Grout Calculator

What decides how much grout you need

Three numbers decide almost everything: the size of your tiles, the width of the joints between them, and how deep those joints are. Grout fills only the gaps, so the amount you need is the total volume of those gaps. Small tiles mean many joints packed into every square foot; large tiles mean a few long, sparse joints. Double the joint width and you double the grout. Double the depth and you double it again.

That is why grout estimates can swing by a factor of four on the same wall. A mosaic of 2x2 tiles at 1/8 inch joints holds several times more grout than 18x18 tile at the same joint width, even though the tiled area is identical. The Tile Council of North America sets the standards for tile installations, and its technical publications explain how joint width is chosen for each tile type.

The grout formula, step by step

The calculation is joint volume times grout weight. Work in inches and square feet, and each step is a full equation:

• Joint length per square foot: 144 divided by tile area in square inches, times the sum of the tile's two sides. For 12x12 tile: 144 / 144 x (12 + 12) = 24 inches of joint per square foot.
• Total joint length: 24 inches per square foot times 100 square feet = 2,400 inches of joint.
• Joint volume: 2,400 x 0.125 x 0.25 = 75 cubic inches.
• Convert to cubic feet: 75 / 1728 = 0.0434 cubic feet.
• Weight: 0.0434 x 100 = 4.34 pounds of grout.

The 100 pounds per cubic foot is a typical density for mixed cement grout, about 1.6 grams per cubic centimeter. Manufacturers publish coverage charts built from the same geometry, including Custom Building Products' grout coverage calculator, and their numbers land near these figures for the same tile and joint sizes.

Grout coverage per square foot by tile size

The table below assumes 1/8 inch joint width and 1/4 inch joint depth, which fits most wall tile. If your joints are wider or deeper, scale the pounds per square foot up in proportion.

Read the last column as bag coverage under these assumptions. The pattern is clear: every doubling of tile size roughly halves the grout per square foot, because joint length per square foot falls as tiles grow.

Worked example: a 100 square foot bathroom wall

Take a standard tub surround: 100 square feet of 12x12 ceramic tile, laid with 1/8 inch joints, on tile that is 1/4 inch thick. Following the steps above, the joints hold 75 cubic inches of grout, which weighs 4.34 pounds. Add the 10 percent waste buffer and the purchase target is 4.34 x 1.10 = 4.77 pounds, call it 4.8 pounds. One 10 pound bag finishes the job with material left over for patching.

Now the same wall in 4x4 tile: joint length per square foot is 72 inches, so total joint length is 7,200 inches and the volume is 7200 x 0.125 x 0.25 = 225 cubic inches. That is 225 / 1728 = 0.1302 cubic feet, or 0.1302 x 100 = 13.02 pounds. With the buffer the target is 13.02 x 1.10 = 14.32 pounds, which means a 25 pound bag. Same 100 square feet, three times the grout, purely because the tiles are smaller.

Bag sizes and how much to buy

Cement grout is sold in small boxes and bags, most commonly 10 pounds and 25 pounds, with some lines offering 5 pound boxes for small jobs. Epoxy grout is sold in kits measured in coverage instead of weight. Match your calculated need, plus the waste buffer, to the nearest bag size.

Buying one size up is usually the smarter move once the job passes half a bag. An opened bag is hard to store, but an unopened one can go back on the shelf or stay sealed in the garage for touch-ups later.

Why you should buy 10 percent extra

The buffer is not padding for bad math. It covers real, predictable losses: grout left stuck to the float and the bucket, joints that swallow more mix where the thin-set squeezed through, and the portion of every mixed batch that hardens before you reach it. Ten percent handles all of that on a typical job.

The stronger reason is color matching. Cement grout gets its shade from pigments, and those pigments drift slightly between production lots. If you run short mid-wall and buy a replacement bag weeks later, the new lot can dry a shade off from the first one, and the seam shows forever. Buying every bag at once, from the same lot, is the only reliable guarantee of a uniform color. This is also why seasoned tilers record the brand, color name and lot number written on the bag before the bag leaves the site.

Sanded vs unsanded grout

The joint width decides which grout family you buy, and the two have different densities, so a coverage chart for one does not transfer cleanly to the other.

Pushing sanded grout into a joint under 1/8 inch usually fails, because the sand grains cannot pack into the narrow gap. Pushing unsanded grout into a 1/4 inch joint cracks as it cures, because it shrinks without aggregate to hold it. When in doubt, the joint width picks the product, and the bag's own coverage chart gives the final word for that specific mix.

Common grout buying mistakes

The costliest mistakes are simple and avoidable. First, estimating from tile count alone and forgetting joint depth: a 1/2 inch deep floor joint holds twice what a 1/4 inch wall joint does at the same width. Second, ignoring the thin-set. If tiles were set with thick comb ridges, thin-set crowds the bottom of the joints and reduces grout need, but if the joints are clean and open the full calculated amount goes in.

Third, mixing by eye. A bag mixed with too much water shrinks as it cures and can leave low, pale joints, which sends people back to the store for more grout than the math called for. Follow the water ratio printed on the bag and mix the whole bag where the job allows. Last, buying grout and sealer at different times. Sealer is cheap, and applying it after the 48 to 72 hour cure protects the color work you just matched, so put both in the same cart.

Estimates from this page are for planning a purchase. The coverage chart on the specific grout bag you buy is the final authority for that product.