How to Measure an Exterior Door Size


Purchasing a new exterior door requires several measurements of the door opening so that a new door will fit in the same space. The door itself may shrink or swell over time and not fit the original measurements. Taking precise measurements enables a homeowner to choose a replacement door that will require little adjustments in order for it to fit in the same opening.

Things You'll Need

  • Measuring tape
  • Paper
  • Pen
  • Open the door and go outside. Measure the width inside the doorjamb from side to side including the trim on each side. Take one width measurement at the top, one in the middle and one at the bottom.

  • Write all three measurements down on paper with a pen marking which measurements they are clearly. The top, middle and bottom measurements are not always the same on a door width.

  • Measure the width of the door opening from the inside of the house in the same manner. Write down all three measurements accordingly.

  • Measure the height of the door opening from the bottom of the doorsill to the header at the top of the door opening.

  • Measure the thickness of the actual old door.

Tips & Warnings

  • Older doors that need replacing may have changes in their original size from weathering. Doors that warp or do not have adequate sealant for protection can either shrink or swell in size.
  • If the replacement door will be a pre-hung door in a casing, remove the trim around the door on the inside and measure horizontally and vertically from the framing on each side to the other side. This process includes measurements for the door casing so that it will fit in the area provided with little additional framing.

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