Adding a new internal door to your home brings plenty of charm and character to any room or corridor. It is also relatively easy to do, providing you have the right tools and follow the correct steps. This includes the preparation process, part of which is measuring out your door and your doorway.

This is a straightforward task, but an important one. You need to ensure that you have the correct measurements before purchasing your door or run the risk of having to make significant cuts to make your door fit. Perhaps even worse, your door could be too small for the doorway, which means it is useless.

This is why measurement is crucial – and in this guide we’ll show you how to properly measure an internal door so it will fit into your doorway with ease. The trick is to get as snug a fit as possible, which will prevent draughts as well as noise travelling between rooms. We’ll cover the tools needed, and what specific dimensions need to be measured on both door and doorway.

Do I Measure the Door or the Frame?

There are a lot of different approaches to measuring your door, but some are notably less effective than others. Despite what some may suggest, it is best to avoid measuring your existing door for reference. What you should do instead is measure your door frame. This is because your door needs to fit completely flush with your frame.

After years of wear, your current door may actually not longer provide this. Not only that, but if your door is wood, it may have changed shape considerably during its lifespan due to minor contraction or expansion. This is caused by changes in temperature and humidity. It means your current door may no longer be the perfect fit it once was.

Is There a Standard Door Size?

We’re often asked this by homeowners wondering if their door is ‘normal’ or ‘unique’ in its size, but the fact of the matter is that there is no ‘standard’ size to use as a reference point when it comes to door measurements.

However, most internal doors do fall within a range – this is 1981 x 457 x 35mm to 2040 x 926 x 40mm. 1981 x 762 x 35mm is a common size, and these dimensions are found in townhouses and new builds throughout the country. You may well find that your doorway matches these dimensions, but you will need to be sure, so measurement is still necessary.

If you’re looking for a fire door, it will be of use to know that our most commonly sold fire doors range from 1981 x 610 x 44mm to 2040 x 926 x 44mm in size. Note the consistency of the 44mm – this is because this is the expected thickness of FD30 rated fire doors.

Should I Be Measuring in Inches, Feet, Metres, or Millimetres? 

With the prominence of American blogs and guides on the internet, there is of course that age-old confusion between the uses of feet/inches and metres/millimetres. It’s particularly confusing in the UK, as we are constantly changing between American measurements (we usually measure our own heights in feet and inches and road distances in miles) and the metric system commonly used throughout Europe, which we use for the majority of measurements these days.

So, which is it for doors? Well, in the UK, manufacturers measure their doors in millimetres specifically. This allows for the most accurate measurements for a snug, flush door fit. As such, it is best to use millimetres when measuring your door. All of our doors are sold with accurate measurements in millimetres.

If you find a door measured in inches, you may well be looking at an American site that may not export to the UK. If you’re set on a door measured in inches, there are numerous online calculators that offer reliable measurement translations, but we’d recommend purchasing a door with millimetre measurements from the get-go.

How to Measure a Doorway

You may spot the term ‘rough opening.’ This is an approximate size for your doorway and includes both the door and the frame. This size is usually calculated by adding about 50-64mm to the actual size of a door. It should be noted that most doors feature a large amount of lipping – the edges of the door that can be planed down considerably to fit a doorway.

When measuring out your doorway, there should be a gap of 3mm around the top of the door before meeting the frame. At the same time, there should be a 9mm gap at the bottom. This will provide the aforementioned draught prevention as well as the noise reduction – the two most important capabilities a new door should possess.

When measuring a door: 

1. Measure the width of the doorway first by taking three measurements – one at the top, one in the middle and one at the bottom. The largest (after adding those extra millimetres) is the width you should be aiming to purchase.
2. Measure the height of the doorway next. Take three – one on the right, one in the middle, and one on the left. The largest measurement is your door height.
3. Measure the depth of your frame. This should be relatively straightforward, as there are only three depths on the mainstream door market – 35mm, 40mm, and 44mm, though the latter is meant for fire doors. Measure the depth of your doorframe and work out which of these sizes is closest to your results.

In Conclusion

So, to recap:

• Measure your doorway, not your door.
• Work out the ‘rough opening’ of your door.
• Take three measurements (one on each side, one in the middle) for height and width.
• It is better to buy a door that is slightly too large than too small, as these can be planed into shape.
• You only need to limit your depth options to one of three sizes.

And this concludes our look into measuring your internal door for fitting. We hope you now have a clearer idea on how this process is best carried out. If you’re looking for guides on other door measurements, check out our quick way to measure out an external door. Perhaps you’re finding the market for doors a little too big to explore effectively and you’re worried about missing the best product for you. This is where our internal door buying guide comes in handy.

Meanwhile, if you have another question about internal doors or door DIY in general, do not hesitate to contact our excellent customer services team. With plenty of DIY knowledge and knowledge on all our products, they’ll be happy to point you in the right direction or answer any queries you may have on your next DIY project.