2021年8月12日 · In the United States, the minimum septic tank size permitted is 1,000 gallons. If you use less than 1,240 gallons per day, a septic tank of 1,900 gallons is recommended. If you use less than 900 gallons per day, you should get a 1,500-gallon septic tank. But if you use less than 700 gallons of water per day, you only need a 1,200-gallon tank.