The geometry behind the number
Grout fills the joint lines, so the math is joint length × joint cross-section. A square foot of 12×12 tile hides about 24 inches of joint line; a square foot of 4×4 mosaic hides 72. That is why mosaics eat three times the grout of large-format tile at the same joint width. The calculator adds 15% waste — tight for straight lays, generous for mosaics.
Sanded or unsanded
Joints 1/8 inch and narrower take unsanded grout — sand grains cannot squeeze into a gap that tight. Wider joints take sanded, which resists cracking. Anything over 1/2 inch needs a high-performance or expansion-joint product instead of more grout.
Depth is part of the math
Grout fills the joint from the tile face down to the substrate — on ⅜-inch-thick floor tile that is a ⅜-deep channel unless you back-butter. Deeper joints scale grout use linearly, so a half-depth back-butter nearly halves the number. Measure the real tile thickness, don't guess it.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- ASTM International — C94 for ready-mixed concrete, C150 for cement and C143 for slump testing.
- American Concrete Institute — ACI 318 code requirements and ACI 302 slab construction guidance.
- Portland Cement Association — mix design, yield and curing data published by the US cement industry.
Figures, formulas and reference tables last verified: 19 September 2026.