How to measure sliding, patio and bi-fold doors correctly
Planning guide for patio access
A large glazed opening onto a patio can look simple on a plan: choose the width, height and door type. On site, however, the interaction of the structural opening, finished floor, wall build-up, travel direction and adjoining patio determines whether the door can be planned sensibly. One measurement is not enough, particularly for sliding and bi-fold doors. Ordering a preferred size too early can create conflicts with flooring, shading, furniture or drainage connections. This guide clearly separates the site survey, planning dimensions and final order dimensions. It helps you document an opening consistently and ask the right questions of designers, installers and suppliers.
Check before enquiring 1
Start by checking the door type at the opening
Your measurement approach starts with the movement the opening needs to provide. A single-leaf patio door needs swing space; a sliding door moves parallel to the wall; a bi-fold system folds several leaves into a stack. This affects more than width: it also affects usable passage width, furniture placement and the operating side. Do not compare overall dimensions alone. Draw a floor plan with the patio, seating, kitchen island or radiators, and mark where people actually walk. Broad glazing can bring in ample light while a narrower moving leaf may be enough for everyday access outside. Conversely, a regularly used route for carrying items outdoors may call for a different layout from occasional balcony access.
For an initial comparison, you can consider a Patio Life lift-and-slide door as an example of a large-format sliding solution alongside a conventional hinged door. Its name alone does not establish dimensions, performance values or equipment; request these separately. The key item is the sketch: which panel should remain fixed, which should move, and on which side should it sit when open? Also note whether full-height glazing should visually continue an adjoining window element. For alterations, establish whether the existing wall opening will remain unchanged or be redesigned. Measured figures have a reliable context only once the intended use, views and movement sequence are clear.
Record structural and clear openings separately
The structural opening is the opening in the building before installation; the clear opening is the unobstructed passage or visible opening within an existing construction. Both figures are useful, but they are not interchangeable. In a new build, measure width at the bottom, middle and top between the reveals, and height on the left, centre and right from the intended datum. In an existing property, linings, old frames, plaster edges and thresholds can conceal the actual building opening. Always state which surface each measurement runs from and to. A photograph marked with measurement points helps avoid later misunderstandings.
Use a rigid tape measure or suitable laser distance meter, and manually check any notable differences. Record every individual measurement in millimetres, not only the largest figure. The smallest dimension may govern the planning, but the reason for variation matters equally: an uneven reveal, sloping floor or uncertain plaster build-up needs a different solution from a uniform opening. Measure the diagonals as well. Unequal diagonals indicate that the opening is not square. This survey does not replace a qualified installer’s site measurement, but it provides a precise basis for discussion and comparing options.
Check before enquiring 2
Set the finished-floor-level datum
For patio doors, a height without a clear reference point has limited value. Use the top of the finished floor as the datum and mark it on the wall and sketch. In a new build, include every layer yet to be installed, such as subfloor build-up, screed and floor finish. In an existing home, establish whether the internal floor finish will remain, be replaced, or be changed for one of a different thickness. Measure from that proposed level to the underside of the lintel. Add the current height to the structural slab if the floor build-up remains undecided. This makes clear whether a figure describes an interim condition or the eventual usable height.
Outside, record a second height sequence: from the same datum across the threshold zone to the top of the future patio finish. Falls, finish thickness and the façade connection belong on the drawing, even if not yet finalised. A door may work indoors while the external junction remains unresolved. A sliding door should therefore be assessed against the planned floor and patio arrangement, not just the structural edge visible today. Ask the designer or installation contractor which information is needed for the proposed installation and who will coordinate the levels. In renovation work, that person should also know whether adjoining rooms are intended to end up at the same level.
Record width, depth and position within the wall
As well as opening width, wall depth affects how the frame, reveal and internal junctions meet. Measure the wall depth at several points and record its build-up where known: structural layer, insulation, lining, plaster or service cavity. Photographing only the visible reveal is not enough. In a multi-layer external wall, the installation position must be coordinated with the full wall construction. Draw the preferred door position within the wall depth too, if this has already been decided. Without it, an external elevation can only be coordinated to a limited extent with internal reveals, shutters or window boards. Clearly mark any unknown layers as unverified. Also check whether visible wall depth matches at the top, bottom and both sides. Differences may indicate changing linings or concealed junctions. A small section sketch showing inside and outside is often more informative than several unlabelled detail photographs.
Measure the available wall sections to the left and right, up to corners, downpipes, posts, power outlets, radiators or other openings. In a side-sliding system, a fixed panel can define the appearance while the moving panel needs no swing space when opened. A side-sliding Siegenia door cannot therefore be assessed simply from its overall width: record in particular which side is intended for the moving leaf and which view is being considered inside and outside. If curtains, shading or fitted furniture are planned, mark their space as well. This makes it easier to identify early whether operation, the handle and access for cleaning will remain practical.
Check before enquiring 3
Always mark opening direction from the correct view
The terms left and right quickly cause errors if the viewing direction is not recorded. Document every door with two simple views: an internal view and an external view. For hinged doors, draw the hinges on the relevant side and show the swing arc. State expressly whether the door opens inwards or outwards. For sliding elements, use arrows for the travel path and identify fixed and moving sections. For bi-fold systems, show the folding points and stacking side on the sketch. A photograph of the existing situation is useful, but cannot replace arrows because it does not reliably show the intended new movement direction.
An outward-opening leaf can save internal space, but it needs an unobstructed area outside. A single outward-opening patio door should therefore be checked particularly against planting, balustrades, steps and furniture. An inward-opening leaf instead takes up space in the room, for example in front of a dining table or bench seat. Do not ask only, “Which side is left?” Ask: from which viewpoint is the side named, where does the leaf move, and can the door open fully? These three details make an order much clearer. For later use with a pram, tray or garden furniture, it is also important to know whether the proposed route works without having to squeeze past.
Measure a bi-fold system as a movement sequence
With a bi-fold door, the number and arrangement of leaves determines how wide the opening is actually released in everyday use. First measure the whole opening, then allow space for the folded leaf stack. Where will it park: inside or outside, left or right? Can it stand there without blocking a corner, light fitting, privacy screen or patio access? Also mark which leaf should serve as the daily access door, if that is intended. This prevents the entire system needing to be opened for a brief trip outside. For wide openings, a scale paper sketch is useful for checking the movement space clearly.
An outward-opening bi-fold door is an example of a configuration where external space and stacking direction need particularly careful planning. Before making the final choice, confirm the specific leaf division, operation and required clearances from the product documents and proposed installation. For an existing patio, also measure distances to balustrades, level changes and canopy posts. A leaf stack that fits on a floor plan may still narrow the main route in real use. In renovation work, also record whether old fittings, linings or an existing canopy affect the desired opening. Check movement before making the visual decision about equal-width panels or glazing bars.
Check before enquiring 4
Assess the passage, not overall width
The overall width of a system does not automatically describe the clear route outside. With hinged doors, the frame and open leaf affect the situation differently from sliding or bi-fold systems. In multi-part systems, one panel may remain fixed or part of the width may be affected by the stacked leaves. Set an everyday-use test for your planning: should a person pass comfortably carrying a tray, will outdoor furniture be moved regularly, does the route lead to an eating area, or is the door mainly for ventilation and views? This use description is more helpful than a general request for “as large as possible”. It directs selection towards a suitable opening type and sensible leaf arrangement.
A single inward-opening patio door can be the simple option where wall width is limited, provided there is clear swing space in the room. A larger system can extend views and access, but requires more coordination with structure, wall junctions and external space. Draw furniture at its real depth rather than ideally; allow for curtains as well as the room required to open a leaf. Test the route with two people travelling in opposite directions if the patio and kitchen are frequently connected. Record the resulting passage requirement as a use objective, not as a self-determined order dimension. Which clear dimensions are ultimately achievable depends on the selected construction and the confirmed site measurement.
Document lintel, reveals and existing conditions carefully
The top of the opening deserves as much attention as the floor. Measure the height to the lintel at both sides and in the centre, photograph visible cracks, services, shutter boxes or unusual junctions, and note their location. During replacement, the old frame may conceal parts of the reveal. Do not remove anything solely to take measurements if this could affect the building fabric or waterproofing. Instead, record visible dimensions and uncertainties. If the new opening is to become wider or taller, this is not simply a door issue: suitable qualified professionals must assess changes to the wall and effects on adjacent building elements.
Also check the flatness of the side surfaces with a long straightedge and record bulges or recessed sections. This matters because a neat measurement alone says nothing about the junction surface. Note whether a window board, radiator, lining or electrical installation is immediately adjacent inside. Outside, brickwork, render, cladding or existing waterproofing can influence detail design. A site survey separated into structural opening, internal junction and external junction gives the installer a better basis for decisions. The final solution should not be inferred from an old door leaf happening to fit the opening today.
Check before enquiring 5
Draw the patio, water route and outdoor space
A door does not end at its frame. Draw the first few metres outside the façade: patio finish, falls, steps, drainage elements, canopy, balustrades, planters and paths. Add the proposed design as well as the existing one if the patio is being renewed anyway. In wet weather, external-space planning must not become an afterthought; junction details need to suit the door project. This guide does not replace technical waterproofing or drainage design. It does, however, help bring the necessary questions into view early: where will the future finished surface sit, where will the area fall, and who coordinates the door, façade and patio?
For a double outward-opening patio door both swing areas should be drawn on the external view. Check whether open leaves obstruct paths, seating or a step. The answer does not necessarily have to be another door model: furniture, a barbecue parking position or the route layout can sometimes be changed. What matters is making the decision consciously. Also note whether an awning, external shading or insect screen is planned so that its installation space and operation can be discussed early. In a terraced-house garden, even a narrow side passage can be decisive; on a generous patio, furniture in front of the glazing may matter more.
Create a process from measurement sheet to order
A useful measurement sheet includes the project address, room name, date, contact person and a clear position number for every opening. Add internal and external photographs, a floor plan, an elevation and every individual dimension in millimetres. Record what already exists and what is planned: new floor finish, altered patio, internal fit-out, shading or cladding. Do not put dimensions from different construction stages in one column. A field marked “open” is better than an assumption. When enquiring, send more than width by height: include the preferred opening type, viewing side, leaf movement, and whether the dimension describes the structural opening or finished construction. The supplier can then ask targeted follow-up questions. Give each revision a clear status and visibly withdraw superseded sheets. Before approval, establish which drawing applies, who entered changes, and whether internal fit-out, façade design and patio planning use the same height datum. This prevents correct individual details from being combined accidentally from different planning stages.
A double inward-opening patio door is only unambiguously requested when hinge sides, the active leaf, room arrangement and height datum are known as well as the overall opening. Before ordering, ask for every position to be checked against your drawing and clarify who is responsible for the final site measurement and junction coordination. Bloomcabin states a five-year commercial guarantee for windows and doors, subject to its terms; delivery takes four to six weeks, subject to order confirmation. The commercial guarantee is separate from any statutory consumer remedies that apply. These details replace neither technical approval nor checks of every project-specific detail. Keep confirmed drawings, measurements and revision records together. With several façade openings, a clear position list helps prevent a correctly measured door from being planned for the wrong location.
Frequently asked questions
Should I provide the smallest or largest opening measurement?
Record every measured value with its measurement point rather than selecting one figure. The smallest available space is often relevant to further planning, but variations must be understood. An irregular reveal, a fall or a floor build-up that is still missing will prompt different follow-up questions. The final order size should be professionally confirmed.
From where do I determine left and right for a patio door?
Always state the viewing direction expressly, for example “viewed from inside”. For hinged doors, add the hinge side and a swing arc. For sliding doors, mark moving and fixed panels and the travel direction. A labelled internal and external view is far more reliable than writing “left-hand” without a drawing.
Can I use an existing door’s dimensions for a replacement?
They are a useful starting point, but not a reliable order size. The old frame may conceal the actual building opening, while new floor finishes, wall junctions or a different opening type can change the planning. Record the visible situation, possible removal limits and proposed finishes. A qualified installer should check the final installation arrangement.
Does the patio need to be fully planned already?
Not completely, but levels and important elements should be known or marked as open. The planned top of the finish, falls, steps, routes and nearby obstructions are important. This information enables coordination of junction details. If external planning is still missing, the door decision should not rely on an assumed finish level.
What should an enquiry include besides width and height?
State the opening type, internal and external view, leaf movement, structural or finished dimension, wall depth and the relationship to finished floor level. Include photographs, a sketch and information on the patio, shading, and adjoining furniture or building elements. For existing buildings, notes on uneven reveals, linings and planned alterations are helpful.
Next steps
Measuring patio, sliding and bi-fold doors well means thinking through later use: who passes through and when, where does the leaf move, at what level do floor and patio meet, and which building elements are adjacent? Treat the opening, wall, interior and outdoor space as one connected situation. Record every measurement with its datum rather than turning one figure into an order size. A clear sketch, photographs and a list of unresolved questions allow the preferred door type to be compared on a sound basis. Final configuration and installation should then form part of coordinated planning with the responsible qualified professionals.
