How to Use This Tool
Enter the tile dimensions, the joint width and the tile thickness. The floor area only scales the answer; the tile size decides the rate.
Grout fills joints, and small tiles are mostly joint
A square foot of 24 x 48 porcelain contains very little joint. The same square foot of 4 x 4 mosaic is criss-crossed with it. Because grout volume follows the length of joint rather than the area of floor, the tile size is the dominant input and the floor size is the least interesting one.
lb per sq ft = ((A + B) / (A x B)) x C x D x 47.9 132 sq ft floor: 24 x 48, 1/16 joint, 1/2 thick 0.094 lb/sq ft 12.3 lb 12 x 24, 1/8 joint, 3/8 thick 0.281 37.0 lb 12 x 12, 1/4 joint, 3/8 thick 0.748 98.8 lb 4 x 4, 1/8 joint, 1/4 thick 0.748 98.8 lb mosaic against large format 8.0x
Joint width and tile thickness both multiply
Both appear linearly in the formula, so doubling either doubles the grout. A thicker tile at the same joint width needs proportionally more because the joint is deeper, which is the part people forget when switching from a 1/4 in wall tile to a 1/2 in floor tile at the same spacing.
Sanded and unsanded are not interchangeable
Below about 1/8 in the joint is too narrow for sand to pack into, so unsanded grout is used. Above that, unsanded shrinks and cracks as it cures, so sanded is required. Choosing by preference rather than by joint width is a common and expensive mistake, and this tool flags which one the joint demands.
The exception runs the other way too: sand can scratch polished marble and some glass, so those surfaces need either a narrow joint with unsanded grout or a specialist product.
Buy more than the calculation
Ten per cent is the usual allowance, and it covers more than spillage. Grout has a working time, so a batch that stiffens has to be discarded rather than retempered with water, and mixing a second small batch from a nearly empty bag risks a colour difference across the floor. As with tile, the whole job should come from the same lot.
What is not counted here
Thinset mortar under the tile is a separate calculation driven by trowel notch size rather than joint width. Sealer, caulk for movement joints and the corner joints that should be silicone rather than grout are all separate too.
