Walk to one of the cabinet openings and open the door, if still installed. Remove all existing cabinet hinges from the cabinets using a screwdriver and set them aside so you have plenty of room to measure.
Stretch the tape measure from the top of the opening to the bottom of the opening to determine the length of the cabinet door.
Reposition the tape measure horizontally and stretch it from the left side of the opening to the right side of the opening to determine the width of the door.
Repeat the process for every door opening on the cabinets and write them down individually on a piece of paper. Label each set of measurements in a consistent manner, such as with letters or numbers, so you know which opening they correspond to.
Look at the strips of cabinet wood located direct to the side of each cabinet opening. If two doors meet these will be thin strips or may also be at the end edges of the cabinets. Measure the horizontal width between the end of the door opening and the other end of the wood area. Divide the number in half so the cabinet door will overlay it by half.
Add the half-overlay measurement determined to the width of each cabinet opening to arrive at the proper width for each door.