Measure the width, height, and depth of your existing frame, not the glass
The most common mistake is measuring the glass itself. You need the dimensions of the frame — the metal or wood border that holds the glass. Measure from the outside edge of the frame on all sides, because that is what determines whether a new door will fit into your opening.
Start with width. Open the door fully and measure horizontally from the outside of the left frame to the outside of the right frame. Write this number down. Then close the door and measure the same span again — the numbers should match. If they do not, your frame is out of square, which matters when you order.
Next, measure height from the top of the frame to the bottom, at three points: the left side, the center, and the right side. Write all three down. If they differ by more than a quarter inch, the frame is not level, and you will need to tell the supplier.
Finally, measure depth — how far the frame sticks out from the wall on both the interior and exterior sides. This determines what size frame the new door needs to have. Measure from the outside wall surface to the outside edge of the frame on both sides.
Key Takeaways
- Measure the outside edges of the frame itself, not the glass panes, at three height points to catch an out-of-level opening.
- Write down all measurements in inches, and note any variation between the three height readings — suppliers need to know if your frame is square.
- Measure the frame depth on both the interior and exterior sides, because new doors must match the thickness of your existing wall cavity.
- If your frame is more than a quarter inch out of square or level, tell the supplier before ordering, as this affects how the door will hang.
Take measurements at the right spots to catch installation problems early
Many frames are not perfectly square or level, and catching that now saves you from ordering a door that will not fit. Measure height at the left edge, the middle, and the right edge. If the left side is 80 inches, the middle is 80.25 inches, and the right is 80.5 inches, that tells you the frame is tilted — and the supplier needs to know.
Do the same for width: measure at the top, middle, and bottom. A frame that is 36 inches wide at the top but 36.5 inches at the bottom is wider at the bottom, which affects how the door will close.
Measure depth in two places — at the top and bottom — because some frames are thicker at one end than the other. This is especially common in older homes where the wall has settled.
Account for the threshold and sill when measuring height
The threshold is the raised lip at the bottom of the door frame. Measure height from the top of the frame down to the top surface of the threshold, not to the floor. This is the actual opening the door glass and frame must fit through.
If you are replacing a sliding glass door with a hinged patio door, or vice versa, the threshold height may change. A sliding door typically has a lower threshold than a hinged door, so measure what you have now and ask the supplier whether the new door type will need a different threshold height.
Write down the frame material and condition
Note whether your frame is aluminum, vinyl, or wood. This matters because some replacement doors are designed to fit into existing aluminum frames, while others require the frame to be removed and replaced entirely. A supplier can tell you which option works for your material, but only if you tell them what you have.
Also note any visible damage: cracks in the frame, rot in wood, or bent aluminum. If the frame is damaged, a new door may not seal properly, and you may need to replace the frame as well. This is not something the measurements will show, but it affects the cost and timeline of the job.
Understand the difference between rough opening and finished opening
The finished opening is what you measure — the outside edges of the frame as it sits in your wall right now. The rough opening is the hole in the wall before the frame was installed. You do not need the rough opening measurement unless you are removing the frame and installing a completely new one.
If you are keeping the existing frame and replacing only the door panels or sash, you only need the finished opening. If the frame is damaged or you want to change the door type, you may need the rough opening, and that requires removing trim and measuring the wall cavity itself — a job for a contractor, not a homeowner with a tape measure.
Record measurements in a format a supplier can use
Write your measurements like this:
| Dimension | Left/Top | Center/Middle | Right/Bottom |
| Height (inches) | 80 | 80.25 | 80.5 |
| Width (inches) | 36 | 36.125 | 36.25 |
| Depth exterior (inches) | 3.5 | — | 3.5 |
| Depth interior (inches) | 3.75 | — | 3.75 |
Include the frame material (aluminum, vinyl, wood), the current door type (sliding, hinged, French), and any damage you see. A photo of the frame from the outside and inside is also useful — it shows the supplier the condition and style without requiring a site visit.
Know when to call a professional instead of measuring yourself
If your frame is severely out of square — more than half an inch difference between the three height measurements — or if you see rot, water damage, or gaps between the frame and the wall, a professional measurement is worth the cost. A contractor can measure the rough opening, assess whether the frame needs replacement, and catch problems you might miss.
Similarly, if you are replacing a door in a historic home or one with an unusual frame size, a professional can determine whether a standard door will fit or whether a custom door is necessary. The measurement fee is usually $50 to $150, and it prevents ordering a door that will not fit.
Frequently Asked Questions
Should I measure in inches or millimeters?
Use inches. Nearly all patio doors sold in North America are sized in inches, and suppliers expect measurements in inches. If you measure in centimeters or millimeters, convert to inches before contacting a supplier — one inch equals 2.54 centimeters.
Do I need to measure the glass, or just the frame?
Measure the frame only. The glass is held by the frame, and the frame is what fits into your wall opening. If you measure the glass, you will order a door that is too small.
What if my measurements are not exact — should I round up or down?
Write down the exact measurement, including fractions or decimals. If one height is 80.25 inches and another is 80.5 inches, tell the supplier both numbers. They will account for the variation when they build or adjust the door. Rounding can hide a problem that will cause the door to stick or not close properly.
Can I measure through a closed door, or do I need to open it?
Open the door fully to measure width. A closed door can be warped or swollen, which gives you a false width measurement. For height, it does not matter whether the door is open or closed, but measure with the door in its normal closed position to be consistent.
What if I cannot fit a tape measure into the corners of the frame?
Use a straightedge — a level, a long ruler, or a piece of straight wood — and measure from the straightedge to the wall or floor. You can also use a laser measure if you have one, which does not require you to fit anything into tight corners.