Measure the masonry opening, not the old frame. Take the width in three places and the height in three places, then use the smallest figure of each. Check both diagonals to see whether the opening is square. Leave the fitting gap your supplier asks for, and never measure the door you are replacing.
Measure the opening. Your old frame has swollen, sagged or been packed out by whoever fitted it, so its dimensions describe a fitting job rather than a hole in your wall.
Take the old frame out of the equation entirely. What your new door has to fit into is brick, blockwork or timber, and that is what you measure.
Most replacement jobs sit in the first row. If you are keeping the frame, be honest about its condition first, because a new leaf hung in a tired frame inherits every fault the frame has.
A steel tape rather than a fabric one. Fabric tapes stretch, and three millimetres of stretch is enough to make a door that does not fit.
Add a spirit level, a pencil, and a notebook or your phone to write in. Take the readings in millimetres throughout, because mixing millimetres and inches is how measurements get transposed.
Get a second pair of hands for anything wider than a single door. Holding a tape level across a two metre opening on your own produces a sag you cannot see.
Work from the structure outward, and write every figure down as you take it. Nobody remembers six measurements accurately by the time they reach the kitchen table.
The smallest figure is the one that governs. A door sized to the widest point of a tapering opening does not go in, and no amount of adjustment afterwards changes that.
Photograph the opening while you are there, including the sill and the head. It answers most of the questions that come up when someone else reads your measurements.
Leave the fitting gap the installation guidance calls for, because the frame needs room to be shimmed square and then sealed. Our own fitting guidance sets it out by door type.
Our guidance also asks for the frame to sit around 3 to 5 cm back from the exterior edge of the wall, and for solid base sheathing behind every fixing point in a rigid material such as waterproof plywood.
Those numbers describe the fitting, not the order. When you order a made to measure door, give us the opening you measured and let us size the frame to it.
Because buildings settle and sliding doors do not tolerate it. Our fitting guidance requires at least a 10 mm air gap between the top of the frame and the wall member above, so the structure can move without pinching the door.
Two further rules come with it, and both matter at the measuring stage rather than later. Do not use rigid sheathing under the fixing points along the head, and do not use mortar as a filling material there.
Expanding foam does not belong on a sliding door installation either. It bows the frame, and a bowed frame gives you a door that drags no matter how carefully it was measured.
The bottom is just as particular. Our guidance calls for the sill to be blocked and left level and straight, with extra blocking at the outer ends and under the fixing points of the bottom rail. Check the level of your opening's sill while you have the spirit level out.
Measure both diagonals and compare them. Equal diagonals mean a square opening. A difference means the opening is out of square by roughly the amount of that difference, and your fitter has to make it up with shims.
Our fitting guidance is explicit about the target. The frame sides must be plumb and level, and the diagonal measurements from corner to corner must be equal.
Record the difference rather than ignoring it. An opening 15 mm out of square is fittable, and knowing about it in advance changes how the frame is packed. Finding out on the day costs an afternoon.
Check the head with the spirit level too. A sagging lintel shows up as a height that is shorter in the middle than at the ends, and that is the figure your door has to clear.
Measure the sill separately, because it changes both the height available and how the door drains. Note whether the existing sill is timber, stone or a formed cill, and whether it slopes away from the building.
The height you record should run from the finished internal floor level to the underside of the lintel. If your floor is being replaced or built up, measure to the level it will be, not the level it is.
Threshold height matters for anyone who will use a wheelchair or a pushchair through the door. Raise it as a requirement when you order rather than discovering it at installation.
Five mistakes account for most doors that do not fit.
One more that is specific to made to measure. A door built to your figures cannot be planed to fit, because the frame is sized to the opening you gave. Check every measurement twice before you order.
