Readings are obtained electronically with a special "drive-by" computer, which picks up the readings emitted by radio frequency through a Meter Interface Unit (MIU). Customers are billed in thousands of gallons so not all dials are used in calculating the bill. The last two digits on the right are dropped when entering the reading for billing, i.e. 547413 is read 6474. The previous reading is subtracted from the current reading to calculate the amount of water used between readings. The meter is read each month.