How Much Glue Do You Need?


You have a project; you will be gluing things, and you can measure the area you will be gluing.  The question then always comes up:  How much glue is needed?

This is something that can be calculated, mostly, and will get you fairly close.  Here’s how to do it:

Glue is a volume [three dimensions], and it will be spread over a surface [two dimensions].  What ties these two together is the *third* dimension of thickness.  That’s one dimension, plus the two of surface area equals the three dimensions of volume; 1 + 2 = 3.  It’s just math.

It just so happens that there’s 231 cubic inches in a gallon of anything.

It had to be *some* number, right?  Well, that’s what it is.

Now, if you sliced and diced all those cubic inches into 1/32-inch-thick layers and then tiled a surface with them, you’d cover almost exactly 48 square feet.  Again, that’s just how the numbers work out.  If you sliced them thinner, such as 1/64 of an inch, it would be close to 96 square feet per gallon.  Call it a hundred, in round numbers.

You know how many square feet of surface area you are gluing.  What’s missing here is the glue-line thickness, and that we have to estimate.  The glue-line thickness depends on how well the pieces fit together, and that, microscopically, means “How rough are the surfaces?”  That’s not rough on the scale of what feels rough to your fingertip, although that enters into it.  That means on the larger scale, how flat are the surfaces?  Actually the way to ask the question is how far do the two surfaces depart from a perfect match to each other as regards flatness?

If you lay two surfaces against each other and don’t push hard, they will touch each other in usually three spots; everywhere else there will be a gap;  sometimes microscopic and sometimes more.  What is the largest gap likely to be, meaning the greatest departure from flatness?  If you say it’s 1/32, then we will figure, on the average, that the glue-line thickness is going to be half of that.  There’s 1/64 of an inch, and coverage is thus about a hundred square feet per gallon, in theory. 

If you are gluing wood that was ripped in a table-saw, the average glue-line thickness may be twice that, or 1/32 inch, and you would think the coverage to be a bit less than fifty square feet per gallon.

There are a couple more factors that will eat into how far the glue will go.  One is how much soaks into the wood.  Some will, and that’s why my application notes about gluing say to put the glue on both surfaces and let it sit a while.  If you don’t do that, then the wood will absorb some of the glue according to its porosity.  The capillary forces that suck the glue into the wood are tens of pounds per square inch, so it can literally wick the glue-joint dry in places;  sometimes almost everywhere.

A dry-glue-joint is almost always caused by  not allowing enough time for the glue to soak into the wood before it is assembled and the glue line squeezed down to a few thousandths of an inch [also not usually a good idea], because then a small amount of absorption can take up all the remaining glue in the joint.

So, some glue will soak in.  How much depends on the kind of wood, the spacing of the growth rings, the type of wood [species] and the angle-of-the-grain in the glue-lines.  Plantation-grown teak, for instance, may have 6-12 growth rings per inch and be rather porous [second-growth redwood is even worse] and you might see a calculated 100 square feet per gallon go down to fifty in practice.  Some wood, on the other hand, may not soak up hardly any.  Some tight-grain woods may have a high natural porosity in spite of having many growth-rings-per-inch.  White Oak, for instance, may surprise you in that regard.

So, there’s a way to calculate usage and some hand-waving to allow for wood porosity, which is invisible until the glue shows you.  There’s another factor…squeeze-out.

The usual practice is to use an excess of glue in a joint, and spread it a bit thicker in the middle of the pieces being glued.  Thus, when you put it all together the air is forced out as the glue flows from the middle of the glue-line towards the edges.  Naturally, that forces out some excess glue, also.  You’ve got to do that to ensure all the air is forced out and the glue-line is completely filled with glue.

The excess drips out and runs to wherever-it-can.  That’s *waste* but you see that it’s necessary to put more glue in the joint that what it will eventually need.  It may be, depending on what you are doing, that the excess glue can be scraped-up as it is forced-out of the glue lines, and used in further gluing.  This minimizes waste.

When reusing glue that has been squeezed out of glue lines, try to avoid getting a lot of wood splinters or sawdust mixed in, as this will create a thicker-than-necessary glue line, and the recycled material will not go as far, so your total glue-usage will not be as efficient as it might.

There’s another factor that will help you to get as much mileage from the glue as possible, and that’s the pot life of the glue itself.  A glue that has a pot life of only an hour in a large mix or 2-3 hours in a small batch in the shade, gives a time-dependent opportunity for you to capture the squeeze-out and spread it on the next pieces you have ready for gluing and assembly.

Smith's Dead-Slow Epoxy Glue™ has a pot life, meaning a working time, of practically all day.  That means your waste will be really the least, as you can gather up drips and reuse them in further glue joints for hours, as the job progresses.

So, in summary, if you had wood with an average gap of a sixty-forth of an inch, you might get fifty to a hundred square feet per gallon of glue.  If you had really nice planed surfaces, you might get three hundred to five hundred square feet per gallon theoretically, but the practicalities of wood absorption and squeeze-out might reduce your actual usage to around 200-300 square feet per gallon.

When you get into your particular project, you will see how your usage varies from these numbers for your particular materials.  This, however, is a guide for starting.

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Steve Smith, reprinted with permission

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