Septic Tank Capacity and Design Flow. RESIDENTIAL SINGLE-COMPARTMENT TANK GROSS SQUARE FOOTAGE SEPTIC TANK CAPACITY DESIGN FLOW MINIMUM DRAINFIELD LENGTH 0 - 1,750 sq. ft 1,000 gals. 720 gal/day 70 1,751 - 2,250 sq. ft 1,500 gals. 900 gal/day 100 2,251 - 3,000 sq. ft 1,750 gals. 1,000 gal/day 120 > 3,000 sq. ft 2,000 gals. 1,000 gal/day 130 RESIDENTIAL DOUBLE-COMPARTMENT TANK GROSS SQUARE FOOTAGE SEPTIC TANK CAPACITY DESIGN FLOW MINIMUM DRAINFIELD LENGTH 0 - 1,750 sq. ft 1,000 gals. 720 gal/day 70 1,751 - 2,250 sq. ft 1,250 gals. 900 gal/day 100 2,251 - 3,000 sq. ft 1,500 gals. 1,000 gal/day 120 > 3,000 sq. ft 1,800 gals. 1,000 gal/day 130 Commercial Septic Tank Capacity. Tank Capacity will be based on a forty-eight (48) hour retention capacity as calculated from the average sewage flow. Two-compartment septic tanks shall be required for commercial establishments having a design flow in excess of 1,000 gallons/day.