Grout Coverage Calculator: Bags & Sq Ft per Bag

How the grout math works

Grout fills the joints between tiles, so the amount you need is the length of all the joints times the joint width times the tile thickness — the grout fills the joint all the way down to the tile bed. A square foot is 144 square inches, and it holds 144 / (tile width × tile length) tiles. Each tile contributes (width + length) inches of joint counted once — every joint is shared by two tiles, so each tile owns half of its perimeter, 2 × (width + length) / 2. Multiply the tiles per square foot by the joint per tile and you get the joint length per square foot: 144 × (width + length) / (width × length) inches. For a 12×12 tile that is 144 × 24 / 144 = 24.00 in, or 2.00 ft, of joint in every square foot. Multiply by the joint width (0.125 in) and the tile thickness (0.375 in) to get the volume: 24.00 × 0.125 × 0.375 = 1.1250 in³ per sq ft. Mixed sanded grout weighs about 120 lb per cubic foot, and a cubic foot is 1,728 in³, so the weight is 1.1250 × 120 / 1,728 = 0.07813 lb per sq ft. Times the area (100 sq ft) that is 7.81 lb, and with 10% waste 8.59 lb — one 25-lb bag. One bag covers 25 / 0.07813 = 320 sq ft.

The worked example, line by line

Setup: 12×12 tile; joint 1/8 in (0.125); thickness 3/8 in (0.375); area 100 sq ft; waste 10%.

Line 1 — joint length. 144 × (12 + 12) / (12 × 12) = 144 × 24 / 144 = 24.00 in = 2.00 ft of joint per sq ft.

Line 2 — volume. 24.00 × 0.125 × 0.375 = 1.1250 in³ of grout per sq ft.

Line 3 — weight per sq ft. 1.1250 × 120 / 1,728 = 0.07813 lb per sq ft.

Line 4 — total. 0.07813 × 100 = 7.81 lb of grout.

Line 5 — with waste. 7.81 × 1.10 = 8.59 lb → ceil(8.59 / 25) = 1 × 25-lb bag.

Line 6 — coverage per bag. 25 / 0.07813 = 320 sq ft per 25-lb bag.

Coverage chart (a): one 25-lb bag, by tile size

Square feet covered by one 25-lb bag of sanded grout, at 1/8 in joints and 3/8 in tile thickness. Smaller tile means more joints per square foot and far less coverage — mosaic is the grout hog of the chart.

Coverage chart (b): joint width, at 12×12 tile

Square feet covered by one 25-lb bag at 12×12 tile, 3/8 in thick, as the joint widens. Coverage is inversely proportional to joint width — double the joint, halve the coverage.

Every branch, checked

6×6 tile, 100 sq ft: 15.63 lb, 17.19 lb with waste, 1 bag, 160 sq ft per bag. Smaller tile doubles the joints, so the same room takes twice the grout as 12×12.

Mosaic 2×2, 1/4 in thick, 40 sq ft: 12.50 lb, 13.75 lb with waste, 1 bag, 80 sq ft per bag. A small backsplash is still a one-bag job — but only just, and only because the tile is thin.

24×24 tile, 100 sq ft: 3.91 lb, 4.30 lb with waste, 1 bag, 640 sq ft per bag. Big tile barely sips grout: a single bag covers six hundred square feet.

18×18 tile, 3/16 in joints, 200 sq ft: 15.63 lb, 17.19 lb with waste, 1 bag. The wider joint and the bigger area cancel out to the same weight as the 6×6 case.

Default tile at 500 sq ft: 39.06 lb, 42.97 lb with waste, 2 bags. At five hundred square feet the second bag appears — the first bag covers 320 sq ft, so a big floor always needs a spare.

Reverse — bags on hand: 1 × 25-lb bag covers 320 sq ft at the default settings. Two bags cover 640 sq ft of 12×12 — or 107 sq ft of 2×2 mosaic, where the joints drink it.

What the defaults assume

The calculator assumes the grout fills each joint all the way to the tile bed — the full tile thickness — and that the joints are uniform in width and depth. It uses a mixed sanded-grout density of 120 lb per cubic foot. Manufacturer coverage charts are built on the same volume math but round to their own bag sizes and test conditions, so expect their numbers to differ by about ±10%. The 10% waste figure covers spillage, wash-off during cleanup, and joints that run a little wide; mosaic, handmade, or uneven tile deserves 15–20% instead. Bag counts always round up, because a half bag does not exist at the store. Rounding convention: all math runs on unrounded values and rounds only for display; coverage per bag is rounded to the nearest whole square foot.

Output · formula · meaning

Who uses this

• The bathroom DIYer: is re-grouting a shower in 12×12 tile and learns 100 sq ft takes 7.81 lb — one 25-lb bag — so the single bag on the shelf is enough, and the second one stays at the store.
• The tile contractor: prices a 500 sq ft floor and learns it needs 39.06 lb, 42.97 lb with waste — 2 bags — so the estimate carries two bags, and the crew never waits on a store run.
• The mosaic backsplash homeowner: picks a 2×2 mosaic and learns the same bag covers only 53 sq ft — six times less than big tile — so the 40 sq ft backsplash still fits in one bag, but only just.
• The large-format installer: sets 24×24 tile and learns one bag covers 640 sq ft — so the grout order for a whole house is a handful of bags, and the budget goes to the tile, not the grout.

Accuracy & limitations

The math is exact for the numbers you enter: joint length is 144 × (width + length) / (width × length) inches per square foot, volume is joint length × joint width × thickness, weight is volume × 120 / 1,728 lb per cubic foot, and bag counts are the with-waste weight divided by 25, rounded up. But the inputs carry the assumptions. The headline assumption is the density: mixed sanded grout at 120 lb per cubic foot. Real products vary — unsanded grout for joints under 1/8 in is a little lighter, epoxy grout is denser, and every manufacturer’s coverage chart rounds to its own test conditions — so expect real coverage to land within about ±10% of this calculator, and check the bag’s own chart before a big order. The calculator also assumes the grout fills the joint to the full tile thickness: tile set with a deep bed, lippage, or joints that were never fully cleaned out will use more, and mosaic sheets on mesh add the joints between sheets on top of the tile joints. It does not subtract for cut tiles, drains, or fixtures — measure the tiled area, not the room. And it assumes square or rectangular tile: hexagons, penny rounds, and other shapes have different joint lengths per square foot, so treat the result as an estimate for those.