Why thickness and density will not do it
A timber slab is solid, so its weight is length times width times thickness times a density and the arithmetic closes. A sectional garage door panel is a folded steel skin with polystyrene or polyurethane inside it and a backer on the other face, and the three materials have densities that differ by a factor of two hundred. Multiply the panel volume by anything and the answer is nonsense.
What does work is a weight per square foot, because that is the unit a panel is actually characterised in. Get that one figure right and everything else on the page is a multiplication.
The default door
A 108 by 84 in opening is 63 sq ft. Four sections of 21 in fill the height exactly, so there is no cut-down top panel. At 3.2 lb per sq ft the panel comes to 202 lb before anything is taken out of it, and 22 lb of hinges, rollers, brackets and struts goes back on. Total 218 lb, which is 109 lb hanging in each of the two cables and 3.47 lb per sq ft over the whole door.
The door weight does not care about the section height at all — it is an area times a weight per square foot, and the height divides that area up without changing it. What the section height does change is the counts. Go to 18 in sections and the same opening takes five rather than four, with the top one cut to 12 in: ten rollers instead of eight, four joints instead of three, and twelve hinges instead of nine. Go to 24 in and it is still four sections, because 84 divided by 24 rounds up to four, with the top one at 12 in.
Glass can make a door lighter
This is the result most people do not expect. Single strength glazing weighs about 1.63 lb per sq ft, and the insulated steel panel it replaces weighs 3.2. Cutting four 12 by 10 in lites into a section takes out 3.33 sq ft of panel worth 10.7 lb and puts back 5.4 lb of glass, so the door gets 5.2 lb lighter, not heavier.
The sign is not a rule, it is a comparison, and it flips the moment the glazing figure passes the panel figure. Quarter inch glass at 3.26 lb per sq ft against the same 3.2 panel is a wash. An insulated glazing unit against a thin uninsulated steel panel is heavier by a clear margin. The page prints the sign for whatever two figures you give it rather than asserting one, because both directions are common and which one you are in depends entirely on the door.
The one number to measure
Panel weight per square foot is worth a pound of accuracy more than everything else on the form combined. One pound per square foot is 63 lb on this door — nearly a third of it — and it goes straight through into the torque on the shaft and the tension in the cables. Hardware is the opposite: 5 lb either way is 2 per cent of the door, and it is not worth agonising over.
The way to settle it is a bathroom scale under one section while the door is apart for some other reason, or the manufacturer literature for that specific panel. Where the answer genuinely matters, the whole door goes on a scale — which is a job for whoever is working on the counterbalance, because a door with its cables off is held up by nothing.
What the section table is and is not
Hardware is spread evenly across the sections in that table, and no door is built like that. The bottom fixture, the lock and the bottom seal are all on section one, and struts land where the manufacturer put them. The even split is there so the column adds up to the total, not because any section actually weighs exactly that. For lifting one panel out, weigh that panel.
Questions people ask
How much does a garage door weigh?
For the default 9 by 7 ft door in the form — 63 sq ft of 3.2 lb per sq ft panel, one glazed section and 22 lb of hardware — 218 lb, which is 109 lb in each cable. But that answer is only as good as the panel figure. An uninsulated steel door of the same size can be under 150 lb and a heavily insulated one with wide glazing well over 250, so the useful thing is to put your own panel weight in.
Why can I not work it out from the thickness?
Because a sectional panel is a hollow shell. A steel skin, foam, and a backer have densities that differ by a couple of hundred times, so multiplying the panel volume by any single density gives a meaningless number. Weight per square foot is the unit a panel is actually described in, and it is the input this page asks for.
Do window sections make a garage door heavier?
Not necessarily, and at the defaults they make it lighter. Single strength glass is about 1.63 lb per sq ft against a 3.2 lb insulated panel, so cutting four lites into a section takes out 10.7 lb of panel and puts back 5.4 lb of glass — the door loses 5.2 lb. Thicker glass, an insulated glazing unit, or a lighter uninsulated panel all reverse that, which is why the page prints the sign rather than a rule.
How many sections will my door have?
The opening height divided by the section height, rounded up, with the top section cut down to make up the difference. An 84 in opening in 21 in sections is exactly four with nothing cut. The same opening in 24 in sections is four with the top one at 12 in, which changes the hinge and roller counts and where the struts sit even though the weight barely moves.
Is this accurate enough to size a counterbalance from?
No, and it is not offered for that. It is a build-up for a door that does not exist yet or is not accessible, and it states no manufacturer specification. Where the weight matters — and for anything to do with a counterbalance it always does — the door goes on a scale, by somebody who works on doors. A wound spring and a loaded cable are both capable of killing.