Grout Calculator

How much grout do I actually need?

Almost everyone buys grout the same way: walk into the store, look at the bag, guess. The guess is usually wrong in one of two expensive directions. Buy too little and you stop mid-wall, mix a second batch hours later and end up with a visible colour break where the two batches meet. Buy too much and the surplus sits in the garage absorbing damp until it is landfill. The quantity is not a mystery, though — it comes from four numbers you already know: the area, the tile size, the tile thickness and the joint width.

The counter-intuitive part is that area matters least. Two bathrooms of identical size can need six times different amounts of grout purely because of the tile format. Grout does not fill the floor, it fills the outline around every tile, so the driver is the perimeter of each piece divided by its face area. Cut a tile in half and you double that ratio.

Worked example: 80 sq ft of 3 × 6 subway tile

A standard subway backsplash, 1/4 in thick, laid with 1/8 in joints. The perimeter-to-area ratio is (3 + 6) ÷ (3 × 6) = 0.5 per inch. Multiply by the joint width and the joint depth: 0.5 × 0.125 × 0.25 = 0.0156 cubic inches of grout per square inch of wall. Add 10% for mixing loss and joints you pack twice, and it becomes 0.0172. Convert to feet — multiply by 144 square inches per square foot, divide by 1728 cubic inches per cubic foot, which is simply dividing by 12 — and you get 0.00143 cubic feet per square foot. Dry cement grout weighs roughly 120 lb per cubic foot, so that is 0.172 lb per square foot, or 13.8 lb for the 80 sq ft wall. Buy two 10 lb bags or one 25 lb bag. One 10 lb bag covers about 58 sq ft of this tile.

The same 80 sq ft in 24 × 24 porcelain

Keep everything else identical — 1/8 in joints, 1/4 in thick — and swap to 24 × 24 slabs. The ratio collapses to (24 + 24) ÷ (24 × 24) = 0.083 per inch, exactly one sixth of the subway figure. The whole job now needs about 2.3 lb of grout, and a single 10 lb bag would cover roughly 349 sq ft. Same room, same joint width, one sixth of the material. That is the single most useful thing to understand about grout: mosaic and subway jobs are grout-heavy, large-format jobs are grout-light, and any coverage figure quoted without a tile size is meaningless.

A typical floor

200 sq ft of 12 × 12 ceramic, 3/8 in thick, 1/8 in joints works out at 0.086 lb per square foot, so about 17.2 lb — two 10 lb bags, or one 25 lb bag with a useful margin for a repair down the line. Notice that thickness pulls in the opposite direction to tile size: a thicker tile means a deeper joint to fill, so the same floor in 1/4 in tile would need only about 11.5 lb.

The formula, stated plainly

Grout per square foot (lb) = [(W + L) ÷ (W × L)] × joint width × joint depth × 1.1 ÷ 12 × 120, with W, L, width and depth all in inches. The bracketed term is the perimeter-to-area ratio, the 1.1 is a 10% waste allowance, dividing by 12 converts cubic inches per square inch into cubic feet per square foot, and 120 is the bulk density of dry cementitious grout in pounds per cubic foot. Manufacturers' printed coverage charts are the same calculation with their own density and waste assumptions, which is why two brands can quote coverage figures 20% apart for identical tile.

Buying, mixing and sealing

Mix in batches you can finish

Cementitious grout has a working life of roughly 30 minutes once water goes in. Mix about a third of a 25 lb bag at a time, let it slake for five to ten minutes, then remix — and do not add more water after slaking, because that thins the mix and produces patchy, chalky colour. Measure water with a jug rather than eyeballing it: water content is the number one cause of colour variation between batches, far more than pigment differences. If a wall will take more than one batch, work to a natural break such as an inside corner rather than stopping mid-field.

Sealing timeline

Sponge off haze while the grout is firm but green, then leave it alone. Light foot traffic after 24 hours, no water for 72. Cement grout should be sealed once fully cured, typically after 48 to 72 hours, and re-sealed roughly annually in showers. Epoxy grout does not need sealing at all, which is a large part of why professionals accept its short pot life and higher price in wet areas.

What this estimate cannot see

It assumes the joint is filled to the full tile thickness. In practice thinset often squeezes partway up the joint, so real consumption on a floor can land 10 to 20% below the calculation — that is deliberate headroom, not an error. It also assumes rectified square edges; cushioned or hand-made tiles have wider effective joints and eat more. It does not deduct outlets, borders or trim pieces, and it treats one bag as one lot, so on a large job order all your grout in a single purchase with matching lot numbers. When in doubt, round up to the next bag: colour matching a shortfall weeks later is far harder than storing a spare.