How Much Grout Do I Need? Formula, Coverage & Worked Examples
Planning estimate — check your grout bag first. Grout fills only the joints, so the tile size matters more than the room size. This guide gives you the formula and a coverage table, then run your job through our grout calculator.
The formula
Joint share = 1 − (tile length ÷ (tile length + joint)) × (tile width ÷ (tile width + joint)). Then pounds = area × joint share × joint depth in feet × density (about 120 lb per cubic foot sanded, 105 unsanded). For 100 sq ft of 12×12 tile with 1/8-inch joints at 3/8-inch depth: joint share = 1 − (12 ÷ 12.125)² = 0.0205, so pounds = 100 × 0.0205 × 0.03125 × 120 = 7.69 lb net, 8.46 lb with 10% waste — one 25 lb bag.
Coverage by tile size
At 1/8-inch joints and 3/8-inch depth, grout per 100 sq ft runs about 3.88 lb on 24×24 tiles, 7.69 lb on 12×12, 15.15 lb on 6×6, and over 22 lb on 3×6 subway tiles. Doubling the joint width roughly doubles the grout. The lesson: never estimate grout from the room size alone — two rooms the same size can need three times as much grout if one is tiled in mosaic and the other in large-format tile.
Sanded vs unsanded
Sanded grout is for joints 1/8 inch and wider: the sand stops wide joints shrinking and cracking as they cure. Unsanded is for joints under 1/8 inch and for polished stone, glass, and metal tile, where sand can scratch the surface. Epoxy grout is stain-proof and waterproof and suits showers and commercial floors, but it is harder to work, sets fast, and is sold by the kit — this guide's pound math does not apply to it.
Depth and waste
Joint depth is usually close to the tile thickness: floor tiles are often 3/8 inch thick, wall tiles 1/4 to 5/16 inch. If you are regrouting over old grout, measure the depth you are actually filling, not the full tile thickness. Waste of 10% covers what stays on the float, in the bucket, and in sponge water; use 15% for mosaics and jobs with many cuts.
Worked examples
- 100 sq ft, 12×12, 1/8-in joints, 3/8-in depth, sanded: 7.69 lb net, 8.46 lb with waste → 1 bag of 25 lb.
- 100 sq ft, 6×6, same joints: 15.15 lb net, 16.66 lb with waste → 1 bag of 25 lb (or 2 bags of 10 lb).
- 120 sq ft bathroom, 12×24, 3/16-in joints, 3/8-in depth: about 0.08 lb per sq ft → 9.6 lb net, about 10.6 lb with waste → 1 bag of 25 lb.
Common mistakes
Three errors cover most grout shortages: estimating from room size without the tile size, using the tile thickness as the depth on a regrout (where the real depth is shallower), and forgetting that a part-bag still has to be bought — grout is sold in whole bags, so 26 lb means two 25 lb bags, not one and a bit.
Frequently asked questions
How much grout do I need for 100 square feet of tile?
For 12×12 tiles with 1/8-inch joints at 3/8-inch depth, about 7.69 lb net — 8.46 lb with 10% waste, so one 25 lb bag. For 6×6 tiles, about 15.15 lb net, 16.66 lb with waste.
Should I use sanded or unsanded grout?
Sanded for joints 1/8 inch and wider; unsanded for joints under 1/8 inch and for polished stone, glass, or metal tile that sand could scratch.
How is grout coverage calculated?
From the joint share of the surface, times the area, times the joint depth, times the grout density — then waste, then round up to whole bags.
Does tile size really change how much grout I need?
More than any other input: at the same joint, 100 sq ft takes about 3.88 lb in 24×24 tile but over 22 lb in 3×6 subway tile.
What waste percentage should I add for grout?
10% standard, 15% for mosaics and jobs with many cuts. Always round up to whole bags — a part-bag still has to be bought.
The rest of the tile job
Grout is the last tile material — the tile calculator counts the tiles themselves, and the laminate flooring calculator covers the non-tile rooms.