How Big Is My Septic Tank?

Alcovy Septic hears “how big is my septic tank?” most often from two people: someone comparing pumping quotes, and someone about to sell a house. Both need the same number, and in Georgia there are three ways to get it — the state rule tells you the minimum, your county record tells you what was actually permitted, and the lids in your yard let you check.

What Georgia’s rule requires

Tank capacity in Georgia is set by Rule 511-3-1-.05 of the on-site sewage management rules. The core requirements:

So a five-bedroom house needs 1,250 gallons. A six-bedroom house needs 1,500. A four-bedroom house with a garbage disposal needs 1,500, not 1,000.

Two things follow that people get wrong. First, bathrooms don’t count — only bedrooms. Second, the rule is a floor, not a target. Installers frequently set a 1,500-gallon tank on a four-bedroom lot because the price difference is small and the pumping interval is much better. That’s exactly why you should look up your real number rather than assume 1,000.

Where your real number is written down

The authoritative answer is the septic permit record held by your county’s Environmental Health office. It lists the permitted tank capacity, the system layout, and usually a plat showing where the tank sits relative to the house.

In Gwinnett these are unusually easy to get. The county holds septic records back to 1959 and has published them free online since 2011 through the GIS Data Browser’s “Septic Tank Parcels” layer — the county’s own quick-start guide walks through it, and our step-by-step version is in how to pull your Gwinnett County septic records.

Gwinnett’s homeowner septic kit also includes a record sheet with blanks for capacity, compartments, tank material and manufacturer — if a previous owner filled it in, it’s in a kitchen drawer. Outside Gwinnett the record lives with the health district serving your county; Rockdale, Walton, Newton and Barrow each hold their own files, usually requested by parcel or address.

Working it out from the lids

If the paperwork is gone, you can get close in the yard. Concrete tanks are rectangles, and volume is arithmetic: length × width × liquid depth, in feet, times 7.48 gallons per cubic foot. A tank about 5 feet by 8 feet with 4 feet of liquid depth holds roughly 1,200 gallons; the same footprint at 5 feet of depth is closer to 1,500. Those dimensions are typical of manufacturers, not mandated by the state, so treat a yard measurement as an estimate.

A few practical tells:

• Two lids on one rectangle, four to six feet apart, is one two-compartment tank — the standard Georgia configuration, with the first compartment holding at least two-thirds of the volume.
• Two separate rectangles usually means a tank plus a pump chamber or an aerobic unit — and an alarm float; ATU and pump system service runs on a different schedule.
• A single lid in the middle often means an older single-compartment tank. Plenty of 1970s and 1980s Gwinnett tanks still are, and they carry solids toward the outlet sooner.
• Lids two to three feet down is normal here; Gwinnett’s kit tells homeowners to probe for a tank at least 10 feet from the house and 2 to 3 feet below grade.

Don’t open a lid to measure depth yourself — tank gases can kill in minutes. Have it measured during a pump-out, when sludge and scum depth get read at the same time.

Why the number actually matters

It sets your price. Pumping is priced by gallons, because the hauler pays by the gallon at disposal. The Lawrenceville pumping cost guide breaks the ranges out by tank size, and the cost estimator will do it from your bedroom count.

It sets your interval. Gwinnett’s kit publishes a table of pumping frequencies by tank size and household size — a 1,000-gallon tank with four people needs pumping roughly every 2.6 years, while a 2,000-gallon tank with the same four can go almost six. That’s covered in how often to pump a septic tank in Georgia.

It matters at closing. A buyer’s inspector compares tank capacity against the bedroom count, and an undersized tank — often because someone finished a basement bedroom without a permit — becomes a negotiation item. Pumping before a home sale and the guide to pumping before listing cover the seller’s side.

It changes if you renovate. Adding a bedroom, or a garbage grinder, changes what the rule requires, and alterations in Gwinnett need a permit — see septic repair permits in Gwinnett.

Make the next answer easier

Once you know your size, write it down twice: on the kit record sheet, and on your pumping receipt. Then make the tank easy to reach. A riser brings the access lid to grade so nobody digs for it again — the riser cost guide runs the payback, usually two or three pump-outs.

If you’d rather someone else settle it, a septic tank inspection locates the tank, confirms compartments and capacity, checks the baffles and outlet filter, and puts it on a document you can hand a buyer or a lender. Questions about a specific property? Get in touch.

Bottom line

Assume 1,000 gallons only if your house has four bedrooms or fewer and no garbage disposal — that’s Georgia’s minimum, not a guarantee. Add 250 gallons per bedroom over four and 50 percent for a grinder. Then confirm the real figure from your county septic record, free in Gwinnett and going back to 1959, and get it written on your next pumping receipt.