A method of estimating time required to charge a vehicle is provided. The method reduces an error in estimating the time required for charging due to a difference between the voltage of an external power supply unit, measured when setting a reservation for charging, and the voltage applied to an on-board charger during actual charging of a battery, and a charging reservation method using the same. The method includes detecting an input voltage to an on-board charger from an external power supply unit and estimating an input current. A charging voltage margin value is determined based on the detected input voltage and the input current and a charging setting voltage is determined by applying the charging voltage margin value to the input voltage. A time required for charging is then calculated based on the charging setting voltage, the input current, and a charging demand of a battery.