Septic Tank Size Calculator

This sizes a household septic system from the two rules that are arithmetic. Enter the bedroom count or the number of people and it gives the design daily flow, the liquid capacity the detention rule asks for, an allowance for a garbage disposal, and the next standard tank that covers it. A third tab turns a design flow and a measured percolation rate into drainfield absorption area, trench length and the reserve area codes want kept clear. The gallons per bedroom, the detention time and the loading rate are all inputs, because the minimum capacity table and the soil table that govern a permit are your local health department's, and they differ from one county to the next.

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Measure In
Gallons here are US gallons. Every figure you enter and every answer below follows this choice.
Codes size a house by bedrooms, not by who lives there now.
Gallons per day, or litres per day, to match your choice above. Commonly 120 or 150 gpd per bedroom; use the figure in your local table. Blank means 150 gpd, which is 568 litres a day.
Liquid capacity has to hold this many days of flow. Two days is the usual rule. Days are days in either unit.
In US gallons. Most codes will not permit a tank below a fixed floor, often 1,000 gallons, and blank takes that figure. Enter 0 if your code sets no floor.
Codes that allow a disposal usually want the tank a half larger.
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Measure In
Gallons here are US gallons. Every figure you enter and every answer below follows this choice.
Gallons per day, or litres per day, to match your choice above. Metered household use is usually 50 to 80 gpd a head, which is 190 to 300 litres; design tables run higher on purpose, and British and Irish practice sizes on 150 litres a head. Blank means 70 gpd, which is 265 litres a day.
Liquid capacity has to hold this many days of flow. Two days is the usual rule. Days are days in either unit.
In US gallons. Most codes will not permit a tank below a fixed floor, often 1,000 gallons, and blank takes that figure. Enter 0 if your code sets no floor.
Codes that allow a disposal usually want the tank a half larger.
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Measure In
Gallons here are US gallons. Every figure you enter and every answer below follows this choice.
Gallons per day, or litres per day, to match your choice above. The same figure the tank was sized on, usually bedrooms x 150 gpd.
Minutes per inch, or seconds per millimetre, to match your choice above. Seconds per millimetre is the same figure a British or Irish percolation test reports as Vp. From a perc test on the site; there is no way to work this out from anything else.
Gallons per square foot per day, or litres per square metre per day, to match your choice above. One gal/sqft/day is 40.75 L/m2/day, so 0.45 becomes 18.34. Leave at 0 to read it off the perc rate instead.
Feet across the trench bottom, which is the only part that counts as absorption area. Blank means 3 feet.
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