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.
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
LATICRETE SPECTRALOCK PRO Premium’s data sheet points to a separate coverage card and the Project Estimator. Without a stable official expanded table to cite the same way as the other presets, it stays omitted rather than scraped or guessed. Cementitious PERMACOLOR Select is included from LATICRETE’s published 10 lb chart.
Tile piece count is a different takeoff — run the tile calculator first, then come back here for joint fill. 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, or ARDEX. Coverage presets are drawn from each manufacturer’s own published technical data, cited below.