Joint volume, not floor area alone
Narrow joints on large tile drink far less product than mosaic with a wide joint. Manufacturers publish coverage as tables — tile size by joint width, for a stated bag or kit size — because density and packaging differ by product line. The honest estimate uses that product’s own chart when your size is listed, then adds waste.
Sanded cementitious, unsanded, and epoxy do not share a single coverage rate. Sanded mixes carry aggregate and are the usual floor pick for joints about ⅛″ and wider; unsanded is meant for narrow joints and soft stone that sand would scratch. Epoxy packs by the kit or gallon, not the same 25 lb bag math as a sanded cementitious line. Mixing tables across products is how people buy a bag that never matches the joint.
Pack size is not the same as coverage
CUSTOM’s Prism ships in a 17 lb box, not the 25 lb bag most sanded lines use, because the recycled-glass filler makes it lighter per unit volume. Its chart and the Polyblend Plus 25 lb chart give identical square footage on every tile size and joint width they share — the smaller box really does cover the same floor. So comparing bag weights across brands tells you nothing about how many you need, and reading a 25 lb row while holding a 17 lb box happens to work here and would not on a different pair of products. Pick the preset that names what is on the shelf.
Prism SCG is the premixed pail version, sold by the gallon and previously branded Fusion Pro. It is a single-component polymer rather than a cement you mix, so its coverage table is its own — a gallon is not a weight and does not convert to one.
Chart cells first — formulas only inside a trusted zone
If your tile width, length, thickness, and joint width match a published cell, you get the published square feet per unit. Off-chart sizes use a joint-volume formula calibrated to that same chart, but only inside a zone where the fit stays tight — tiles about 3″ or larger on the short face and joints ⅜″ or narrower. Outside that zone you get a warning instead of a silent guess.
On the high-error cells we checked before shipping the formula path, the freeform estimate usually over-counts bags — it skews toward extra material, not short. MAPEI Keracolor S/U on mosaic and wide joints is a called-out exception that can under-estimate; the UI warning says so when that product and geometry combine. Joint depth on the charts usually equals tile thickness; if thinset fills the bottom of the joint, lower the depth and you will need less grout.
What we refuse to invent
SPECTRALOCK PRO Premium was a documented gap here for a while: its data sheet (DS-681) gives packaging but sends you to a separate coverage card for the numbers. That card turned out to be DS 254.3, the LATICRETE Grout and Sealant Color Card, and it carries the full table — so the preset is now sourced rather than guessed. LATICRETE READY-TO-USE Grout is on the same card and is still left out: three tile rows is too thin to fit a formula to, and a preset that is wrong between its own chart cells is worse than no preset. If you already know the brand, the LATICRETE grout calculator and the MAPEI grout calculator each carry that maker's full range with the published charts printed out in full.
Tile piece count is a different takeoff — run the tile calculator first, then come back here for joint fill. Chart-versus-formula walkthrough: How Much Grout Do You Need. Grout sealer is a coating with its own coverage, not joint volume, so it is not folded into the bag count. The 10% waste default is a labeled trade convention for cleanup and whole packs; quarry tile often wants more.
Not affiliated with MAPEI, LATICRETE, TEC Specialty Products, ARDEX, or CUSTOM Building Products. Coverage presets are drawn from each manufacturer’s own published technical data, cited below.