Grout Calculator

How much grout: the formula

Grout fills the joints, so the amount is the volume of the joints. Each tile has one joint along its length and one along its width, as deep as the tile is thick. Per square foot of floor, with every size in inches:

This is the formula grout makers print. MAPEI’s is (A + B) ÷ (A × B) × C × D × 1.6 = kg/m², with sizes in millimeters,^[1] and ARDEX uses the same form with 1.5.^[2] The last number works as a density in kg per liter (ARDEX calls it a coverage figure; reading it as a density is ours); MAPEI’s 1.6 is 99.9 lb per cu ft, so the calculator’s default is ρ = 100. As a check, ARDEX’s own example (150 × 150 mm tile, 5 mm thick, 3 mm joints) comes out at 0.30 kg/m² in the US form too, which is 0.061 lb per sq ft.

The calculator fills the joint to the full tile thickness, as the makers’ coverage charts do. When you set tile, MAPEI and TEC ask you to leave at least 2/3 of the tile’s depth open for grout, so a real joint may take a little less.^[3][4] It then adds 10% for waste and cleanup, the figure LATICRETE calls customary.^[5]

Example: a 15 × 12 ft floor in 12 × 24 × 3/8 in tile

1. Joint volume: (24 + 12) ÷ (24 × 12) × 0.375 × 0.125 × 144 = 0.8438 cu in per sq ft
2. Weight: 0.8438 ÷ 1,728 × 100 = 0.04883 lb per sq ft (4.88 lb per 100 sq ft)
3. Area 180.00 sq ft, with 10% extra: 180.00 × 0.04883 × 1.10 = 9.67 lb
4. Bags: 9.67 ÷ 25 = 0.39 → 1 × 25 lb bag

With CBP’s Polyblend Plus selected (ρ = 110) the same floor takes 10.63 lb; with 3/16 in joints instead of 1/8 in, 14.50 lb. Either way one 25 lb bag is enough.

Example: a subway tile backsplash

40 sq ft of 3 × 6 × 1/4 in tile with 1/16 in joints takes unsanded grout. With MAPEI Keracolor U (ρ = 73): 1.1250 cu in per sq ft ÷ 1,728 × 73 = 0.04753 lb per sq ft, and 40.00 × 0.04753 × 1.10 = 2.09 lb, one 10 lb bag. In pre-mixed grout it is 40.00 × 1.1250 × 1.10 = 49.5 cu in ÷ 231 = 0.21 gal, one 1 gal pail.

Why grout makers’ charts disagree

US grout makers print coverage charts rather than the formula. Working back from each chart gives a nearly constant ρ for every tile and joint in it, so they are built the same way; they just use different densities. For the same 12 × 12 × 3/8 in tile with 1/8 in joints, a 25 lb bag covers 400 sq ft by LATICRETE’s chart and 348 by CBP’s, 15% apart.^[6][7] Whether that is the powder or rounding on the safe side, the data sheets do not say.

The ρ values are our calculation from the charts, not figures the makers publish. Unsanded grout varies even more: MAPEI Keracolor U works out to about 73 and CBP’s non-sanded Polyblend Plus to about 105, so the same joints take 1.44 times as much CBP grout by weight. If your brand is in the list, pick it; if not, the default of 100 sits inside the range of the sanded and universal cement grouts above (96 to 110); for an unsanded grout, pick the closest unsanded preset.

Formula against the makers’ charts

Square feet per 25 lb bag. The formula column uses ρ = 100 with no extra.^[3][6][7]

Coverage per 25 lb bag

Square feet one 25 lb bag covers at ρ = 100, with no extra, computed by the calculator. For a 10 lb bag multiply by 0.4. Cells are blank where the joint is under the ANSI minimum for tile with a side over 15 in: 1/8 in for rectified tile and 3/16 in for calibrated tile.^[9]

Small tiles and wide joints use grout fast: a 2 × 2 × 1/4 in mosaic with 1/8 in joints takes 4.0 times as much per sq ft as 12 × 12 × 3/8 in tile. Doubling the joint width doubles the grout.

Sanded or unsanded

The traditional rule is sanded grout for joints wider than 1/8 in and unsanded for narrower ones.^[10] At exactly 1/8 in either works: CBP rates its sanded grout from 1/8 in and its non-sanded up to 1/8 in.^[7] The calculator says which one your joint takes. Unsanded grout is often sold in smaller bags, and the calculator switches to a 10 lb bag when the joint calls for it. Check the product’s rated joint range:

• Tile that sand scratches. CBP’s technical bulletin says polished stone, bisque, metal and other soft tile may still need grout without sand, whatever the joint width, and to test first.^[10] Try the grout on a spare tile if you are unsure.
• Lugged subway tile. Tiles with small spacer lugs on the edges leave a 1/16 in joint when stacked, but in a running bond the joint can close to 1/32 in, too narrow for sand.^[10]
• Joints wider than 1/2 in are past most grouts’ range; MAPEI rates Keracolor S to 5/8 in.^[3] ANSI A108.02 never allows a joint under 1/16 in.^[9]

Pre-mixed and epoxy grout

Pre-mixed grout comes in pails, so the calculator figures it by volume: the joint volume times the area, divided by 231 cubic inches per US gallon. CBP’s Fusion Pro chart comes out at 96% to 97% of the full joint volume, so figuring by volume comes within a few percent of the chart, and the extra in the calculator covers the difference:^[13]

Epoxy grout is sold in kits. Choose pre-mixed in the calculator to get the volume, then compare it with the kit: LATICRETE’s SPECTRALOCK PRO full unit makes 0.8 gal,^[14] which is 184.8 cu in, or about 164 sq ft of 12 × 12 × 3/8 in tile with 1/8 in joints (our calculation; we have not checked LATICRETE’s own epoxy chart). CBP’s chart gives 187 sq ft for its 9.5 lb CEG-Lite kit at the same tile and joint.^[15] For epoxy, LATICRETE adds 10% for waste, spillage and cleanup, and another 10–15% when grouting unglazed quarry tile.^[14]

Related calculators

• Tile calculatorTiles, boxes, grout and thin-set for a floor or wall
• Square footage calculatorAdd up rooms and odd shapes in feet and inches
• Wallpaper calculatorRolls by wall width, height and pattern repeat
• Project materials listPrint everything you added from each calculator

Prepared by the Yard & Build editors. The formulas behind this calculator are checked against hand-worked answers every time the site is built. Grout data sheets and coverage charts checked October 5, 2026.