The gap is a bigger share than it looks
A tessera does not occupy its own size on the board. It occupies its size plus one gap in each direction, because every piece carries half a gap on all four sides and there are two halves to a gap. So a three quarter inch square with a 3/32 gap takes up 0.84375 by 0.84375 inches, which is 0.712 square inches for a piece whose face is 0.5625.
That means only 79 percent of the surface is glass. On a 12 inch tile with the same gap it is 98.5 percent. The mosaic is about fourteen times as much gap by area, and since the gap fills with grout of some colour, it is doing fifteen times as much to the finished colour of the work. People choose a grout and then wonder why the piece looks nothing like the loose tesserae did in the tray.
The piece count follows the same arithmetic: 144 divided by 0.712 is 202 pieces per square foot, against 0.98 for the 12 inch tile.
Nipped pieces, and why the count is not just area over module
Borders, curves and any place the grid runs out need cut pieces, and a cut piece starts life as a whole tessera. If both halves get used the tessera did the work of two placements; if the offcut goes in the bin it did the work of one. That is what the yield field controls, and it is the difference between two quite different answers on a design with a lot of edge.
On the defaults, 12 square feet at 202 pieces per square foot is 2427 placements. A quarter of those are cut pieces, so 607 placements come from about 303 nipped tesserae and 1,821 come from whole ones. That is 2124 tesserae consumed, and 2337 after the ten percent allowance — about an eighth fewer than treating every placement as a fresh tessera, which sounds small until the design is mostly curve and the cut share is seventy percent.
Setting the yield to 1 is the honest choice for anyone new to nippers. A tessera does not break where you want it to until it does, and the ones that break wrong are not offcuts, they are losses.
By the pound, by the piece, or on mesh
Loose tesserae are usually sold by weight, which means the count has to become pounds before you can order. A piece weighs its face area times its thickness times the density of the material, and the density is the number worth getting right: glass is near 156 pounds per cubic foot, but ceramic, porcelain, marble and smalti all sit somewhere else, and a supplier who knows their stock beats any general figure.
On the defaults, a three quarter square at a quarter inch thick is 0.1406 cubic inches. At 156 pounds per cubic foot, which is 0.0903 pounds per cubic inch, that is 0.0127 pounds a piece, and 2337 of them come to 29.7 pounds — 2.47 pounds per square foot of finished mosaic. That is the tesserae alone; substrate, adhesive and grout are on top.
Mesh-mounted sheets change the question entirely. The sheet sets the gap for you, so the coverage is the sheet area and the piece count is informational. What the sheet does not solve is the joint where two sheets meet, which is set by hand and has to match the machine-set gaps either side of it. That joint is the tell on sheet-mounted work and it is worth planning where it falls.
Where this page stops
It counts tesserae and turns them into a buying unit. It does not work out adhesive or grout, because the tile calculator already does that from the same tessera size and joint width, and duplicating it here would only mean two pages that could disagree with each other. Put the same numbers in there and you get bags and pounds.
It also does not judge a substrate. What a mosaic can be set on, in what location, and whether it will stay there is a question about the specific substrate, the specific adhesive and where the piece is going to live, and it is answered by the data sheets for those products, not by a piece count.
The dust nobody mentions
Cutting glass or stone tesserae produces fine dust, and wet cutting produces a slurry that dries into the same dust on every surface it lands on. Nippers throw fragments hard enough to reach your face across a bench. Fresh glass edges cut quietly. Those are the hazards; the handling of them is covered by published guidance and by the safety data sheet for whatever you are cutting, and no part of that belongs in a calculator.
Questions people ask
How many mosaic tiles are in a square foot?
Divide 144 by the module, which is the tessera plus one gap in each direction. Three quarter inch squares with a 3/32 gap occupy 0.84375 inches square, so the module is 0.712 square inches and there are 202 pieces to a square foot. Change the gap to an eighth and it drops to 188. The gap moves the count much more than people expect on small pieces, which is the whole reason the field is separate from the tessera size.
Why is so much of a mosaic surface grout?
Because the gap is a fixed width and the pieces are small, so the ratio between them is nothing like it is on floor tile. Three quarter inch tesserae with a 3/32 gap leave 79 percent glass and 21 percent gap. The same gap on a 12 inch tile leaves 98.5 percent tile. That is why the grout colour on a mosaic is a design decision rather than a detail, and why a sample board grouted the way the real piece will be is worth the afternoon.
Do I need to buy extra for the pieces I nip?
You need to buy for the tesserae consumed, which is not the same as the placements made. Every cut piece starts as a whole tessera, and whether that tessera yields one usable piece or two depends on the shape you need and how well it breaks. The page asks for that yield and for what share of the field is cut. On the defaults, 2427 placements come out of 2124 tesserae because half the cut work comes off the offcuts.
How do I turn a piece count into pounds?
A tessera weighs its face area times its thickness times the density of the material. Three quarter inch squares a quarter inch thick are 0.1406 cubic inches each; at 156 pounds per cubic foot that is 0.0127 pounds, so 2337 of them is 29.7 pounds. The density is the number to check — 156 is about right for glass and wrong for ceramic, porcelain, marble and smalti, all of which sit somewhere else. Your supplier has the figure for what they sell.
Does this work out the thinset and grout as well?
No, on purpose. The tile calculator already takes a tile size, a joint width and an area and gives bags of setting material and pounds of grout, and building a second version of that here would only create two pages that can disagree. Put the same tessera size and gap into it and you will get the setting materials for the field this page has counted the pieces for.