How often should you pump your septic tank? A calculator

The honest answer to "how often do I pump?" depends on two numbers: how big the tank is and how many people use it. A couple in a 1,500-gallon tank and a family of six in a 750-gallon tank are on completely different schedules. Pick your numbers below for an estimate, then read the adjustments — several of them matter more around Fort Smith than they do in most places.

Where the numbers come from

The estimate uses the pumping-frequency table published by Penn State Extension (Karen Mancl's work on septic tank maintenance), which most extension services and septic pros still use as a starting point. It assumes a conventional tank and typical household water use. It answers one question: how long until sludge and scum fill about a third of the tank, the point where solids start washing out to the drain field.

Years between pump-outs. The calculator caps its answer at five years: a tank that "won't fill" for twelve years still has baffles, lids, and an outlet filter that can crack or clog in the meantime.

Don't know your tank size?

• Look at the old pumping receipt. Most pumpers write the gallons they took out.
• Ask the county health unit. Systems permitted through the Arkansas Department of Health have a permit on file with the Sebastian or Crawford County health unit, and it usually lists the tank size. Very old systems may have no record.
• Go by bedrooms as a rough guess. Tank size is sized off bedrooms, so a newer two- or three-bedroom house is most often on a 1,000-gallon tank, and bigger houses on larger ones. Older rural homes can be on 500- or 750-gallon tanks — use the smaller number if you're unsure.

River Valley adjustments

• Clay drain fields are less forgiving. Tight Sebastian and Crawford County clay absorbs effluent slowly, and solids that slip past a full tank seal it faster. If your field has ever been soggy, lean toward the short end of your estimate.
• Aerobic systems run on a different clock. An aerobic treatment unit needs regular inspection of the aerator, chlorinator, and spray heads whatever the sludge level; this table does not apply to it. Follow the service contract and the manufacturer's schedule.
• Garbage disposal. The calculator shortens the interval by about a third if you use one — disposals add a lot of solids that bacteria break down slowly.
• Company coming, or a house sale. A tank near its limit can tip over during a crowded holiday week, and buyers' inspectors often ask for a recent pump-out and inspection. Schedule ahead rather than in an emergency.
• Signs override the calendar. Gurgling drains, sewage smell, or wet ground over the field mean call now, whatever the calculator says — see septic backing up and drain field problems.