The garbage disposal rule, state by state

Search long enough and you'll find the same sentence everywhere: "Add 50% to your tank size if you have a garbage disposal." It is the most-copied sentence in septic sizing — and across the ten states we've verified from the official rules, it is exactly right in two of them.

Where ×1.5 is real

Georgia (Rule 511-3-1-.05(3)) and North Carolina (15A NCAC 18E .0801) both multiply the required capacity by 1.5 with a disposal. If you're in those states, the famous rule is your rule.

Where the add-on is different

Florida adds a flat 250 gallons — on a 900-gallon base that's +28%, not +50%. New York doesn't do arithmetic at all: the disposal counts as one extra bedroom, so you move one row down the table. On a 3-bedroom home that's +0 gallons (still 1,000); on a 4-bedroom it's +250.

Where nothing is added — because it's already counted

Pennsylvania's design flows state outright that they provide for garbage grinders, laundry and dishwashers — the disposal is inside the number, and multiplying again double-counts it. Missouri's Table 4 footnote says the same thing in so many words. And Virginia's tank section (§815) contains no disposal provision at all: the ×1.5 column in Virginia rows on copied charts is decoration.

What to do with this

Run the calculator with the disposal box checked and unchecked — it applies your state's actual treatment, and the result line tells you which rule fired. And if an installer quotes you a disposal upsize, ask which section of the state rule requires it. In four of our ten states, there isn't one.

Checked against the state rules cited on our state pages, Nov 10, 2026.