Check your county permit record or a past pumping invoice; a pump-out measures it for certain. For new and replacement tanks, Ohio's OAC 3701-29-12 sets minimums of 1,000 gallons for one or two bedrooms, 1,500 for three, and 2,000 for four or five.
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Ohio sizes septic tanks by bedrooms, statewide. Under Ohio Administrative Code 3701-29-12, in effect since January 1, 2015, a tank installed under a permit today needs a minimum liquid capacity of 1,000 gallons, in one or two compartments, for one or two bedrooms; 1,500 gallons in two tanks or compartments for three bedrooms; 2,000 gallons in two tanks or compartments for four or five bedrooms; and at least 2,500 gallons for six or more, with added capacity for each bedroom. In a two-compartment tank, the first compartment must hold between one half and two thirds of the total. The same rule requires an effluent filter at the outlet that retains solids larger than one sixteenth of an inch, watertight risers to grade with at least a 12-inch opening, and lids that weigh at least 50 pounds or are secured. Those sizes follow from the state's design flow of 120 gallons per bedroom per day under OAC 3701-29-11. Local health districts may adopt stricter standards, so a permit in Hamilton, Butler, Warren, or Clermont County can ask for more but not less. Many older tanks in the region went in under earlier rules and are smaller than a new one would be. Clermont County Public Health's service reports flag that situation by noting a system would be bigger if installed under today's regulations, and advise watching water use rather than treating it as a violation. Size matters most for pumping: Ohio State University Extension estimates that a household of four fills a 750-gallon tank in about 1.8 years, a 1,000-gallon tank in about 2.6 years, and a 1,500-gallon tank in about 4.2 years. To find your own number, look at the county permit file, the gallons on your last pumping invoice, or the manufacturer's label on newer tanks, which the rule requires. A septic inspection confirms capacity and condition, and our pumping frequency FAQ turns the size into a schedule.