It is not necessary to make a verbal intention. Instead, once a person has this in his heart that he is fasting and he does not eat or drink anything, nor does he indulge in intimate relations with his spouse-his fast will be valid. Also merely partaking of Sehri (in preparation for the fast) also equates to the intention of the Fast1. If a person makes a verbal intention by saying: “O Allah Ta’ala! I will fast for Your pleasure tomorrow”, even then there is no harm in this2.
Also remember that in the month of Ramadhan the intention of Fast could be made right until Nisfun-Nahaar-Share’e (Shar’ee mid-day) if one has not eaten or drunk anything after Sehri time up to that time. In order to calculate Nisfun-Nahaar-Share’e one should note the time of Subh Saadiq (true dawn) and the time of sunset. After counting the number of hours between these two times, divide this number in half. This will give you Nisfun-Nahaar-Shar’ee 1.