Ask for sliding glass doors size in feet and you will get a tidy list of numbers — 5 ft, 6 ft, 8 ft wide, 6 ft 8 in tall. But those numbers are only the marketing label. The door that actually arrives at the site is built and measured in millimetres, and the hole cut in your wall is a third, different measurement. This guide decodes all of them so you can order, frame, and install without an expensive return trip.
Most ordering mistakes happen because the same word “size” is used for four different measurements. Keep them apart before you cut a wall:
On a 6 ft (72 in) two-panel door, the clear passage is only about 30 to 33 inches — under a third of the nominal width, not half. That gap is exactly where most returns and restacking fees are born. Put the actual frame size in millimetres on your purchase order; the foot label alone is not enough.
The conversion is exact: 1 inch = 25.4 mm and 1 foot = 304.8 mm. What trips people up is that catalogues round to the nearest millimetre, so a 72 in unit becomes an 1829 mm unit on the drawing. The arithmetic below is exact; the catalogue rounding is what creates the small gap between the imperial label and the metric shop drawing.
| Nominal (ft–in) | Inches | Exact mm | Catalogue mm | Typical panels |
|---|---|---|---|---|
| 5 ft 0 in | 60 | 1524.0 | 1524 | 2 |
| 6 ft 0 in | 72 | 1828.8 | 1829 | 2 |
| 8 ft 0 in | 96 | 2438.4 | 2438 | 2 |
| 9 ft 0 in | 108 | 2743.2 | 2743 | 3 |
| 12 ft 0 in | 144 | 3657.6 | 3658 | 3 or 4 |
| 16 ft 0 in | 192 | 4876.8 | 4877 | 4 |
Heights follow the same rule. The most common stock height is 6 ft 8 in (80 in = 2032 mm); 82 in is about 2083 mm, and 8 ft (96 in) is 2438 mm. Confirm the exact height against the supplier’s drawing — tall openings change both the header depth and the glass weight.
The simplest layout is the double sliding door — one fixed leaf and one operable leaf on a two-track system. Because the leaves overlap, the clear passage is always less than half the nominal width. Add panels and the clear passage changes again depending on which leaves move.
| Layout | Nominal width | Approx. clear passage |
|---|---|---|
| 2-panel (1 fixed + 1 slide) | 5 / 6 / 8 ft | 22–24 / 30–33 / 43–45 in |
| 3-panel (fixed + 2, or OXO) | 9 / 12 ft | ~1/3 to ~2/3 of width by layout |
| 4-panel (2 fixed + 2 slide) | 12 / 16 ft | up to ~half when two leaves stack |
A 4 panel sliding door is the format that reaches 12 to 16 ft openings, and because two leaves can stack to one side it gives the widest clear passage of the standard runs — which is why it is the default for great rooms and pool decks.
In millimetres, the figure that matters for your wall is the frame wall width — the depth the closed door occupies in the jamb. Build the rough opening to this number plus its reveal, not to the foot label. The table below lists the real ALPES thermal-break sliding door series.
| Series | Frame wall width | Single-sash limit | Profile |
|---|---|---|---|
| SL155 (four-track) | 155 mm | 2800 × 2600 mm, ~7.5 m² | 6060-T66, 2.2–2.5 mm |
| SL126 / 182 (two / three-track) | 126 / 182 mm | up to 2500 × 3000 mm, ~7 m² | 6060-T66, 2.0 mm (to 3.0 mm) |
| SL112 / 167 (single / two / three-track) | 112 / 167 mm | up to 220 kg per sash | 6060-T66, 1.8–2.5 mm |
One honest catch: frame depth is a series property, not a function of track count. The four-track SL155 is 155 mm deep, which is actually shallower than the three-track SL126/182 at 182 mm. Do not size the wall jamb by counting tracks — read the series drawing.
A size table can list 16 ft, but the door only works if the rollers and glass carry the weight. The load ceiling is what actually caps the biggest single leaf:
Past these figures you are into engineered multi-slide runs that need a structural header and a signed load path. The number on the size chart is a starting point; the hardware rating is the limit.
| Your opening | Route |
|---|---|
| Up to 8 ft, single configuration | Stock / standard track system |
| 9–16 ft, multiple panels | Configurable multi-track, standard series |
| Over 16 ft, or over ~10 ft tall, or non-rectangular | Custom, engineered with signed load path |
| Coastal / high-wind climate | Confirm performance spec with the supplier first |
When you write the order, give the supplier the millimetre figures, not the foot label:
| Nominal | Total unit width (mm) | Rough opening (mm) | Approx. clear passage (mm) |
|---|---|---|---|
| 5 ft 0 in | 1524 | ~1540 | ~580 |
| 6 ft 0 in | 1829 | ~1845 | ~790 |
| 8 ft 0 in | 2438 | ~2454 | ~1090 |
| 12 ft 0 in | 3658 | ~3675 | by panel layout |
| 16 ft 0 in | 4877 | ~4895 | by panel layout |
Before you commit the wall, confirm the exact dimension of sliding door you need with the supplier’s engineering team — glass weight, wind load, and track count all shift the final millimetre drawing. State the actual frame size in mm on the purchase order, keep the foot label for conversation only, and the install will be a weekend job instead of a return.
