How much grout do you need?

How do I calculate how much grout I need?

Grout fills the joints, so it depends on the tile size, the joint width and the tile thickness: the share of the floor taken by joints, times the area, times the depth of the tile. For 100 square feet of 12 by 24 inch tile, ⅜ inch thick, with ⅛ inch joints, the joints hold about 5.4 pounds of sanded grout — one 25 lb bag with plenty left over.

How many square feet does a 25 lb bag of grout cover?

It depends entirely on the tile. Custom Building Products' chart for Polyblend Plus sanded grout gives 348 square feet for 12 by 12 inch tile with ⅛ inch joints, but only 54 square feet for 1 by 1 inch mosaic. Small tiles have many more joints per square foot.

Sanded or unsanded grout?

Sanded grout for joints ⅛ inch and wider — the sand keeps it from shrinking and cracking in wide joints. Unsanded grout for joints narrower than ⅛ inch and for polished stone and glass tile that sand could scratch.

Why do small tiles need so much more grout?

Every tile adds its own joint on each side. The same 100 square feet in 3 by 6 inch subway tile, 5/16 inch thick with ⅛ inch joints, takes about 17 pounds of grout — three times as much as large-format tile.

How much extra grout should I buy?

About 10% for spills, cleanup and the grout left in the bucket. Buy all of one color at once — a bag from another batch can dry a slightly different shade.

Does joint width matter more than tile size?

Both matter, and joint width is easy to underestimate. On 120 square feet of 12 by 24 inch tile, ⅜ inch thick, ⅛ inch joints take about 6.5 pounds of sanded grout, while ³⁄₁₆ inch joints take about 9.7 pounds, roughly 49% more, because wider joints take a bigger share of the floor area and are filled to the same depth.

How much grout does a 30 square foot backsplash take?

For 3 by 6 inch subway tile, 5/16 inch thick, with ⅛ inch joints: about 5.2 pounds before any extra. A small job like this is easy to run short on because the bucket and tools hold some mixed grout; buy a full bag rather than counting on the estimate.

How the amount is calculated

The joints form a grid around every tile. For a tile of length l and width w with joint width j, each tile owns a cell of (l + j) × (w + j), and the part of that cell outside the tile is joint. So the joint share of the floor is 1 − (l × w) ÷ ((l + j) × (w + j)): 2.05% for 12 by 12 inch tile with ⅛ inch joints, but 6.0% for 3 by 6 inch tile. Multiply that share by the area and the tile thickness to get the volume of the joints, then by the weight of grout per unit volume. That weight is calibrated to the Polyblend Plus sanded grout chart, whose 348 square feet per 25 pound bag for 12 by 12 inch tile with ⅛ inch joints and ⅜ inch thickness is the anchor.

The calculation assumes the joints are filled to the full depth of the tile and that the grout has the same density as in the chart. It does not include grout lost to waste, so the result is the amount to fill joints, and the calculator adds a margin when you ask for one.

Worked example

A bathroom floor of 120 square feet in 12 by 24 inch tile, ⅜ inch thick, with ⅛ inch joints needs about 6.5 pounds. That is a quarter of one 25 pound bag, but a floor that size would normally be bought as a whole bag and the leftover sealed for repairs. For the wall, the same arithmetic with 3 by 6 inch subway tile at 30 square feet gives about 5.2 pounds, which is far more per square foot than the floor.

Mistakes to avoid

Entering the tile thickness wrongly is the most common: the depth of the joint is the thickness, so ½ inch stone tile needs a third more grout than ⅜ inch porcelain at the same joint width. Measuring the joint width from the spacers instead of the actual installed joint is another; rough-edged or handmade tile often needs wider joints than the nominal one. And do not use the bag coverage printed for large tile on a mosaic job: coverage drops from hundreds of square feet to dozens as the tile gets smaller.

Sources: the grout weight per volume is calibrated to Custom Building Products' coverage chart for Polyblend Plus sanded grout; the joint-share arithmetic is geometry. Manufacturers' charts for other grouts differ, so check the coverage printed on your bag. Related tool: tile calculator.