The plate is the plaque
A 24 by 18 rectangular plate is 432 square inches. At 3/16 thick that is 81 cubic inches. A 3/4 inch raised border at 3/32 proud adds about 5.7 cubic inches, and 120 raised characters at a 3/4 cap height, covering a fifth of their box at 3/32 proud, add about 0.76. The total is roughly 87.5 cubic inches, and the flat background is 92.6 percent of it.
| Element | Cubic inches | Share | At 548 lb/cu ft |
|---|---|---|---|
| Background plate | 81.00 | 92.6% | 25.7 lb |
| Raised border | 5.70 | 6.5% | 1.8 lb |
| Raised lettering | 0.76 | 0.9% | 0.2 lb |
| Whole plaque | 87.45 | 100% | 27.7 lb |
Add a sixteenth of an inch to the plate and you add 27 cubic inches, which is more than four times the border and the lettering put together. Every argument about how much text a plaque can carry is, in weight terms, an argument about nothing.
Why bronze markers are thin and stone bases are not
Cast bronze sits somewhere around 548 pounds a cubic foot depending on the alloy, against about 170 for granite — a little over three times as dense. The same 24 by 18 face in bronze at 3/16 weighs about 25.7 pounds. In granite at the same thickness it is about 8. Take the granite to a thickness that would survive being walked on, say 4 inches, and it is 170 pounds; take the bronze to 4 inches and it is close to 550, and the metal cost is out of the question long before the weight is.
That is the whole reason for the usual construction. The stone or concrete base provides the mass and the bearing, and the bronze provides a surface that holds an inscription for a very long time in a thickness measured in sixteenths.
Density is an alloy question, so it is an input
There is no single figure for bronze. The alloys used for architectural and memorial castings differ in tin, zinc and lead content, and the density moves with them. The default here is a common working figure and it is deliberately editable, because the foundry pouring your plaque knows what is in the crucible and no calculator does. A ten percent error in density is a ten percent error in every number on the page.
The fixing count is arithmetic, not a specification
Perimeter divided by spacing, rounded up, with a floor of four. That is all the fixing figure is. What the anchors have to be, how long, set in what, and whether the wall or the stone behind them will take the load is a question for the plaque maker and for whoever owns the substrate. A plaque that comes off a wall is a serious matter and it is not one that a division sum has any standing to comment on.
The same applies to the stone face the plate needs. The calculator gives the area at the margin you asked for, which is a layout number. Whether that stone is the right size, the right material and permitted where it is going is settled by the cemetery or the building owner, in writing, before anything is cast.
Questions people ask
How much does a bronze plaque weigh?
Almost entirely a function of face area and plate thickness. A 24 by 18 plate at 3/16 with a border and normal lettering comes to about 28 pounds. Halve the thickness and you halve the weight; double the face area and you double it. The raised border and the letters together are usually under ten percent of the metal, so any estimate that starts from the plate and ignores the wording will be close.
What is the density of cast bronze?
It depends on the alloy, and the spread is wide enough to matter. The figure used here as a starting point is around 548 pounds a cubic foot, which is a little over three times granite and about a tenth more than steel. Higher tin and lead content pushes it up. Since the whole calculation is linear in density, take the number from the foundry rather than from a general table if the weight is going to be relied on.
Does the amount of lettering change the weight much?
No, and that is the useful finding. Raised letters are shallow and they cover only a fraction of their own character box — perhaps a fifth for a normal Roman. On a 24 by 18 plaque, 120 characters at a 3/4 cap height come to less than a cubic inch of bronze, about a quarter of a pound. What lettering costs is pattern work and hand finishing, both of which are labour and neither of which appears in a weight calculation.
How many fixings does a plaque need?
The page divides the perimeter by whatever spacing you enter and rounds up, with a minimum of four. That is a layout starting point and nothing more. What the fixings must be, how deep they set, what adhesive or anchor system is appropriate and whether the substrate will hold them is decided by the plaque maker and the anchor manufacturer, and by whoever owns the wall or the stone. Get it from them.
Can I mount a bronze plaque on an existing headstone?
That is a question for the cemetery first and the monument dealer second. Whether anything may be added to an existing memorial, whether bronze is permitted in that section, and who is allowed to do the work are all cemetery rules, and they vary between cemeteries and change over time. The arithmetic here will tell you how much stone face a given plate needs and what it weighs; it has nothing to say about whether you are allowed to fix it there.