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How to Measure Your Prehung Front Door

Prehung front doors include both the door and the frame that supports the door. Taking accurate and detailed measurements of your door’s rough opening and door jamb is critical for a successful installation. Follow this guide on how to measure a door for replacement. Looking to order custom doors? We offer virtual consultations over the phone, video or email.

Exterior Door Measurements

Man measuring a door

Learning how to measure a door for replacement is crucial to finding the best option for you. If you purchase a door with a jamb that’s too small there will be an opening where the frame doesn’t cover the gaps completely. If it’s too large, the door frame will need to be trimmed to fit in the framing. If the jamb size is incorrect, the door won’t close properly and can cause drafts and insects to enter more easily.

Most front doors open inward, which is called an inswing but if your door opens to the outside that’s called an outswing. You’ll also have to determine whether your door swings right or left, commonly known as door handing. Ordering a replacement door will depend on the type of front door you already have.

Here is a list of the important measurements you’ll need for a prehung door:

  • Door Handing
  • Inswing/Outswing
  • Rough Opening Height (smallest measurement)
  • Rough Opening Width (smallest measurement)
  • Jamb Depth

Common Door Terminology:





Door Handing (Left or Right)

Your door swing and doorknob location must be determined looking at your door from the outside.

Inswing Door

The door swings inward into the house. Most exterior doors are inswing.

Outswing Door

The door swings outside, away from the house.

Threshold Nose

Threshold Nose

The door threshold is usually a piece of wood or stone at the bottom of your door. The threshold nose is the front facing cap.

Rough Opening Header

Rough Opening Header

The rough opening is the structural frame surrounding your door. The header is the top of your door frame.

Rough Opening

There are a few required dimensions to fit a prehung door, which includes both the door frame and slab. Prehung doors have a variety of standard sizes with recommended rough opening sizes, however you should measure your own rough opening down to a fraction of an inch. For a proper fit you need to specify a standard door size no larger than the smallest dimension of your rough opening.

Door Sizes

Actual Door Slab Size

Required Rough Opening Size

  Door Width Door Height Opening Width  Opening Height
30" Prehung Door 31-1/2" 81-1/2" 32-1/2" 82"
32" Prehung Door 33-1/2" 81-1/2" 34-1/2" 82"
34" Prehung Door 35-1/2" 81-1/2" 36-1/2" 82"
36" Prehung Door 37-1/2" 81-1/2" 38-1/2" 82"

How to Measure your Door’s Rough Opening

Measuring a door jamb

1. Remove interior moulding that surrounds the old door (top and sides).

Pry it away from the door jamb with a wide, flat pry bar. This will reveal the door’s rough opening (the structural frame that surrounds the door unit).

Measuring a door jamb

2. Measure the door’s rough opening height.

To get this measurement without removing the old door, simply measure from the bottom of the threshold nose to the bottom of the rough opening header. Check the height at the centre and both sides and record the smallest measurement.

Rough Opening

3. Measure the width of the rough opening.

Do this three times – on the centre, top and bottom edges – and record the smallest measurement.

Measuring a door jamb

Measure Door Jamb Depth

This is the measurement of the wall thickness that your new door will be filling in. This is the inside edge of the door jamb to the outside edge of the door jamb (where the hinges are).

Ordering Front Doors

Measuring a door jamb

Whether you want to configure your dream door or need help taking care of the entire process, our team can help you choose a front door that’s right for you. Book an appointment with a windows and doors consultant for more information.

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