How To Measure A Door
You may be surprised by the many different sizes available when buying your new doors. There are some "industry standard" sizes that almost all suppliers use. However, these aren't always universal, especially in the UK market, where older doors are more likely to be irregularly shaped.
Because of this, it's always worth measuring your existing doors before buying new ones, rather than just relying on standard sizes, to ensure you're making the right purchase.
We've compiled a simple guide to help you get the right measurements and avoid common mistakes that can trip up the inexperienced.
How to measure for new internal doors
When measuring for new internal doors, the easiest thing to do is measure the door you’re replacing. However, if the door doesn’t fit well within the frame, there is no current door to measure, or it is a new opening without a frame, you’ll have to measure the gap and make some adjustments as we’ve outlined below instead.
Whatever your situation, you’ll need to measure the width, height and thickness of the door or the opening.
Step 1: Measure the width
- Use a measuring tape to measure the width at three points: the top, middle, and bottom.
- Some older doors and door openings aren’t perfectly rectangular, so take a few measurements and use the largest number as your door width.
- Check that you only include the door and not other parts, such as weather stripping.
Step 2: Measure the height
- Run the tape measure from the top of the door to the bottom in a straight line. Also measure this in a few different places in case of varying heights.
- Double-check that you’re not including extra door parts, such as a door sweep, in your measurements.
Step 3: Measure the door thickness
- Run your tape measure from the edge of one side to the edge of the other to work out the thickness of your door.
- Make a note of the number, then measure the door jamb on the frame to check that it’s a similar size, as you may need to know both figures.
Extra considerations for measuring an internal door
If measuring between the frame:
If your opening already has a frame fitted, your measurement would be "frame-to-frame".
Measure the width of the space from left to right, but deduct 2-3mm from either side to allow it to open and close. This will also ensure your door doesn't crack or warp during the winter months when doors are prone to slight swelling due to humidity changes.
If there’s no door or frame:
If it's a new opening, it may not have a frame fitted. When you come to measure this, it is usually known as the "brick-to-brick" opening. Measure the rough opening size (width and height) and consider any adjustments for framing.
Tip: Once you have all three measurements, repeat the process for each door you want to replace. Not all internal doors in your house will be the same size or thickness, so measure each door individually, just in case.
How to measure for a new external door
External doors are measured the same way as internal doors but might require additional steps or considerations to get the best fit.
Measuring for a new door in an existing frame
If you plan on attaching a new door to an existing frame, it's best to measure the new frame rather than the old one for a more flush fit.
Measure the following:
- Width
- Height
- Thickness
- Side panels (if needed)
Replacing the door and frame
If you plan on replacing the door and the frame, measure the structural opening, brick to brick.
When measuring a rough opening, don't forget to consider your new frame. We recommend taking 2 inches off the width and around 2.5 inches off the height of the brick opening to allow a good fit.
You may also need to deduct room for draught excluders from your measurements, which can be up to 25mm in height.
Tip: The height measurement is particularly important for external doors to ensure the weatherproofing fits snugly and keeps drafts out.
How to measure for bifold doors
For bi-fold doors, you will need to measure the full width and height of the opening, including the frame.
- Measure the width at the top, middle and bottom and use the smallest width for your order.
- Measure the height at different points in the opening and use the smallest measurement.
- Measure the diagonal from corner to corner to check that the opening is an even rectangle. The two numbers should be very close.
- Bifold doors need precision to operate smoothly. Measure the opening width and height carefully, and ensure that the floor is level for proper sliding or folding.
For larger openings, especially with glass doors, there may also be specific UK building regulations to consider, such as safety glass requirements. Always consult a professional for guidance if unsure.
What if I can’t find the right door size?
If you’re struggling to find the right size or are not 100% sure of your measurements, we recommend buying an internal door slightly larger than your opening, which can be trimmed down to fit. For external doors, use the smallest measurements.
The threshold for adjusting your doors differs from manufacturer to manufacturer, so check this before purchasing. We've also compiled a guide on the standard door sizes available to help you make the right choice.
Can I just measure the door that I'm replacing?
When buying a replacement door, measuring the existing door is the easiest way to get the necessary measurements. However, we recommend checking the door to ensure it fits snugly in the frame and has no issues with rubbing on the frame or gaps around the edges.
Shop internal and external doors from Leader Online
Once you have your measurements, you can start looking for the perfect door for your space. At Leader Online, we have a wide range of internal and external doors, as well as door frames, that are sure to suit your needs. We even offer bespoke doors that can be built to your specifications.
Call our friendly Door Experts in our sales team for more advice. We'll know what size you need and can even discuss pricing and availability with you over the phone.
