Where the pound per square foot comes from
Ordinary soda-lime glass has a density of about 2.5 grams per cubic centimetre. Convert that and you get roughly 156 pounds per cubic foot. Divide by twelve and you have the figure worth carrying in your head: about 13 pounds per square foot for every inch of thickness. Quarter inch glass is a quarter of that, near 3.25 pounds a square foot. Half inch is 6.5. Three quarter is 9.75.
The density field on the page is there because that 2.5 is a round number for a family of glasses rather than a constant. Low-iron runs fractionally different, borosilicate is lighter, and a fabricator who has weighed their own stock has a better number than any published one. If nobody has given you a figure, leave it at 156 and treat the answer as good to a couple of percent.
Thickness is linear, area is where the weight hides
Everybody knows thicker glass is heavier, and everybody adjusts for it. Almost nobody adjusts for area with the same instinct, because a large pane does not look proportionally heavier than a small one — it just looks large. So the mistake runs one way. A quarter inch mirror at 4 by 8 feet is 32 square feet at 3.25 pounds, which is 104 pounds. A three quarter inch shelf at 2 by 3 feet is 6 square feet at 9.75 pounds, which is 58.5. The shelf has three times the thickness and just over half the weight.
The large thin one is also worse to handle for reasons that have nothing to do with the pounds. It flexes, so two people cannot lift it at the ends without bowing it. It catches wind outdoors. It has four feet of unsupported edge in the middle where nobody is holding it. The calculator prints both of those comparison figures at the make-up you entered for exactly this reason.
Laminated and insulated make-ups
For a laminated pane the arithmetic is the two lites of glass plus the interlayer, and the interlayer is a small correction. A 0.030 inch PVB layer at roughly 1.07 grams per cubic centimetre works out around 0.17 pounds a square foot, which on a half inch laminated build is about three percent of the total. It is worth including for honesty and not worth arguing about.
For an insulated unit the air space weighs nothing at all, so the weight is simply the two lites. What the page leaves out is the spacer, the desiccant inside it and the edge seal, which together are a pound or two on an ordinary residential unit and more on a large commercial one. If the difference matters, the fabricator can give you a shipping weight for the exact unit, and that number beats any calculation.
Turning pounds into a crew and a set of cups
The last section divides. It takes the per-person figure you typed in and reports how many people that implies, and it takes the cup rating you typed in, divides it by the derating you chose, and reports how many cups that implies. Neither number is a recommendation, because the inputs are not ours. There is no published lift limit on this page and there is no cup rating on it either.
What is worth saying about cup ratings is that they are measured on clean, dry, flat glass at a stated orientation. Dust, condensation, a curved pane, cold weather, a coating on the surface and an angle other than the tested one all change the picture, which is why nobody who does this for a living treats the number on the cup as the number in the field. The divisor field exists so you can put your own margin in and see it applied.
The part that is not arithmetic
Glass fails suddenly. There is no creaking, no sag, no warning — it is intact and then it is not. Annealed glass in particular breaks into long shards, and the edges of a pane are dangerous even while it is whole, which is why glazers wear cut-resistant gloves to carry a pane that is not going anywhere near a saw. Tempered glass will not take a cut, a hole or a notch after tempering, and attempting one detonates the pane, so every dimension has to be right before it goes in the oven.
Where a location requires safety glazing, and what qualifies there, is set by the code your jurisdiction has adopted and by the inspector. This page will not tell you, because the answer varies by state, by city, by code edition and by the specific location in the building. Ask your glazier and your building department before ordering, not after.
Questions people ask
How much does a sheet of glass weigh per square foot?
About 13 pounds per square foot per inch of thickness for ordinary soda-lime glass, which comes from a density near 2.5 grams per cubic centimetre, or roughly 156 pounds per cubic foot. That makes quarter inch about 3.25 pounds a square foot, half inch about 6.5 and three quarter about 9.75. Nominal thickness is not always what arrives, so if the weight matters closely, ask the fabricator what they actually cut to.
Which is heavier, a big thin pane or a small thick one?
Usually the big thin one, and by more than people expect. Weight goes up in a straight line with thickness but it goes up with area as well, and area is the dimension that grows fastest on a real job. A 4 by 8 sheet in quarter inch is 104 pounds. A 2 by 3 shelf in three quarter inch is 58.5. The page prints both comparisons at whatever make-up you entered so you can see it on your own numbers.
Does laminated glass weigh much more than the same thickness of plain glass?
Slightly less, actually, for the same overall thickness, because the interlayer is lighter than glass. PVB is roughly 1.07 grams per cubic centimetre against 2.5 for the glass, so a 0.030 inch layer contributes about 0.17 pounds a square foot where the same thickness of glass would contribute about 0.39. On any normal build the difference is a couple of percent and it is included here for completeness rather than because it changes a decision.
How many suction cups do I need to lift a pane?
That is not a question this page answers, and it should not be answered by a calculator. What it does is divide: it takes the rating you read off your own cups, divides it by whatever derating figure you choose to apply, and reports how many that implies for the weight it calculated. Cup ratings are established on clean, dry, flat glass at a stated angle, and the field conditions that matter are the ones you can see and the calculator cannot.
Is the insulated unit weight here the shipping weight?
No. It is the two lites of glass and nothing else. Spacer, desiccant and edge seal are left out, which on an ordinary residential unit is a pound or two low and on a large commercial unit rather more. If you are working out what a delivery weighs or what a crew has to carry, ask the fabricator for the shipping weight of the specific unit — they have it, and a measured figure beats a calculated one every time.