Measure the width of the door frame at the top of the doorway inside the edge of the exterior trim — not the doorjamb — closest to the outside surface of the home. Measure to the nearest 1/8 inch, and write the number down.
Repeat the measurement of the width of the doorway from Step 1 twice more, but these times, measure across the middle of the doorway and then across the bottom. Write these numbers down. Select the smallest of the three numbers as your width.
Measure the height of the doorway inside the frame running along the left side, down the center and on the right side. Write these numbers down as you take them, keeping to the nearest 1/8 inch, and choose the smallest of the three as the height.
Take the smallest width measurement from Step 2 and the smallest height measurement from Step 3 with you to the home center where you’ll be buying your door. Select a door that is no larger in either direction than your measurements. Consult a sales associate for help in determining the right door for your home, if necessary.