A sash window that is 5 mm out can be the difference between a clean installation and costly site adjustments. Knowing how to measure window sash properly gives you a reliable starting point for a replacement quote, whether you need supply only or a fully fitted solution.
The key is to measure the right part of the window for the job. A moving sash, the opening it travels within and the complete box frame are different dimensions. Do not assume that the visible glass size, brick opening or an old supplier’s label is enough to order from.
First, establish what you are replacing
A traditional vertical sliding sash window has two moving panels: the upper sash and lower sash. These slide within a timber box frame, usually with sash cords, weights or a modern spring balance system concealed at the sides. Some windows have one fixed sash and one moving sash, while others are modern tilt-in designs made to resemble traditional sliding windows.
If you are replacing the complete window, the critical dimensions are normally the existing box-frame width and height, along with the depth of the frame and details around the internal reveals. If only one damaged sash is being renewed, you need precise measurements of that sash as well as its thickness, profile and operating arrangement.
For an initial quotation, clear overall measurements and photographs are usually more useful than trying to identify every historic detail. Final manufacturing dimensions should always be confirmed before an order is placed, particularly on older London and Essex properties where openings are rarely perfectly square.
Tools you need to measure a window sash
Use a rigid steel tape measure rather than a fabric tape. You will also need a pencil, paper or notes app, a spirit level if available, and a camera. A ruler or digital calliper can help when measuring sash thickness, glazing bars and existing glazing units.
Take every dimension in millimetres. For example, record 1,245 mm rather than 124.5 cm. This avoids unnecessary conversion errors when the window is being specified and quoted.
If access is difficult, do not lean out of an upper-floor window or remove beads, cords or trims that you are not confident reinstating. Photograph the area and arrange a professional survey instead.
How to measure window sash width and height
Start by deciding whether you are recording the sash itself or the complete window frame. Label each measurement clearly. A simple note such as “front bedroom, complete frame” prevents confusion once several windows are involved.
Measuring the complete box frame
For a full replacement, measure the internal width of the existing box frame at three points: near the top, through the centre and near the bottom. Measure from the inside face of one box lining to the inside face of the other, not from decorative architrave or plaster.
Record all three figures. The smallest measurement is often the most relevant because it shows the tightest part of the opening, but do not reduce dimensions yourself to allow for fitting tolerances. A supplier or installer needs the actual measurements to assess the right manufacturing size and installation allowance.
Next, measure the height on the left, centre and right. Take this from the underside of the head of the frame down to the sill or the lowest fixed point of the box frame. Again, record every result. Uneven settlement, worn sills and previous alterations can make a noticeable difference across an older opening.
Measure the diagonal from top left to bottom right, then from top right to bottom left. If the diagonals differ, the opening is out of square. That does not necessarily prevent replacement, but it should be flagged early so the specification and fitting method can account for it.
Measuring an individual sash
If only the upper or lower sash is being replaced, measure the timber sash itself rather than the clear opening. Open the window enough to access the edges safely, then measure the sash width at the top, middle and bottom. Take the height at both sides.
Measure from the outer edge of the sash timber, including the meeting rails, but excluding temporary paint build-up where possible. Heavy layers of old paint can add several millimetres and disguise the true profile. Note this on your enquiry rather than scraping or forcing a stiff sash.
For a double-hung window, measure upper and lower sashes separately. They may appear identical but can differ in height, rail design or glazing-bar layout. In particular, check the meeting rail where the two sashes overlap.
Measure depth, thickness and clearances
Width and height alone are not enough for a like-for-like sash replacement. Measure the thickness of the moving sash at an exposed edge, usually at the meeting rail or side stile. Older timber sashes can vary, and a new sash must work correctly with the existing parting bead, staff bead and pulley positions.
Also record the overall depth of the box frame from the internal face to the external face where accessible. This helps establish whether a replacement can retain the existing box or whether a complete new box-frame window is more practical.
If you are upgrading from single glazing to double glazing, depth matters even more. Modern sealed units require space, and a slim double-glazed unit may be appropriate where a standard unit would alter the sightlines or make the sash too heavy. The final choice depends on the window style, conservation expectations, required thermal performance and the condition of the existing frame.
Check how much clearance there is around the moving sash. Look for rubbing marks, swollen timber, gaps and loose beads. A sash that currently sticks may not simply be oversized - the box could be distorted, the cords may have failed or layers of paint may be restricting movement. Include photographs of these details with your measurements.
Do not measure from the glass alone
The visible glass area is useful for matching glazing bars, but it is not the sash size. The glass sits within timber rebates and is held by putty or glazing beads. Ordering from the glass measurement can leave you with a sash that is substantially too small.
For a period-style window, note the number and arrangement of panes. Photograph the window straight on from inside and outside. This makes it easier to match horn details, sash-bar positions, meeting rails, hardware and the overall proportions that give a property its character.
You should also record whether the existing window has Georgian, Victorian or other decorative glazing patterns. Exact historical classification is less important than a clear photograph and accurate description of what needs to be retained.
Account for sills, reveals and surrounding finishes
A replacement sash window has to fit the building, not just the old frame. Measure the internal reveal depth and inspect the external sill, brickwork, render and any stone surrounds. A rotten timber sill, cracked render or damaged brick reveal may need repair before or during fitting.
For flats and upper-storey properties, access is another practical consideration. A large glazed sash may require careful handling, lifting equipment or particular delivery arrangements. Mention floor level, parking restrictions and access constraints when requesting a quote. This is especially relevant on busy London streets, where installation time and access planning affect the job as much as the window dimensions.
If the property is listed or sits within a conservation area, do not presume that any replacement specification will be acceptable. Requirements vary by local authority and by the existing window’s significance. Retaining slim sightlines, matching glazing-bar patterns and selecting an appropriate finish may be necessary, even when modern double glazing is the goal.
A practical measuring record for your quote
For each window, provide a room or location reference, quantity, width and height measurements at three points, frame depth, sash thickness where relevant, and two diagonal checks. Add internal and external photographs, plus close-ups of any damaged areas, horns, glazing bars and hardware.
State whether you require supply only or supply and fitting. Trade customers who already have an installer may only need made-to-measure windows delivered to site. Homeowners and project managers may prefer a complete installation service, particularly where old boxes, sills or surrounding finishes need assessment.
Avoid rounding every figure to the nearest 10 mm. Record what the tape shows. A measurement of 1,243 mm is more useful than 1,240 mm when the project is reviewed, even though final dimensions may later be adjusted for fitting.
When a professional survey is the better option
Self-measurement is ideal for obtaining an early budget figure and comparing like-for-like specifications. It is less suitable when the windows are badly decayed, the openings are visibly distorted, access is unsafe or you are replacing a large number of windows across a refurbishment.
A survey can confirm the manufacturing sizes, assess frame condition and identify the details that affect price: glazing choice, sash profile, timber-style finish, hardware, access and installation work. It also avoids a common mistake - measuring an existing window as though it is square when it has been settling for more than a century.
Send clear measurements, photographs and your project location, then let the specification be checked against the actual opening. Bespoke Glaze can use those details to give practical guidance on the right replacement route, helping your project move towards a window that fits, performs and looks right from the first installation.