To measure for a sliding door, measure the width of the opening at the top, middle and bottom and the height on the left, middle and right, then take the smallest of each as your working size. That gives a reliable figure for budgeting and comparing quotes, but the manufacturing size is always fixed by a survey, because the frame has to be made slightly smaller than the opening and a surveyor checks things a tape measure alone will not show.
This guide explains what to measure, the difference between the structural opening and the frame size, the tolerances involved, and what to look for at floor level and above the opening. If you are buying supply-only, it also explains why the sizes you give matter so much.
What do I need before I start?
A steel tape measure at least 5 m long, a long spirit level (1.2 m or more is ideal), a pencil and paper, and ideally a second person to hold the end of the tape. Photos of the opening from inside and outside are useful to send with your enquiry.
If there is an existing door in place, you are measuring around it, and the true opening is partly hidden by the frame, plaster and trims. For a replacement, the most useful figure is usually brick to brick from outside, between the faces of the reveals, because that is closest to the structural opening.
Structural opening or frame size: what is the difference?
The structural opening is the hole in the wall: brick to brick across, and from the underside of the lintel down to the floor or sill. The frame size is the overall size of the door frame that is made to go into it. The frame is made smaller than the opening so it can be packed level and square, fixed and sealed.
The difference is the fitting tolerance. A typical allowance is around 10 mm off the width and the height, roughly 5 mm each side, depending on the system, the fixing method and how true the opening is. An opening that is out of square may need a larger allowance. Always say which of the two your figures are, because mixing them up is one of the commonest reasons a door does not fit.
How do I measure the width and height?
- Step 1: Measure the width at the top of the opening, just below the lintel, from brick face to brick face.
- Step 2: Measure the width again in the middle and at the bottom, just above the floor or sill.
- Step 3: Write down all three widths and use the smallest.
- Step 4: Measure the height on the left, in the middle and on the right, from the underside of the lintel to the floor or sill.
- Step 5: Write down all three heights and use the smallest.
- Step 6: Measure both diagonals, corner to corner. If they differ by more than a few millimetres, the opening is out of square.
- Step 7: Check the floor across the opening with a spirit level and note any fall from one side to the other.
- Step 8: Photograph the opening inside and out, including the lintel area and the floor at both sides.
Measure in millimetres, not centimetres or inches, and do not round up. If your three readings differ by more than about 10 mm, note that too, because it tells the surveyor the opening is not square or the brickwork is out of line.
Why does the floor level matter?
The bottom track of a sliding door must be level for the panels to roll smoothly, stay put when open and seal properly when closed. If the floor or sill slopes across the opening, the frame has to be packed up at the low end, which raises the threshold slightly, or the floor has to be adjusted before fitting.
Lay your spirit level across the opening and note how much one end is higher than the other. Also compare the finished floor height inside with the ground or patio outside, because that decides which threshold options are realistic. Our sliding door thresholds guide explains the choices.
What should I check above the opening?
Look for a lintel above the opening. Most door openings have a steel or concrete lintel carrying the wall above; older homes may have timber, or occasionally very little at all behind the old frame. Signs of trouble include cracks running up from the corners of the opening, a sagging head, or an old door that has become hard to open because it is being squeezed.
If you want the new door wider than the existing opening, that is structural work. It needs structural calculations, a new steel beam or lintel and a Building Regulations application in England, or a building warrant in Scotland; Wales and Northern Ireland have their own building control processes. Our guide to widening an opening covers what is involved.
Why does a survey fix the final size?
Your own measurements are ideal for budgeting, but the size the door is made to comes from a survey. The surveyor measures the opening precisely, checks it for level and square, looks at the lintel and the reveals, confirms the threshold detail and notes anything that affects fitting, such as cavity closers, render, cladding or heating pipes close to the opening.
Surveys are free for new doors and replacements. Because the doors are made to measure, nothing is ordered until you have confirmed the surveyed specification in writing. That step protects you: goods made to the customer's specification are excluded from the usual 14-day cancellation right under regulation 28(1)(b) of the Consumer Contracts Regulations 2013, so the written sign-off is where the size and specification are locked in. Your rights under the Consumer Rights Act 2015 are unaffected.
I am buying supply-only. What am I responsible for?
If you buy a door supply-only, you or your builder are responsible for the sizes. We make the door to the dimensions you confirm in writing, so check whether you are giving the structural opening or the frame size, and say which. A door made to the wrong size cannot usually be altered or returned, because it was made to your specification.
You are also responsible for checking that the opening is sound, level and ready, that the lintel is adequate, and that the installation meets the Building Regulations, which in England means using an installer registered with a competent person scheme or notifying Building Control. Our supply-only sliding doors page sets out what is included and how delivery works.
Common questions
Should I give the structural opening or the frame size?
Give the structural opening, brick to brick and lintel to floor, and say clearly that is what it is. That lets the supplier apply the right fitting tolerance for the system. If you give a frame size instead, label it as such, because mixing the two up can leave a door too big for the opening or with gaps too wide to seal neatly.
How much smaller than the opening is the door frame?
Typically around 10 mm smaller in both width and height, which leaves roughly 5 mm each side for packing, fixing and sealing. The exact allowance depends on the system, how the frame is fixed and how true the opening is. An opening that is out of square or level may need a larger allowance, which the survey will pick up.
Can I measure for a replacement with the old door still in?
Yes, for budgeting. Measure from outside between the faces of the brickwork either side of the existing frame, and from the underside of the lintel to the sill, but remember the true opening is partly hidden by the frame, plaster and trims. The surveyor checks the reveals and fixing points more closely before the size is fixed.
What if my floor is not level?
A small slope can be taken up by packing under the frame at the low end, but that raises the threshold slightly. A larger slope may need the floor or sill adjusting before fitting. Note the difference across the opening with a spirit level when you measure, and send a photo, so the surveyor can plan the threshold and any making good.