How to correctly measure a window for installing blinds (horizontal, vertical, roller, day-night)? 

Introduction. Accurate window measurements are the key to successful blind installation. Correct measurements determine whether the blinds will fit and function properly. Even a few extra millimetres can result in the structure not fitting into the window opening or unwanted gaps remaining at the edges. In this article, we will explain in detail how to measure different types of blinds – horizontal, vertical, roller blinds and day-night systems. The tips will be useful for apartment and house owners, as well as those planning to install sun protection systems in their office. By following our recommendations, you will be able to take accurate measurements yourself and order blinds of the perfect size.

Preparing for measurement: tools and general tips

Before you start measuring, it is important to prepare the necessary tools and consider a few rules for accurate measurement:

• Tools. You will need a metal tape measure (a steel tape measure does not stretch and provides better accuracy than a fabric tape measure), a pencil and paper (or a smartphone) to record the results, as well as a spirit level and a stool/ladder if you need to measure high openings. It is advisable to enlist the help of an assistant if you need to measure a wide window or opening – this will make the process easier.
• Measure several times. Each parameter (width and height) should be measured at least twice for verification. If you get different results, measure a third time and take the most consistent value. Take the time to measure the width and height in several places: for example, the width of the window opening at the top, in the centre and at the bottom; the height on the left and right. This will help to identify any distortions in the opening. For internal installation (blinds in an opening or on a frame), use the smallest of the values obtained – this will ensure that the structure fits exactly. For external installation (blinds on a wall or opening), take the largest size – so that the blinds completely cover the opening.
  
• Accuracy to the millimetre. Do not round the measurement results. Record the exact values to the millimetre. Even a few millimetres of difference can affect the appearance and performance of the finished product. If you are unsure between two values, it is better to check the measurement again than to order blinds of the wrong size.
  
• Width × height – don't mix them up! It is customary to write down the width first, then the height of the product. For example, 1200×1500 mm means a width of 1200 mm (120 cm) and a height of 1500 mm (150 cm). Do not mix up these parameters in your notes and when placing your order. After completing all measurements, carefully check your notes.
• Avoid common mistakes. Decide in advance how your structure will be mounted (see below for mounting methods). Do not forget to take into account the features of the window: whether there is a window sill, whether there is a handle that may interfere with the canvas, whether the frame is deep enough for internal mounting. If you plan to install blinds on plastic windows without drilling (on the sash), make sure that the selected system allows for this method and take into account the thickness of the brackets. A common mistake is to measure only the glass without adding allowances, or, conversely, to measure the entire opening when only the sash parameters are required. Therefore, carefully read the following sections, which describe the measurement methods for each type.

Choosing the installation method: on the opening or on the sash

Before proceeding directly to measurements, determine how your blinds will be installed. There are two main mounting methods:

• Installation on the opening (external installation). In this case, the blind rail is fixed above the window opening – to the wall above the window or to the ceiling. The fabric (slats) not only cover the window itself, but also cover part of the wall around it. This method is most often used for vertical blinds and large roller blinds, as well as when the window frame is shallow or the opening is uneven. The advantage of external installation is that the blinds completely cover the window and visually enlarge it. The disadvantage is that the structure will cover part of the wall and may block access to the window sill. For correct measurement when installing on an opening, you need to know the clear size of the window opening and add allowances (this is described in detail in the sections for each type).
  
• Installation on the sash (internal installation on the frame). Blinds are installed directly on the window frame (on each sash separately) using small brackets or self-tapping screws. This method is popular for horizontal blinds and compact roller blinds (including day-night systems) on plastic windows. Advantages: the window can be opened freely even when the blinds are down, and the window sill remains open (can be used for flowers, etc.). Measurements for installation on the sash differ in that the size of the glass part of the window (or the distance between the inner edges of the glazing beads) is measured, taking into account a small overlap on the frame to completely cover the glass. Details can be found in the relevant sections below.
  

Note: There is also installation inside the window opening (inside the niche). This is a type of internal mounting where the blinds or roller blinds are installed flush with the frame inside the opening (for example, on the upper horizontal bar of the window frame or the slope). The measurement principle for installation in an opening is similar to installation on a sash: you need to know the internal width and height of the niche and make a small indent so that the finished product fits freely between the slopes. In the following sections, we will also consider this option for some types of blinds.

Horizontal blinds: measurement features

Horizontal blinds (classic blinds with horizontal slats) are most often installed on plastic windows separately on each sash. This option is convenient for both home and office: the design is compact, inexpensive, and allows you to adjust the light by turning the slats. They can also be installed above the window opening if you need to cover a large window completely. Let's look at both methods of measuring horizontal blinds:

• Installation on the sash (on the window frame). The most popular method for metal-plastic windows is to attach the blinds to each sash. The question most often arises of how to measure blinds for plastic windows with this type of installation. To take the measurements correctly, measure the width of the glass – from the inner edge of the left glazing bead to the inner edge of the right glazing bead. Add ~1 cm to the width obtained (so that the slats overlap the frame slightly and do not leave a gap at the sides). Next, measure the height of the sash – from the top of the glazing bead (at the top of the glass) to the bottom of the lower glazing bead. Add ~3 cm to the height – this allowance will be used for the upper blind rail (which is attached slightly above the glass) and for the lower fasteners or slat fixings. As a result, the blinds will completely cover the glass and partially cover the glazing beads, ensuring maximum effective blackout without gaps. Additional tip: choose the control side (cord or turn stick) opposite the window handle – this way, the blinds will not interfere with opening the sash.
  
• Installation on an opening (above the window). If you plan to install a single wide horizontal structure that will cover the entire window opening, the measurements are taken differently. Measure the total width of the window opening (from the left to the right). It is recommended to add an allowance of ~5 cm to this size (so that the blinds protrude approximately 2–3 cm on each side and are guaranteed to cover the window width). Next, determine the height – measure from the top edge of the opening (or from the point where the cornice will be attached to the wall/ceiling) to the window sill. It is recommended to add about 5 cm to the height so that the bottom slats cover the window sill or remain slightly below the opening line. Horizontal blinds are quite light, so there is no need to add a large overlap – a few extra centimetres at the top and sides are sufficient. If you are attaching the cornice to the ceiling, the height can be measured from the ceiling to the window sill. Note: make sure there is enough space at the top for the brackets and that the lowered slats will not hit any objects on the window sill.

Vertical blinds: how to measure the opening for the slats

Vertical blinds consist of vertical slats and are usually installed on large windows, balcony doors or office partitions. They can be mounted outside the opening (to the ceiling or above the window on the wall) – this is the most common method – or inside the window recess. Vertical blinds are rarely installed on window sashes, as they are bulky and designed to completely cover the opening. Let's look at two options:

• Installation on the opening (to the ceiling or wall). First, measure the clear width of the window opening. To ensure that the blinds completely cover the window and look attractive from the side, add approximately 20 cm to the width of the opening – this is ~10 cm on the left and right for overlap. This way, the outer slats will extend beyond the opening and will not let light through at the edges when closed. Next, determine the height: if you plan to mount the blinds on the wall above the window, measure from the mounting point of the cornice to the window sill. Add about 10–15 cm to the height of the opening at the top so that the cornice is positioned above the opening (this way, the slats will not partially block the view when closed). The bottom edge of the slats is usually made at the level of the window sill or 1–2 cm higher so that the slats do not rest on the window sill. If you plan to attach the curtain rod to the ceiling, then the height of the blinds will be equal to the distance from the ceiling to the window sill (or to the desired level if there is no window sill). Please note: for external installation, take into account any protruding elements – window handles, radiators under the window sill, etc. If the window sill or handle protrudes significantly, order extended brackets so that the slats hang at a distance from the obstacle and move freely.
  
• Installation in an opening (inside a niche). If you decide to install vertical blinds inside the window opening, you must take measurements very carefully, taking into account the gaps for the slats to move. Measure the width of the opening at the top (where the cornice will be) and at the bottom. From the values obtained, select the minimum and subtract ~1 cm – this will be the required width of the blinds, which is guaranteed to fit between the slopes. Next, measure the height of the opening on the left and right; take the smallest measurement and subtract 1–2 cm. This way, the slats will be slightly shorter than the height of the niche and will not rub against the window sill or floor. As a result, the blinds will fit evenly inside the opening, but remember: this installation method requires a perfectly even opening. If the walls are crooked or skewed, it is better to choose external mounting, as any unevenness will be noticeable in closed vertical blinds (the slats may hang unevenly).
  

Roller blinds: measurements for different installation methods

Roller blinds (fabric roller blinds) are a modern and versatile option for sun protection. They consist of a single piece of fabric that is rolled onto a roller. Roller blinds are suitable for both residential interiors and offices, and can be mounted on a frame or above a window. Correct measurement of roller blinds ensures easy installation and no gaps at the edges. Let's look at how to measure a window for a roller blind in three cases: when mounting on the sash, inside the opening and on the opening.

• On the sash (each window sash separately). Many roller blinds (especially Mini systems) are designed to be installed directly onto a plastic frame without drilling. To order a roller blind of the right size, first measure the width of the glass (from the edge of the left glazing bead to the edge of the right). Add approximately 3–4 cm to this width – this allowance will allow the curtain fabric to cover the glass and partially overlap the glazing beads to prevent light gaps on the sides. (Usually, 1.5–2 cm overlap on each side is sufficient, but keep in mind that too wide an overlap may interfere with the window handle). Then measure the height of the sash – from the top edge of the frame to the bottom. The height of the roller blind is usually taken as equal to the height of the sash (or 1–2 cm more if you want the closed fabric to extend below the level of the glass). This is sufficient, as the roller is attached at the top directly above the glass, and the fabric can be lowered slightly below the glazing bead at the bottom. Important: make sure that the depth of the glazing beads (the distance from the glass to the edge of the frame) allows the roller blind to be installed on the sash – some cassette systems require a few centimetres of flat surface. If the glazing bead is very rounded or narrow, a different type of fastening may be required.
• In an opening (inside a window recess). Installing a roller blind inside an opening requires very precise measurements, as the fabric must fit between the slopes. Measure the inner width of the opening (it is best to do this at the top, in the middle and at the bottom and take the smallest value). Subtract ~0.5–1 cm from the resulting width for the gap. Similarly, measure the height of the recess on both sides and use the smallest figure, reducing it slightly (by 0.5–1 cm). This way, the finished roller blind will fit freely into the opening and be attached either to the top of the frame or to the ceiling of the niche. Please note: if the window has a pull-out handle or a protruding window sill, make sure that they do not interfere with the movement of the fabric. It may be worth choosing external mounting so that the blind passes in front of the handle.
  
• On the opening (external surface mounting). When you need to cover the entire window opening with a roller blind (for example, in a kitchen or office with a wide window), take measurements with a margin. First, determine the width of the opening from corner to corner. Add ~10 cm to this measurement (so that the fabric extends approximately 5 cm on each side and completely covers the opening). Then measure the height from the mounting point (ceiling or wall above the window) to the window sill. We recommend adding ~15–20 cm to the height. Part of this allowance will be used to attach the roller blind above the opening (it is worth attaching the roller blind slightly above the upper edge of the window so that it does not interfere with the opening of the sash, if there is one). The remaining centimetres will ensure that the lowered blind covers the window sill and leaves no gaps at the bottom. If the window sill does not need to be covered, you can add less – ~10 cm so that the fabric simply covers the glass completely. Tip: for wide windows, it is better to install two roller blinds to ensure that they are perfectly horizontal. Make sure that the wall mountings are sturdy and can support the weight of the roller blind.
  

Day-night blinds: specific measurements

Day-night systems are a type of roller blind consisting of a fabric with strips of transparent and opaque fabric. By moving the double-layer fabric, you can adjust the level of light in the room. Measuring day-night roller blinds is generally similar to measuring regular roller blinds, but there are a few nuances:

• On the frame (on the sash). Most day-night blinds for plastic windows are made as compact cassette systems that are attached to the sash. Measurements are taken in the same way as for a regular roller blind: measure the width of the glass at the edges of the glazing beads and add ~3 cm (1.5 cm on each side) – this is enough for the striped fabric to completely cover the window glass without interfering with the window fittings. The height of the product is measured from the top edge of the frame to the bottom (or to the desired closing point). Usually, the height of the day-night system is made equal to the height of the entire sash so that when the blinds are lowered, the stripes alternate evenly across the entire window area. Check that there is enough space on the frame to attach the cassette at the top (some systems require about 4–5 cm of flat surface above the glass). If there is not enough space or the glazing bead is very sloped, you can choose an open type day-night blind – it is attached to brackets, and the frame requirements are the same as for conventional roller blinds.
  
• In the opening (inside the niche). Installing day-night blinds inside the opening is less common, as the striped structure looks best in a cassette design on the sash. However, if you want to install the system in the opening, follow the recommendations for roller blinds: measure the internal dimensions of the opening and subtract 1–2 cm for gaps. Please note that even small misalignments will be noticeable – the stripes should be parallel to the window sill. Therefore, check the mounting location with a level and, if possible, mount the cassette strictly horizontally. If the window opening is uneven, it is better to switch to external installation to avoid gaps.
  
• For an opening (external installation). When installing a day-night system above a window (on a wall or ceiling), use the same measurements as for roller blinds. The only thing to keep in mind is to make sure that there is enough space at the top of the blind to fully extend the double fabric. Add a few extra centimetres to the required height if you want the transparent strips to completely cover the opening when in the open (day) position. In general, take the width 8–10 cm larger than the opening (so that the strips overlap the edges) and the height 15–20 cm larger, taking into account the dimensions of the cassette.

Tips for avoiding mistakes when measuring

When measuring any blinds, keep in mind some practical tips to help you avoid annoying mistakes:

• Write everything down immediately. Don't rely on your memory — write all the numbers down in a notebook right away. It is advisable to indicate next to them which window and which part they refer to (for example: ‘kitchen, left window, width 1205 mm, height 1480 mm, wall mounting’). This is especially important if you need to measure several openings.
• Take into account fittings and batteries. Before measuring, check that nothing protrudes into the area where the blind will be installed. Window handles, transom windows, air conditioners, heating pipes under the window — all of these can either interfere with installation or be covered by the blinds. You may need to slightly shift the mounting point or choose a different type of blinds/brackets.
• Double-check before ordering. Once you have all the necessary measurements, do a ‘double check’. Imagine that the blinds are already installed and mentally check that there are no problems (are there enough overlaps, will the product be too wide and bump into furniture, etc.). If any of the measurements are in doubt, re-measure the relevant parameter to be sure.
  

Finishing touches: professional measurement and custom-made blinds

Correct measurements are half the battle when it comes to installing blinds. If you have done everything carefully and accurately, you will get a design that fits perfectly into your window opening, provides effective protection from the sun and looks aesthetically pleasing. If you have any doubts, you can always seek help from professionals.

Our company manufactures custom-made blinds according to individual sizes and offers professional measurement and installation services. Experienced craftsmen will come to you, take all the necessary measurements and install the blinds quickly and efficiently. You are guaranteed that the design will fit your window perfectly, whether in an apartment, private house or office. Order custom-made blinds from us and let your windows always look stylish and functional!