Septic Tank Size Calculator

Septic tank size calculator: a lookup by bedrooms

A septic tank size calculator looks up the typical minimum tank size for the number of bedrooms in the house. A 3-bedroom home is usually listed at 1,000 gallons, a 4-bedroom at 1,200, and a 5-bedroom at 1,500. Above 6 bedrooms it adds a flat step per bedroom. Your local health department sets the actual rule.

Bedrooms are used because the wastewater a house can produce depends on how many people the house is built to hold.

How the lookup works

You enter the number of bedrooms. The calculator rounds it to a whole number and returns the matching tank size from the table below. A 1-bedroom house gets the 2-bedroom size of 1,000 gallons, with a note that it is the smallest tank most codes allow. A house with 7 or more bedrooms starts from the 6-bedroom size of 1,500 gallons and adds 250 gallons for each bedroom past 6.

Worked example, a three-bedroom house

A new 3-bedroom home is planned on a rural lot.

• Bedrooms: 3
• Typical minimum tank: 1,000 gallons
• In cubic feet: 1,000 ÷ 7.48 = 133.7 ft³

That volume is only a starting point for budgeting. The permit office tells you the number you must meet.

Worked example, a larger house

A family builds a 5-bedroom house, including a finished basement with two bedrooms.

• Bedrooms: 5
• Typical minimum tank: 1,500 gallons
• In cubic feet: 1,500 ÷ 7.48 = 200.5 ft³

Counting the basement rooms matters. If a room could be used as a bedroom, many offices count it as one, even when the owner calls it an office. Ask the office how it counts.

Typical minimum tank by bedrooms

Bedrooms	Typical minimum tank
1	1,000 gal (smallest tank most codes allow)
2	1,000 gal
3	1,000 gal
4	1,200 gal
5	1,500 gal
6	1,500 gal
7	1,750 gal
8	2,000 gal

These are commonly cited baseline figures, not a code requirement. Your jurisdiction may demand more.

What else the permit looks at

• Soil. A percolation or soil evaluation tells the designer how fast the ground takes water, and that decides the size and type of drain field.
• Site. Distances from wells, streams, property lines and the house are set by local rules.
• Water use. Disposals, soaking tubs and water-hungry fixtures can change the requirement.
• Future plans. If you may add bedrooms later, size for them now, because a larger tank is cheaper at install time than a swap later.

Common mistakes

• Buying a tank before the permit. The office may require a different size or type.
• Counting only the rooms called bedrooms. Offices differ on what counts as one, so ask yours.
• Treating the tank as the whole system. The drain field is the bigger part of the design.
• Skipping a licensed professional. A septic system affects groundwater and public health, so a licensed designer or installer should do the design, and an inspector should check the work.
• Forgetting upkeep. Tanks need periodic inspection and pumping, and the schedule depends on the size and household.

Planning around the number

Treat the tank size as the floor for your budget. Ask the local health department or a licensed septic installer three things before you commit. What is the required minimum for your bedroom count? What kind of tank material do they allow? And does the site need a pump or an advanced treatment unit because of the soil or the slope?

Costs rise with tank size, but the tank is often a smaller share of the job than the drain field and the excavation. If you are choosing between a 1,000 and a 1,200 gallon tank and the rules allow both, ask how much extra the larger one costs installed. A bigger tank gives solids more settling time, which many owners like, though the permit still sets the minimum.

Related pages

If you need to know how many gallons a tank holds in another shape, see the tank volume calculator. The pipe volume calculator is useful for plumbing runs, and the GPM calculator helps with flow rates.