Thinset vs Grout: What's the Difference?
Two materials every tile job needs — but they do completely different things at different stages of installation. Confusing them is the most common tile-setting mistake.
Thinset: the adhesive under the tile
Thinset mortar (also called tile adhesive or thin-bed mortar) is a cement-based adhesive that bonds tile to the substrate. It is spread onto the substrate with a notched trowel before the tile is placed. The notch size determines how thick a bed of mortar is combed onto the surface — a 1/4×1/4 in notch covers about 90 sq ft per 50 lb bag, while a 1/2×1/2 in notch (used for large-format tile) covers only 45 sq ft per bag.
Thinset must fully cure — typically 24 to 48 hours — before any weight is placed on the tile or grouting begins.
Grout: the filler between tiles
Grout is applied after the thinset has cured and fills the joints between tiles. It is spread with a rubber grout float in diagonal strokes, then cleaned with a damp sponge. Grout is porous and water-resistant — not waterproof. A penetrating sealer reduces staining and moisture absorption, but true waterproofing requires a membrane beneath the tile assembly, not just sealed grout. Grout also allows slight movement between tiles and determines the final colour of the joint lines.
Grout quantity is determined by tile size, joint width, and joint depth — not just area. A 2×2 mosaic with 1/8 in joints uses exactly 6× more grout per square foot than a 12×12 tile with the same joints — pure geometry.
At a glance
| Property | Thinset Mortar | Grout |
|---|---|---|
| Purpose | Bond tile to substrate | Fill joints between tiles |
| Applied with | Notched trowel | Rubber grout float |
| When in sequence | First — before tile is set | Second — after thinset cures |
| Key variables | Area, trowel notch size | Tile size, joint width, joint depth |
Calculate grout for your tile job
The calculator below is the grout estimator — enter your tile size, joint width, and area to see exactly how many pounds and bags you need:
Tile & Area
Grout Joint
Grout Type & Bag Size
How the grout formula works
Step 1 — joint length per unit area (TCNA/Laticrete geometry)
joint_len_per_area = (tile_L + tile_W) / (tile_L × tile_W) [1/in] Each tile shares half its perimeter with its neighbours, netting (L+W) of joint per tile face area L×W. A 12×12 yields 24/144 = 0.167/in; a 2×2 mosaic yields 4/4 = 1.0/in — 6× more grout for the same area.
Step 2 — grout weight per square foot
lb/sq ft = joint_len_per_area × joint_width × joint_depth × density (lb/in³) × 144 Density: sanded 104 lb/ft³ (= 0.0602 lb/in³), unsanded 90 (= 0.0521), epoxy 110 (= 0.0637). Multiplying by 144 in²/ft² converts the per-in² result to per-ft².
Step 3 — total pounds and bags
grout_lb = lb/sq ft × area × (1 + waste%/100) bags = ⌈ grout_lb / bag_size − 1e−9 ⌉ The −1e−9 float guard prevents a spurious extra bag when the result lands on exactly a whole number due to floating-point rounding.
Related tools
Frequently Asked Questions
No. Thinset mortar is an adhesive designed to bond tile to a substrate. It is applied under the tile with a notched trowel and is not formulated to fill joints between tiles. Grout is a cementitious filler applied after the tile is set and the thinset has cured. Using thinset as grout would result in poor joint fill, colour issues, and a surface that cannot be cleaned.
No. Grout has no significant adhesive strength — it is designed to fill joints, not bond tile to a surface. Tile set in grout alone would detach quickly. Always use a proper thinset mortar or tile adhesive to bond tile, then grout the joints after the mortar has cured (usually 24 hours).
Thinset always goes first. The sequence is: prepare substrate → spread thinset with a notched trowel → set tile with spacers → let thinset cure (24–48 hours typically) → apply grout → clean up grout haze. Never grout before the thinset is fully cured; premature grouting can cause tile to shift and grout to crack.
Modified thinset contains polymer additives (latex or acrylic) that improve flexibility, adhesion, and moisture resistance. Unmodified thinset is pure Portland cement, sand, and water-retention additives. Some tile installations require unmodified thinset — for example, when setting tile over membranes or using certain epoxy grouts. Check your tile and membrane manufacturer specifications.