Septic Tank Size Calculator

Find a planning estimate for the right septic tank size for your home, based on bedrooms, occupants, soil type and garbage disposal use. No signup, no cost.

Find a planning estimate for your home in under a minute.

How many bedrooms?

Bedrooms drive the code minimum — even if fewer people live there. For 7+ bedrooms, choose Custom and enter the number.

Occupants

Soil type

Garbage disposal used regularly?

Planning estimate only. Method: bedroom minimums commonly seen in many US codes (1 bed: 750 gal, 2 bed: 750–1,000 gal, 3 bed: 1,000 gal, 4 bed: 1,250 gal, 5 bed: 1,500 gal, 6 bed: 1,750 gal; for 7+ bedrooms, add 250 gal per bedroom over 6), checked against a 150 gallons per bedroom per day design flow. Drainfield size needs a percolation test. County and state rules govern.

Disclaimer: All results are estimates for planning only. County and state rules govern — check with your local authority.

How to use this calculator

Choose the number of bedrooms. For 7 or more bedrooms, choose Custom and enter the number.
Enter how many people live in the home, choose the soil type, and say whether a garbage disposal is used regularly.
Click Calculate. Read the tank size in gallons, the design flow check, and the drainfield range.

How the result is calculated

The tool uses common US code minimums as a planning guide: 1 bedroom 750 gallons, 2 bedrooms 1,000 gallons, 3 bedrooms 1,000 gallons, 4 bedrooms 1,250 gallons, 5 bedrooms 1,500 gallons, 6 bedrooms 1,750 gallons, and for 7 or more bedrooms 1,750 + (bedrooms – 6) x 250 gallons. Design flow is checked as bedrooms x 150 gallons per day.

Worked example: 3 bedrooms, 4 people, Loamy soil, no disposal = 1,000 gallons, 450 gallons per day, Mid range drainfield.

Frequently asked questions

Is 1,000 gallons enough for a 3-bedroom home?

As a planning minimum, yes in this guide – but the final size is set by your county or state rules, and by occupants, disposal use, and soil. Always confirm before you build or replace a system.

Why bedrooms, not just people?

Bedrooms reflect the home’s potential occupancy. A home can gain occupants later, so code minimums are based on bedrooms even if fewer people live there now.

Does a garbage disposal change the size?

It can. Regular disposal use adds solids and can mean a larger tank or a shorter pumping interval. Confirm this with your county if you use one regularly.

Why does clay soil need a larger drainfield?

Clay drains more slowly than sandy or loamy soil, so the drainfield usually needs to be larger. A percolation test is needed to size a drainfield properly.

Does this calculator replace county approval?

No. This is a planning estimate only. County and state rules govern, and a professional must confirm the final design.

When to call a professional

Call a licensed septic professional for a new install, a replacement, slow drains, odours, wet or surfacing areas, alarms, or a percolation test. Do not guess a drainfield size from this tool alone.