 2-4/use
Shower or tub 5 25-60/use
Toilet flush 3 4-7/use
Bathroom lavatory sink 2 1-2/use
Water softener regeneration 5 50/100/cycle
Backwash filters 10 100-200/backwashing
Outside hose faucet 5 depends on run time
Reverse Osmosis water treatment * 3-10 gallons of wastewater per gallon of treated water produced

First count the number of times you do each activity and for how long.  Take the gallons per minute flow rate (GPM) and multiply by the total number of minutes.  This gives you how much water was used per activity type.  Add all the types together and you will have a great approximation of your waste water production.  Add in 100 gallons per day per person (or more to err on the side of caution).  Once you have calculated you sewage production, and have considered many of the variables discussed above, add another twenty percent to your total.  This is your buffer for the unexpected.  It could be an unexpected house guest, or a very large gathering at you home something that is out of the ordinary.  Determining the size of the tank involves planning for some unexpected or unlikely events, remember the tank has an approximate lifespan of twenty-five  years.  A lot can and will happen over the next twenty five years.

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22 thoughts on “How Big Of A Septic Tank Do I Need?

  1. Pingback: How Big of a Septic Tank do I Need? | Preparedness Blogs

    • The age of home doesn’t matter very much. What matters is size of home and/or how many people are in the home.
      Next time you have your tank pumped out. Ask the guy how big the tank is.

  2. I am buying a 4 bedroom home , i know nothing about septic they are putting in a new 1250 gallons tank is that acceptable for the 1800 sq foot home that we are purchasing

    • That size is adequate for a four bedroom house. The minimum reccommended size for a four bedroom house is 1200 gal, so 1250 gives you a little more room (but not much)

    • I’m not sure what the requirements are in India, however 200 us gallons per person is a good idea. Dont forget about guests. If you frequently have more people then include them in the calculation.

  3. I am buying 3 bed room house 1120 sq feet its have one bath and tank size is 1000 sq ft can I add other one or two baths.

  4. I have a 3 bed 2 bath house with a 1bed 1 bath guest house. It is sitting on a 1000 gallon septic right now. We were told we need to upgrade to a 1250, but looking at our water bill we average at about 150-175 gal/day. Do we really need a bigger tank?

  5. supposed you are going to use a graywater system along with the septic system how much longer would that prolong intervals of pumping?

    • If by pumping you mean sludge pumping? Then it won’t increase the interval significantly because most solids will come from the sewage as opposed to the greywater.

  6. I am buying a house advertised as 5 bedrooms ( and priced as such)

    However, septic inspection has come back to say it has only a 1000 gallon tank

    There will be 4 adults all of the time and occasionally an extra 1 or 2 staying for maybe a few weeks at a time

    House was originally built in 1965 as 3 beds with another 2 added later ( around 1970) but I guess the septic was not considered or upgraded at that time

    This leaves me concerned obviously in terms of capacity but as well as longer term – not compliant potentially with local requirements as well as the difficulty I would have selling

    I am waiting to hear back from the seller, but don’t think this is going to end well

    Any views on this at all please?

    Thanks

    Paul

    • I am not surprised that the septic system was not upgraded when the addition was added to the house. They are either forgotten because they are hidden underground or ignored due to cost. Check the local regulations for houses built when your house was built. It may only be a cost you need to incur when you need to replace the system. I have also seen it possible and legal to replace an inadequate septic system if you are installing an exact replacement.

    • Also check the age of they system. 1965 is the pushing to upper range for the life of a septic system. If it has not been replaced since 1965, have it inspected. If it needs replacing then subtract that cost from the price you offer.

  7. Thanks for the suggestions – appreciated

    Hoping to go the the town planning dept next week and will update you

    The inspection just carried out says the tank is 1000 gallons and appears to be operating ok

    However, only 2 people living there for several years, so no real impact

    We immediately have 4 plus visitors and guests

    Everything I have read says you cannot advertise a property as 5 beds when you only have septic capacity for 3 beds

    As much as we want the house, I think this will be a big problem for us

    Good point on the age of the system as well

    Unless the seller is going to replace the system to a 5 bed unit, then the deal is likely dead…………..and don’t think he will

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