What Size Septic Tank Do I Need? Sizes by Bedroom Count

A typical three-bedroom house needs a 1,000-gallon septic tank, and that is the code minimum for three bedrooms in many places. Bedroom count is the biggest driver of tank size, because codes assume each bedroom can house two people and size the system for the water they could use, not for who lives there today. But there is no national rule: each state, and sometimes each county, sets its own minimum, so the number on your permit is the one that counts.

Septic tank size by number of bedrooms

The table below shows common tank sizes. The code column is from a widely used model plumbing code; the typical column reflects what many installers set where the local rule is a 1,000-gallon minimum. Neither replaces your local permit.

Where the local minimum is 1,000 gallons, a one- or two-bedroom home still gets a 1,000-gallon tank. Some places add capacity for each bedroom past three; ask the permit office how it counts.

How the sizing works

Designers estimate the flow the home could produce on a busy day. A common planning figure is about 150 gallons per bedroom per day, so a three-bedroom home is designed for about 450 gallons a day. The tank is then sized to hold roughly two days of flow, which gives solids time to settle before the liquid moves on to the drain field. That is why a four-bedroom home lands near 1,200 gallons. These are planning figures. The permit office applies its own table, and some use higher flows.

State minimums vary: check before you buy

Do not size a tank from a chart on the internet, including this one. Rules differ in important ways:

• North Carolina sets the minimum capacity of any septic tank at 1,000 gallons, with more required for larger homes.
• Washington requires tanks with at least two compartments and sets 1,000 gallons for homes up to four bedrooms.
• South Carolina sets a 1,000-gallon minimum, which its rule treats as enough for homes of up to four bedrooms.
• Many other states set 1,000 gallons as the floor for new tanks, and some counties require more.

Rules change, and local health departments may be stricter than the state. Your state page on this site is a start; the county or local health department that issues septic permits has the final word.

One compartment or two?

A single-compartment tank is a simple box. A two-compartment tank has a wall with an opening in it that splits the tank in two. Solids settle in the first compartment, which is usually the larger one, and the liquid that moves to the second compartment is clearer. That helps keep solids away from the drain field. Some states require two compartments, or two tanks in series, and a few treat the second compartment as a way to meet the total volume. If you are buying a tank, ask which your county requires. If you already own one, a pumper needs to pump every compartment.

When to go bigger than the minimum

• A garbage disposal: food scraps add solids. Some designers add capacity, and others recommend a larger tank where a disposal is planned.
• More people than bedrooms suggest: a large household, a home office with clients, or an in-law suite.
• Room to add bedrooms later: it is cheaper to size up when the tank is installed than to dig it up.
• Heavy water use: a soaking tub, multiple dishwashers, or frequent laundry days.
• Longer gaps between pump-outs: more volume gives more room for sludge. See how often to pump a septic tank for a table by tank and household size.
• A vacation home with big weekend crowds: peaks stress a system more than the yearly average.

Going bigger does not fix a weak drain field. The tank only holds and separates; the soil does the treatment. If the field is the limit, a larger tank will not help. Our guide to drain field replacement cost covers what that costs.

How to find the size of the tank you have

• The septic permit or as-built drawing from the county or local health department.
• An old pumping receipt that lists the gallons pumped.
• A label or stamp on the tank lid or the tank itself.
• Your next pump-out: ask the pumper to measure the tank and write the size and the number of compartments on the receipt.

Older homes often have tanks smaller than today's minimums. A 750-gallon tank in a four-bedroom home is not necessarily a violation, but it will fill faster and needs more frequent pumping. The price for a pump-out is in our septic tank pumping cost guide.

What to know when you buy a tank

• Approval: tanks usually have to be on the state or county list of approved tanks. Ask for the model, the capacity, and proof of approval.
• Material: concrete is heavy and strong, while plastic and fiberglass are lighter and may need extra care in high ground water. Your installer will know what works in your area.
• Lids and risers: ask for risers that bring the lids to ground level. They cost more once and save on every pump-out.
• Effluent filter and baffles: confirm they are included.
• Inlet and outlet position: tanks are made for specific depths and pipe positions, so the installer should match the tank to your site.

When the cheap bid costs more

A tank that is too small for the bedroom count will not pass the permit, so a bid for a smaller tank is not a savings. The same goes for a tank without the filter, the risers, or the approved label, and for one set too deep or too shallow for the site. The tank is only part of the job; the field, the trenching, and the permit make up much of the price. For the whole-system picture, see the septic installation cost guide.

Red flags

• A contractor who sizes the tank without checking your permit or the bedroom count.
• A tank that is not on the approved list, or that has no capacity marking.
• A promise that a smaller or cheaper tank will "work fine" for two people.
• No permit pulled for a tank replacement.
• No license number, or one that does not match the work.

Finding a licensed pro

Installers are licensed or certified by the state or county, and the requirements differ. You can search licensed contractors near you. GroundRoster lists businesses whose license class covers septic work, checked against state and county license rosters, and does not collect or publish prices. Confirm a license with the agency that issued it before you hire, and ask the contractor to show the tank size the permit calls for. To compare real local numbers, request free quotes and we will route the job to licensed septic contractors who serve your area.

Common questions