Why Should You Know How to Measure for Double Glazing Sizes?

If you want new double glazing, knowing your double glazing sizes is very important. If you have an installer, he will send a technical surveyor. However, if you plan to install it yourself, you need to be doubly sure of your double glazing sizes. If you don’t, you may face delays. What’s more, you might even lose money.

The correct measurement of double glazing sizes is essential for home improvement projects

How, you ask?

It’s simple. Let’s say you measure your double glazing. However, your measurements aren’t accurate. Maybe you measured the visible fitting instead of measuring the actual aperture. Maybe you failed to take the 10mm fitting allowance into account. Whatever it was, somehow, your measurements are off.

When you place your order, you will use these inaccurate measurements. When your double glazing arrives, it will not fit your apertures. Now, you can’t return them and get a refund. You will lose the money you spent. Moreover, you will need to place another order with the correct double glazing sizes.

Since double glazing may take anywhere from 14 days to 21 days to be manufactured, you also lose all that time.

As you can see, getting accurate measurements is quite important. However, how do you actually take proper measurements? Let’s find out…

What Should You Keep in Mind When You Measure for Double Glazing Sizes?

Measuring your double glazing is a simple matter if you keep certain points in mind. Your double glazing sizes don’t mean the size of the fitting. In fact, you need to measure your opening, or aperture. This is a common mistake people make. Additionally, to get an accurate figure, you can’t rely on one set of measurements. Let’s see what you need to keep in mind when measuring your doors and windows.

Always Measure from Outside

This is an easy point to forget, but is very important. It is essential to remember that your double glazing is fitted from the outside. Therefore, if there is a difference in measurement between the inner and outer aperture, it can be a problem.


Because if your inner aperture is slightly wider than your outer aperture, your double glazing won’t fit. Alternatively, your double glazing might be too small and not fit properly. This is why you should always measure double glazing sizes from the outside.

The only exception to this rule is measuring bay and bow windows. You need to take both inner and outer measurements for them.

Bay and bow windows require inside and outside measurements for proper double glazing sizes

Measure Brick-to-Brick

Again, the double glazing sizes are not measured on the fitting themselves. They are, in fact, the measurement of the aperture. This is why it is important that you measure from brick to brick, and not just the visible frame.

Your double glazing is manufactured according to the size of the aperture. The width of the frame can vary according to the material. For example, a uPVC frame will be thicker than hardwood or aluminium frames.

If you measure the width of the sash or the panel, you won’t take into account the width of the frame itself. As a result, if your old windows are a different material from what you are ordering, you will have improper double glazing sizes.

Take 3 Sets of Measurements

The measurement of any opening in a house is not absolutely precise. There can be variations, albeit minor, at various points in the aperture. It could be as minor as a few millimetres, or as drastic as an imperceptible increase or decrease along the length or width. To counteract discrepancies caused by this, you should always take at least 3 sets of measurements.

If you are measuring the length, take the left edge, right edge, and centre for your double glazing sizes. For the width, measure along the top, bottom, and centre. This give you a more or less precise assessment of the aperture’s width. Finally, just take out the average of the three figures to get your final measurement. To do that, simply add the 3 figures, and divide the sum by 3.

Additionally, remember to subtract 10mm from the final double glazing sizes. This is for fitting allowance, which is 5mm on either side.

Measure Inclusions as Well

In addition to the double glazing sizes, you need to measure the inclusions as well. This includes the width of the cill, and any frame extenders or additional fittings. If possible, measure the width of the sealed double glazed unit as well. You may even break this down into the width of the panes of glass and the space between them.

For example, a 20mm unit will have 2 panes of 4mm glass separated by a 12mm gap.

You will need these double glazing sizes when you order your sealed double glazed unit. If you have measured the glass from edge to edge, specify in your order that these are ‘tight’ measurements. This way, the company will know to deduce the fitting allowance. The more detailed you are, the more accurate your double glazing sizes will be.

What Should You Do If You Aren’t Sure of Your Double Glazing Sizes?

Double glazing installation companies will measure double glazing sizes before ordering

Measuring your double glazing sizes may not be rocket science. However, if it’s your first time, you may understandably be nervous. Even minor mistakes in measuring your double glazing sizes might mean a wasted unit. This is why, if you aren’t sure, get some expert advice.

One way of doing it is to get your installation company to come and do a full technical survey. They will send in an expert surveyor who will not only measure your double glazing sizes, but also assess your double glazing needs for Building Regulations compliance.

However, it might be that you aren’t hiring a double glazing installation company. You may have your own carpenter or glazier who will install your double glazing. Alternatively, you may want to do it yourself, like we mentioned earlier.

If you are hiring a trusted local carpenter, you could ask him to come and measure your double glazing sizes. He will be experienced at this. Not only will you get accurate measurements, but also tips on how to do it yourself the next time.

Alternatively, if you want to try your hand at installing your fittings yourself, you can pay a carpenter to measure your double glazing sizes. Alternatively, you can contact us. Leave us a message online and we shall get back to you. Our expert staff is highly knowledgeable about all aspects of double glazing sizes. They will talk you through the process. Moreover, they will also recommend trusted local installers if you need them.

Frequently Asked Double Glazing Questions

Frequently Asked Double Glazing Questions

Do UPVC Windows Need Trickle Vents?

Trickle vents are not a requirement, however they do offer additional ventilation without the need for opening your windows so can be a valued add on.

How Long Will It Take To Install My Casement Window?

Because every job is different, it is best your talk to your double glazing installer who will be able to give you an exact timescale.

Are There Other UPVC Casement Window Styles?

Other UPVC casement window styles include the French casement window which is a more traditional take on the standard casement style.

Will I Have A Choice of Casement Window Locks?

A high security lock will be fitted as standard with your new casement window but your double glazing installer will also offer a choice of security upgrades.

What Are Flush Casement Windows?

Flush casement windows are where the unit sits flush to the frame, creating a super sleek look.

Are There Different UPVC Casement Window Sizes?

UPVC casement windows come in a wide range of sizes to suit your property. You can specify your casement window size in our window cost calculator to get a unique window price!

Can I Choose My Casement Window Handles?

Your double glazing installer will offer you a choice casement window handles and casement window hardware options so you can achieve the perfect look.

What is a Casement Window?

A casement window is attached to its frame by one or more hinges. A casement window is hinged at the side allowing them to open extremely wide. These windows can double up as emergency exits and are perfect for hard to reach places within the home, with many home owners installing these windows behind their kitchen sinks.

Where Can I Get A Tilt and Turn Windows Price?

You can get a tilt and turn windows price using our window cost calculator which will give you a free and unique quote within a matter of minutes.

How Does The Tilt and Turn Window Mechanism Work?

The tilt and turn window can be ‘tilted’ inwards due to its unique turning action which allows the window to be easily accessed from both inside and out. All it takes is a swift turn of the handle.

How Much Do French casement windows cost?

The cost of French casement windows will vary depending on installer but you can get an average casement windows cost using our free online quoting engine

How do UPVC Sash Windows Work?

UPVC vertical sliding sash windows are built to either slide vertically downwards or upwards. This opening function makes the UPVC sash window much easier to use compared to traditional wooden windows where the only choice is double hung and single hand.

Are UPVC Sash Windows Easy To Use?

UPVC sash windows are very easy to use. Their spiral balance mechanism allows for smooth operation so you can open and close your windows with ease. UPVC sash windows are also incredibly low maintenance and require very little cleaning.

Why Should I Choose a UPVC Sash Window?

UPVC sash windows are incredibly attractive and make a stunning addition to any property and are relatively low maintenance thank to their UPVC design. You can enjoy a traditional aesthetic for many years to come thanks to their long standing properties.

Replacement Sash Windows In A Conservation Area

If your home is a listed property or within a conservation area then you will need to seek planning permission.

Bay or Bow Window?

Simply put, it really depends! Bay windows offer often much welcome and useful extra storage space to the frontage of homes. On the other hand, bow windows provide a curved shape alternative in terms of look compared to bay windows. They are both beautiful options and can add much character and charm to the right property.

Can Bay Windows Look Modern?

Bay windows can easily be either modern or traditional depending on the combination of glass and / or coloured uPVC profiles now available. Subsequently, its easy for these windows to seamlessly blend in with contemporary or old properties.

Can Double Glazed Windows Reduce My Heating Bills?

When you install double glazing in your home or commercial property, it is likely that the extra heat retention will lessen the need to use central heating systems. In turn, this could mean you see a drop in heating bills. If you are concerned about your energy bills, it is easy to compare business energy or home energy online.

What Makes Flush Sash Windows Different?

Flush sash casement windows are specifically designed to imitate the appearance of traditional wooden timber windows by appearing to have the same sashes that finish 'flush' with the outside face of the window.

Should I replace my old sash windows?

Sash windows are a key feature for many traditional period buildings. Sash windows were traditionally built using timber, which is fairly high maintenance and has a tendency toward rotting and warping. Over time, these windows can become draughty and insecure. uPVC sash windows are a great way to experience the benefits of modern double glazing, without sacrificing the aesthetic of your home. uPVC sash windows will not rot or warp and require little maintenance. They are also thermally efficient, which means that your home will not require as much heating.

How much does sash window restoration cost?

The cost of sash window restoration will depend on the survey result. Standard draught proofed sash windows (without sill replacement) usually starts at about £500 per window. The time taken will also depend on the survey result.

Can I install double glazing into my original sash windows?

This will depend upon what type of window you have. Replacing existing moving sashes with new, double glazed replicas might to be possible.

How much does it cost to replace a sash window?

The cost of replacing a sash window will depend on many factors including: style, size, material and glazing. A wooden sash window will cost significantly more than a uPVC sash window. A wooden sash window can cost up to £1000. Wooden windows are more expensive to manufacture so expect to pay that bit more. uPVC sash windows can be designed to mimic wood and are much cheaper in price. The starting price for a standard uPVC sash window is on average £525.

Instant Double Glazing Prices