The grout coverage formula
Grout fills the joints between tiles, so the amount depends on how much joint there is. Each tile contributes its perimeter as grout line, shared with its neighbors, which is why the formula uses tile length plus tile width. Multiply that by the joint width and the joint depth to get the grout volume around one tile, scale it by the number of tiles per square foot (144 divided by tile length times tile width), and multiply by the floor area. All measurements go in inches.
The last step converts volume to weight. Mixed grout weighs about 124 pounds per cubic foot, and the powder gains roughly 30 percent of its weight in mix water, so one pound of powder fills about 18 cubic inches of joint. Divide your cubic inches by 18 and you have pounds of powder. Round up to the nearest bag size, because grout is cheap and a second trip is not.
The formula assumes joints are filled to about three quarters of the tile depth, which matches how most installers work. If your joints run deeper or wider than the numbers you plug in, treat the result as a minimum.
Worked example 1: 100 square feet of 12 by 12 tile
Take a living room floor: 100 square feet of 12 by 12 inch tile with 1/8 inch joints at 3/8 inch depth. Step one: 100 times (12 plus 12) times 0.125 times 0.375 times 144, divided by (12 times 12). That is 100 times 24 times 0.125 times 0.375 times 144, divided by 144, which equals 112.5 cubic inches of joint. Step two: 112.5 divided by 18 is 6.3 pounds of powder. One 10 pound bag, or one small premixed tub, covers it with room to spare.
Notice the answer is small. A single 25 pound bag, by the same math, covers about 400 square feet of 12 by 12 tile at 1/8 inch joints. People overbuy grout for big tile constantly; the bag chart looks scary, but the joints are thin.
Worked example 2: 50 square feet of 4 by 4 tile
Now a bathroom floor: 50 square feet of 4 by 4 inch tile, same 1/8 inch joints at 3/8 inch depth. Step one: 50 times (4 plus 4) times 0.125 times 0.375 times 144, divided by (4 times 4). That is 50 times 8 times 0.125 times 0.375 times 144, divided by 16, which equals 168.8 cubic inches. Step two: 168.8 divided by 18 is 9.4 pounds of powder.
Half the floor area, yet half again as much grout as the big tile job. Small tiles pack far more grout line into each square foot: nine 4 by 4 tiles fit where one 12 by 12 goes. This is the rule to remember: when the tile size drops, the grout bill climbs. Buy the 25 pound bag for this job.
Worked example 3: a shower floor of 2 by 2 mosaic
Shower floors are the extreme case. Take 13.3 square feet of 2 by 2 inch mosaic with 1/8 inch joints at 1/4 inch depth (mosaics are thinner, so the joints are shallower). Step one: 13.3 times (2 plus 2) times 0.125 times 0.25 times 144, divided by (2 times 2). That works out to 59.9 cubic inches. Step two: 59.9 divided by 18 is 3.3 pounds of powder.
A whole shower floor needs barely 3 pounds of grout, yet it uses grout at the highest rate per square foot of any surface in the house. Pair this with the shower tile measurement in our shower tile guide, and one 25 pound bag covers the entire shower, walls included, with plenty left over.
Sanded vs unsanded: which grout to buy
Use sanded grout for joints wider than 1/8 inch. The sand gives the grout body so it does not shrink and crack in wide joints. Use unsanded grout for joints 1/8 inch or narrower, and for polished stone or glass tile that sand would scratch. The coverage formula is the same either way; the bag you pick depends on joint width, not square footage.
Premixed grout in tubs costs more per pound but skips the mixing and cleanup, which suits small jobs like a backsplash. Powdered grout in bags is far cheaper per square foot and suits floors and showers. Whichever you choose, check the bag's own coverage chart against the formula: if they disagree by a lot, trust the wider number and keep the receipt.
Buying: round up and keep the receipt
Grout powder keeps for months in a dry place, so overbuying by one bag size is a cheap insurance policy. Underbuying means a second bag that may come from a different batch, and grout, like tile, can shift shade between batches. For the three examples above: buy a 10 pound bag for the 100 square foot 12 by 12 floor, a 25 pound bag for the 50 square foot 4 by 4 floor, and one 25 pound bag for the whole shower. Once you know the tile count from the tile calculator, run the joint formula and buy with confidence.