For 100 sq ft of 12 × 24 in porcelain tile, ⅜ in thick, with 3/16 in joints, you need about 8 lb of cement grout including 10% extra, so one 10 lb bag. Grout is sold by weight but used by volume: the amount depends on how much joint there is per square foot, which grows with smaller tile, wider joints and thicker tile.
How the grout is calculated
Every tile has half its perimeter's worth of joint around it. Per unit of area that works out to (L + W) ÷ (L × W), with L and W the tile's length and width. Multiply by the joint width and the joint depth (the tile thickness, since grout fills the joint to the face) to get the joint volume per square foot, then by the area and by the grout's density.
pounds = (L + W) ÷ (L × W) × joint width × tile thickness × area × density × (1 + waste %)
With L, W, joint and thickness in inches, the result per square foot is in cubic inches after multiplying by 144; divide by 1,728 for cubic feet. Cement grout comes out at about 100 lb per cubic foot of filled joint: that is the figure MAPEI's own Keracolor S coverage chart works out to on every tile size from 4 in up. Ready-to-use grout is simpler still: one gallon fills about one gallon (231 cu in) of joint.
Worked example
These are the calculator's defaults.
- Joint per sq ft: (24 + 12) ÷ (24 × 12) × 0.1875 × 0.375 × 144 = 1.27 cu in.
- For 100 sq ft with 10% waste: 1.27 × 100 × 1.10 = 139.2 cu in, or 0.0806 cu ft.
- Weight: 0.0806 × 100 lb = 8.1 lb, so one 10 lb bag. A 25 lb bag would cover about 341 sq ft of this tile.
Change only the joint to ⅛ in and the grout drops to 5.4 lb; switch to a 2 × 2 in mosaic, ¼ in thick, at the same 3/16 in joint and the same 100 sq ft needs about 43 lb. Small tile is where people run out.
Coverage from the manufacturers' charts
The presets use each product's own chart. We back-solved the density from every chart cell for tile 4 in and larger; on those sizes the calculator lands within about 5% of the published figures, usually within 2%.
| Tile, joint | MAPEI Keracolor S, per 25 lb | MAPEI Ultracolor Plus FA, per 10 lb | LATICRETE PERMACOLOR, per 8 lb |
|---|---|---|---|
| 12 × 12 × ⅜ in, ⅛ in | 383 sq ft | 156 sq ft | 128 sq ft |
| 12 × 24 × ⅜ in, ⅛ in | not listed | 207 sq ft | 170 sq ft |
| 24 × 24 × ⅜ in, 3/16 in | 510 sq ft | 207 sq ft | 170 sq ft |
| 6 × 6 × ¼ in, ⅛ in | 285 sq ft | 117 sq ft | 97 sq ft |
| 2 × 2 × ¼ in, ⅛ in | 93 sq ft | 39 sq ft | 34 sq ft |
Source: MAPEI technical data sheets for Keracolor S and Ultracolor Plus FA, LATICRETE data sheet DS-250 for PERMACOLOR Grout. Charts are for estimating; the makers note that real joints vary.
Mapei grout: Keracolor S, Keracolor U or Ultracolor Plus FA
Keracolor S is sanded and made for joints from ⅛ in to ⅝ in; MAPEI says not to use it in joints under ⅛ in. Keracolor U is the unsanded version for 1/16 in to ⅛ in joints, and its chart gives noticeably more area per pound (212 sq ft per 10 lb on 12 × 12 in tile at ⅛ in). Ultracolor Plus FA is a fine-aggregate grout for 1/16 in to ¾ in, sold in 10 and 25 lb bags, so it covers both tight rectified joints and wide rustic ones.
Laticrete grout: PERMACOLOR
PERMACOLOR Grout comes in 8 and 25 lb bags for joints from 1/16 in to ½ in. PERMACOLOR Select is sold as a base bag plus a separate color kit and its data sheet has no coverage chart, so we do not guess one; for Select, the PERMACOLOR Grout preset is the closest published figure.
Pre-mixed buckets
MAPEI Flexcolor CQ (1 and 2 gallon pails) and CUSTOM Fusion Pro (1 gallon) cost more per joint but need no mixing and no sealing. On 12 × 12 in tile at ⅛ in joints a gallon covers 208 sq ft of Flexcolor CQ and 199 sq ft of Fusion Pro.
Choosing the joint width
The joint is not only a look. ANSI A108.02 (4.3.8) requires the joint to be at least three times the real size variation of your tile, never under 1/16 in. For tile with any side over 15 in, 4.3.8.1 sets an average minimum of ⅛ in for rectified tile and 3/16 in for calibrated (pressed-edge) tile. The check under the result tests your joint against these rules and against the joint range on the grout's data sheet. Below ⅛ in, use unsanded or fine-aggregate grout; sand cannot pack into a narrow joint.
Before and after grouting
Grout goes in after the thinset has cured, usually 24 hours; the thinset calculator covers the mortar and the trowel. Leave movement joints at walls, changes of plane and around the tub unfilled and seal them with silicone instead: the caulk calculator sizes the tubes. To count the tile itself for a whole room, a shower or a backsplash, start with the tile calculator, the shower tile calculator or the backsplash tile calculator.