How Much Grout and Mortar Do I Need for Tiling?

Thin-set is estimated by trowel size, because the notch depth sets how much mortar stays on the floor. A 50 lb bag of modified thin-set covers roughly 95 ft² with a 1/4 × 1/4 in square notch (mosaic and small tile), about 75 ft² with a 1/4 × 3/8 in notch (typical 12×12), and 60 to 70 ft² with a 1/2 × 1/2 in notch or a euro-notch used for large-format tile. Back-buttering large tile — recommended above 12 in on a side — cuts that coverage by another 15–20%.

So a 200 ft² floor in 12×24 tile, back-buttered, at 65 ft² per bag with the coverage penalty applied, needs about 200 ÷ 55 = 3.6, so 4 bags. Buy the extra bag; running out mid-floor with mortar curing on the substrate is a genuinely bad afternoon.

Grout works from geometry instead. The volume of grout equals the joint width × joint depth × total joint length, and total joint length rises sharply as tile gets smaller. Rather than compute it, use the published coverage: a 25 lb bag of sanded grout covers roughly 100–125 ft² of 12×12 tile with 1/8 in joints, about 60–70 ft² of 12×12 with 1/4 in joints, and as little as 40 ft² of 2×2 mosaic.

Match the grout type to the joint. Unsanded grout for joints under 1/8 in and for polished stone that sand would scratch; sanded grout for 1/8 in and wider; epoxy or high-performance grout for showers, backsplashes and anywhere staining matters. Never thin grout with extra water to stretch a bag — it weakens the joint and causes uneven colour.

Add the small items that always get forgotten: spacers in the joint size you chose, a bucket of pre-mixed grout haze remover, silicone caulk matched to the grout colour for every inside corner and change of plane (grout cracks there, caulk does not), and sealer for cement grout unless you used epoxy.