Grout Calculator

Joint width decides it. Use unsanded grout for joints 1/8 inch and narrower: the fine mix packs into tight lines and will not scratch polished stone or glass. Use sanded grout from 1/8 inch up to about 1/2 inch, because the silica sand resists the shrinkage cracking that plagues wide unsanded joints. At exactly 1/8 inch either will work and most manufacturers list both. If you are grouting polished marble or glass tile at 1/8 inch or wider, pick a fine-sand or high-performance formula rather than standard sanded grout, and test it on a spare tile first.

Grout fills joints, so consumption tracks joint length per square foot. A 12" × 12" tile puts about 24 inches of joint into every square foot; a 12" × 24" puts about 18 inches; a 24" × 48" plank only about 9 inches. Roughly speaking, doubling tile size halves grout use. That is why one 25 lb bag might cover a couple hundred square feet of small mosaic and well over 500 square feet of large-format plank at the same joint width. Tile thickness matters too: a 3/8 inch porcelain plank holds three times the grout of a 1/8 inch glass mosaic.

Epoxy grout is a two-part resin and hardener. It is stain-proof, chemical-resistant, and never needs sealing, which makes it standard in commercial kitchens and popular in showers and around ranges. It costs roughly three to five times as much per square foot as cementitious grout and has a working time of about 45 to 60 minutes, so you mix small batches and clean as you go. It also does not shrink, so it holds wide joints well. The pounds figure here is based on cementitious grout; epoxy ships in kits sized by volume, so use the kit coverage chart instead.

Cementitious grout color comes from pigment blended into each production batch, and batches drift. Two bags of the same color from different lots can cure noticeably different, and the change shows up as a stripe across the floor. Buy every bag at once, confirm the lot or batch number printed on the bags matches, and if you must mix lots, dry-blend all the powder in a clean bucket before adding water. Mixing water matters just as much: an extra splash in the second batch lightens the cured color even within one lot. Measure water by the cup, not by eye.

It is a planning estimate built from joint geometry — joint length per square foot times joint width times tile thickness — converted at about 0.058 pounds of bagged grout per cubic inch (a 25 lb bag yields roughly 0.25 cubic feet of mixed grout), plus 10 percent for what stays in the bucket, on the float, and in the sponge. Manufacturer coverage charts are usually more conservative because they assume overfilled joints, eased tile edges, and cleanup loss; some list roughly half the coverage this math gives. Check the chart on the bag you actually buy and round up. Grout is cheap next to tile, and running out mid-floor risks a color mismatch you cannot undo.