How to Measure Internal Door Sizes

If you’re installing new internal doors for the first time, it’s essential to get the sizing right. Especially at a time when energy costs are soaring. The tiny gaps between the door leaf and the frame can allow hot air to escape from your living space in the colder months and leak out cool air when the sun is out. What’s more, getting your door sizes wrong can lead to difficulties opening and closing and disproportionate wear on the door leaf.   At Doors & More, we don’t just want to help you find the perfect internal door for your living space. We want to make sure that you find the perfect fit for your doorway. While internal doors come with a few centimetres of lipping to trim your doors to size, you don’t want to waste time, effort, and timber on a door that’s the wrong size for your frame. Especially since excessive trimming may expose the core beneath the timber veneer.  Here, we’ll look at how to measure your door perfectly and ensure peace of mind when you hang your new internal doors. 

How to Measure an Internal Door

As with any form of carpentry, precision measurements are very important. Especially before you start trimming your door to size. Estimation can lead to difficulties opening and closing your doors once they are hung, and the frustration of trimming and re-hanging your new doors. Likewise, overestimating or being overzealous with trimming may lead your internal doors to lose their insulative properties.  When measuring your internal doors, you will need:

• A  tape measure
• A pencil 
• A piece of scrap paper

You may also need to trim your doors to size, so it’s advisable to also have:

• A hand wood plane
• Sandpaper

Step 1— Measuring the door’s height

When measuring your internal door’s height, we recommend measuring inside the vertical sides of the frame. Remember, door frames are not completely symmetrical, so it’s best to take three measurements. One from the left side, one from the right side and one from the middle. The longest length will give you the required height.  You will also need to consider the impact of your floor coverings when measuring the height of your internal doors. 

Step 2— Measuring the width

Your door frames are unlikely to be the exact same height all the way across. Likewise, they may be different widths from top to bottom. Judging your internal door’s width on a single measurement may leave you with a lot of trimming to do. With this in mind, it’s best to take three measurements from inside the door frame. Take one from the top, one from the middle and one from the bottom. The widest measurement is the one you’ll use to determine the width of your new door. 

Step 3— Measuring thickness

The thickness of internal doors is usually much more standardised than the height and width. Most internal doors are 35mm and some are 40mm in thickness. The only exception to this is fire doors. These are thicker and heavier, with a more robust core to contain the spread of fire. Fire doors are commonly placed in doorways connecting living spaces to areas of shared occupancy like corridors or stairwells to facilitate a safe fire escape route. Fire doors must be hung in a fire door frame, using appropriate hinges. 

What is the standard size for internal doors?

There is no standard size for internal doors, although most fall within a common size range. 1981 x 762 x 35mm is arguably the most common door size. However, internal doors commonly range between 1981 x 457 x 35mm to 2040 x 926 x 40mm, while internal fire doors range from 1981 x 610 x 44mm to 2040 x 926 x 44mm. 

Can I Measure the Existing Door?

When measuring internal doors, many are unsure whether they should take down and measure the existing door leaf, or instead measure the door frame.  At Doors & More, we recommend basing your measurements on the inside of your door frame rather than your existing door leaf. Your existing door may not be perfectly flush with the frame, and basing your measurements on the door frame will help to ensure a fit that is at least as good as (if not better than) your existing door.

How Much of a Gap do I Leave Between the Door and Frame?

Getting the gap between the internal door leaf and the frame just right is hugely important. Especially when installing a fire door.  To help provide good thermal and sound insulation for your internal doors, there should be a gap of around 2-3mm between the door leaf and frame at the top and sides with a gap of around 9mm at the bottom.  This is enough to make sure that the door offers fair insulation while also ensuring that it opens and closes properly.  Please note that when hanging a fire door the gap between the leaf and the frame should never be more than 4mm or less than 2mm. We strongly recommend hiring a trained professional to install your internal fire doors. 

What if I’ve Chosen the Wrong Measurement?

If you have chosen the wrong measurement, you may return your doors to us within 14 days of receiving them. See our returns policy for more details. When in doubt, do not try and trim your doors down to size. We will be unable to accept returned doors unless they are in resellable condition. 

How to Trim Your Internal Doors

Assuming that you only need to remove a couple of millimetres from the sides of your doors, trimming them is a fairly straightforward job, requiring little more than a steady hand and some patience. 

• Make pencil marks on both sides of the door where you intend to trim, so you can ensure that both sides are even  
• Work inwards from the outer edge of the door to avoid splitting the wood grain
• Keep checking the fit as you go, lining the door leaf up with the frame
• Sand down the edges after you’ve achieved the correct height and width to ensure a perfectly smooth fit

Find the perfect fit with Doors & More

Once you know the size of door you’re looking for, the fun part can begin. Take a moment to browse our huge range of internal doors from the UK’s leading manufacturers. We have internal doors in a huge range of styles, materials and finishes from Deanta, JB Kind, Jeld Wen, PM Mendes and more!