To measure a sliding glass door, measure the opening - not the old door. Take the width at three heights and the height at three points, use the smallest number of each, check both diagonals, then check the sill for level and the jambs for plumb. Send those numbers in inches and I turn them into a frame size and rough opening.
The most common mistake is measuring a door that is already installed, then ordering a new one to match, as if every panel size were standard. A European sliding or lift-and-slide door is built to your opening, so the measurement is the order. Get it right once and the shop drawing is a formality; get it wrong and the error travels all the way to the container.
What Do You Need Before You Measure?
A 25 ft tape, a 4 ft level, a pencil and a way to write down numbers where you can find them again. Measure in inches to the nearest 1/8 in. If the drawings are in millimeters, convert once and keep both; the conversion table is in the rough opening guide.
Decide first which job this is, because the starting point differs:
- New opening (framing not built yet). You are choosing the size. Measure the plan, then confirm on the framed opening before ordering.
- Replacement of an existing slider. Measure the rough opening behind the old frame if you can see it, or the existing frame if you cannot yet.
How Do You Measure the Width?
- Measure the width at the top, middle and bottom of the opening, framing to framing (or, on a replacement, from the inside face of one side to the other).
- Write all three down. Use the smallest for sizing.
- If the three differ noticeably, note it. It tells the framer where the opening is out of plumb. The full window-and-door measuring steps are in the rough opening guide.
A single measurement is a guess. Openings in older houses are rarely the same width at the sill as at the head.
How Do You Measure the Height?
- Measure from the finished floor or the top of the sill pan to the underside of the header at the left, center and right.
- Use the smallest.
- Write down what the floor is. Finished flooring that is not installed yet changes the height, so say whether you measured to subfloor or finished floor. More door topics are in Doors & Openings.
Height is the number that gets people. A European lift-and-slide sill can sit lower and shallower than a typical US patio door threshold, so the height at the sill line needs its own confirmation. I do that on the shop drawing, not from a photo.
Is the Opening Square, Level and Plumb?
Measure both diagonals corner to corner. If they are within 1/8 in of each other, the opening is square enough to shim. If they are not, tell me the difference.
Then check the sill for level along its full length and the jambs for plumb. Sliders and lift-and-slide doors are the least forgiving products on a job: a sill that is out of level can make even a premium door feel heavy to push. The large-glass lift-and-slide guide covers why the track needs a rigid, level base.
How Does the Measured Opening Become a Door Size?
A bracket-mounted European door frame is smaller than the rough opening by a clearance on each side for leveling, shimming and a foam gap. The working range I use for planning is 1/2 in to 3/4 in per side, as set out in the rough opening guide. A nailing-fin unit follows a different detail, so which one applies is decided up front.
A worked example, using the same convention: if the smallest width you measure is 96 in, a 2400 mm frame (94 1/2 in) leaves 3/4 in per side. European doors are built to the opening, so you do not have to round to a modular size; the shop drawing carries the actual frame dimensions in millimeters.
Two things to keep in mind:
- Size and weight set limits. Panel dimensions and glass weight are bounded by the system, which is why a very wide opening may become three panels instead of two. The largest published panels are in the patio door sizes guide.
- Sizing is confirmed on the shop drawing before production.
What Is Different When You Replace an Existing Sliding Door?
Replacing a slider is a survey job. Check these before you order:
- What is behind the old frame. Siding, trim and flashing hide the real rough opening. Strip enough to see it before the number goes on an order.
- Whether the opening can take a wider or taller unit. A new door can only grow if the header and framing allow it; a wider opening may change the header and framing. That is a structural question for your engineer or framer, not a measurement.
- The sill and the water path. Old sill pans, membranes and flashing decide how the new sill is detailed. See retrofitting European windows into an existing home for the survey side.
- Hand and operating panel. Stand outside, look at the door and note which panel moves. I mark it on a sketch rather than in words, so there is no mix-up between "left" and "right".
- Screens, tracks and hardware on the old unit are a separate measurement. They do not carry over to a European door.
What Should You Send?
One sheet per opening, in this order:
- Opening ID that matches your window and door schedule
- Width at top, middle and bottom
- Height at left, center and right
- Both diagonals
- Sill level and jamb plumb notes
- Floor condition (subfloor or finished) and the wall build-up
- Photos of the opening from inside and outside
- A sketch of the operating panel and the direction it slides
Send that with the plan or the schedule and I will return the frame size, the rough opening and any size that looks tight. The same sheet applies to aluminum, uPVC, wood and aluminum-clad wood doors; for timber see wood sliding patio doors.
FAQ
How do you measure a sliding glass door? Measure the opening width at the top, middle and bottom and the height at the left, center and right, then use the smallest of each. Check both diagonals are within 1/8 in and the sill is level.
How do you measure a patio door for replacement? Measure the rough opening behind the old frame, not the old door. If you cannot see the framing yet, measure the existing frame, then confirm the true opening once the trim is off and before you order.
What is the rough opening for a sliding glass door? It is larger than the door frame by a clearance that depends on the product and mounting method. For a bracket-mounted European door I plan on 1/2 in to 3/4 in per side and confirm it on the shop drawing.
Do I measure the old door or the opening? The opening. European doors are built to the opening dimensions, so the old door's size matters only as a hint to where the framing is.
Can I get a wider sliding door than the old one? Sometimes, if the framing allows it. A wider opening may change the header and framing, which is a question for your framer or engineer before any door is ordered.
Have an opening to measure? Send me the numbers through the quote form or run the size through the estimator first.
Kai, your window guy!