A method for controlling a washing machine which includes a rotating washing tub in which laundry is placed and a motor to rotate the washing tub, the method comprising: performing primary rotation of the motor; after performing the primary rotation of the motor, turning a first current flowing in the motor off; detecting a primary laundry amount on the basis of the primary rotation of the motor; supplying, if the detected laundry amount is equal to or greater than a predetermined laundry amount, a predetermined amount of water or water corresponding to a predetermined water level to the washing tub in such a manner that the laundry becomes wet; performing secondary rotation of the motor; after performing the secondary rotation of the motor, turning a second current flowing in the motor off; and detecting a secondary laundry amount on the basis of the secondary rotation of the motor, characterized in that the method further comprises: displaying a calculated operation time on the basis of the detected secondary laundry amount or laundry quality, wherein when the first current flowing in the motor is turned off, the motor is nonlinearly decelerated during a first period, linearly decelerated during a second period after the first period, and nonlinearly decelerated during a third period after the second period, wherein the detecting the primary laundry amount detects the primary laundry amount using a deceleration speed or time of the motor during the second period. performing primary rotation of the motor; after performing the primary rotation of the motor, turning a first current flowing in the motor off; detecting a primary laundry amount on the basis of the primary rotation of the motor; supplying, if the detected laundry amount is equal to or greater than a predetermined laundry amount, a predetermined amount of water or water corresponding to a predetermined water level to the washing tub in such a manner that the laundry becomes wet; performing secondary rotation of the motor; after performing the secondary rotation of the motor, turning a second current flowing in the motor off; and detecting a secondary laundry amount on the basis of the secondary rotation of the motor, characterized in that the method further comprises: displaying a calculated operation time on the basis of the detected secondary laundry amount or laundry quality, wherein when the first current flowing in the motor is turned off, the motor is nonlinearly decelerated during a first period, linearly decelerated during a second period after the first period, and nonlinearly decelerated during a third period after the second period, wherein the detecting the primary laundry amount detects the primary laundry amount using a deceleration speed or time of the motor during the second period. displaying a calculated operation time on the basis of the detected secondary laundry amount or laundry quality, wherein when the first current flowing in the motor is turned off, the motor is nonlinearly decelerated during a first period, linearly decelerated during a second period after the first period, and nonlinearly decelerated during a third period after the second period, wherein the detecting the primary laundry amount detects the primary laundry amount using a deceleration speed or time of the motor during the second period.