Grout Calculator

Grout is the one tile material people cannot estimate by eye, because the quantity depends on four things at once: how much area you are tiling, how big the tiles are, how wide the joints are, and how thick the tile is. Change any one and the answer moves a long way. Enter them below and the calculator returns pounds of grout, 25 pound bags, and the thinset underneath. If you do not have the area yet, the room calculator works it out from the dimensions.

How much grout you need depends on joint length, not area

The thing to understand first is that grout fills the gaps between tiles, so what matters is the total length of joint, not the size of the floor. And joint length per square foot depends entirely on tile size.

Small tiles mean a lot of joints. Large tiles mean very few. Over the same 100 square feet:

Tile size	Grout per sq ft	One 25 lb bag covers
3 x 6 in subway	0.205 lb	122 sq ft
6 x 6 in	0.137 lb	183 sq ft
12 x 12 in	0.068 lb	366 sq ft
12 x 24 in	0.051 lb	488 sq ft
24 x 24 in	0.034 lb	731 sq ft

All at a 1/8 inch joint and 3/8 inch tile thickness.

A subway tile job takes six times the grout of a 24 by 24 job over the same floor. That is the single most useful thing to know before you order, and it is why a coverage figure quoted without a tile size is meaningless.

The formula

Grout fills a channel around each tile. Work out the volume of that channel per square foot, then convert volume to weight:

Pounds per sq ft = 8.75 × (L + W) ÷ (L × W) × J × D

Where L and W are the tile's length and width in inches, J is the joint width in inches, and D is the joint depth, which is normally the tile thickness. Everything stays in inches here, unlike the area itself; the square inches to square feet converter covers that conversion if you need it.

The 8.75 is not arbitrary. There are 144 square inches in a square foot and 1,728 cubic inches in a cubic foot, and cement grout weighs roughly 105 pounds per cubic foot. Put those together, 144 ÷ 1,728 × 105, and you get 8.75.

Worked example. A 12 by 12 tile, 3/8 inch thick, with 1/4 inch joints:

(12 + 12) ÷ (12 × 12) = 0.1667
× 0.25 joint × 0.375 depth = 0.015625
× 8.75 = 0.137 pounds per square foot
A 25 pound bag therefore covers about 183 square feet

Checking the formula against a manufacturer

Worth being straight about this: the major manufacturers do not publish a formula. They publish empirical coverage tables, because grout density varies between products and a single constant cannot describe all of them.

So the formula above is an estimating tool, and it is fair to ask how close it lands. LATICRETE's grout coverage chart gives real figures for PERMACOLOR SELECT per 25 pound unit:

Spec	LATICRETE published	This formula	Difference
12 x 12 x 3/8 in, 1/4 in joints	195 sq ft	183 sq ft	6% conservative
18 x 18 x 3/8 in, 1/8 in joints	581 sq ft	549 sq ft	6% conservative

Two very different tile sizes and joint widths, and the formula tracks within 6 percent on both, consistently on the conservative side. That is the direction you want an estimate to err: you buy slightly more grout than the published table suggests rather than running out with a bathroom half grouted.

LATICRETE's own note on those tables is worth repeating: coverage is "approximate, based on nominal tile size, and is provided for purposes of estimation only," and they recommend adding 10 percent for waste, plus another 10 to 15 percent for unglazed quarry tile, which drinks grout through its porous face. The calculator above builds the 10 percent in.

Joint width: pick it before you order

Joint width has as much influence on the quantity as tile size does, and it is not a free choice.

Joint	Typical use
1/16 in	Rectified porcelain, precision-cut edges only
1/8 in	The general-purpose default for most wall and floor tile
3/16 in	Larger format tile, or tile with slight size variation
1/4 in	Handmade, saltillo, quarry and rustic tile with irregular edges
3/8 in and up	Slate, flagstone and other natural stone with uneven edges

Tiles with a nominal size vary slightly between pieces, and the joint is what absorbs that variation. Going narrower than the tile allows is how you end up with a visibly crooked grid. Going wider than you planned means you need more grout than you ordered, so decide the joint before the delivery, not on the day.

Sanded or unsanded

Not a quantity question, but it changes what you buy.

Unsanded grout for joints 1/8 inch and narrower. It is smoother and will not scratch polished surfaces like marble or glass.
Sanded grout for joints 1/8 inch and wider. The sand stops the grout shrinking and cracking as it cures across a wider gap.

At exactly 1/8 inch either works, and the tile surface usually decides it. Use unsanded on anything polished or soft, sanded everywhere else.

Thinset and mortar

The layer underneath is estimated a completely different way: not by tile size but by trowel notch, because the notch sets how thick a bed you comb out.

Trowel notch	Coverage per 50 lb bag
1/4 x 1/4 in square	85 to 110 sq ft
1/4 x 3/8 in square	60 to 100 sq ft
1/2 x 1/2 in square	42 to 55 sq ft
3/4 in U-notch	34 to 44 sq ft

Those ranges come from a published tile setting coverage chart, and the calculator above uses the low end of each, so it errs toward ordering enough.

The notch is chosen by the tile, not by preference. Small mosaic and wall tile go down with a 1/4 by 1/4. Standard floor tile up to about 12 by 12 uses a 1/4 by 3/8. Large format tile, 15 inches and over on a side, needs a 1/2 by 1/2 to get full coverage under a panel that is never perfectly flat. Natural stone and very large panels move up again.

Back-buttering changes the number. On large format tile you skim the back of each piece as well as combing the substrate, which can add 30 to 50 percent to the mortar you use. Factor that in before you buy if you are setting anything over 15 inches.

Worked example: a subway tile shower

Three shower walls totalling 105 square feet, tiled in 3 by 6 subway at 5/16 inch thick with 1/8 inch joints, set with a 1/4 by 1/4 trowel.

Grout per sq ft: 8.75 × (3 + 6) ÷ (3 × 6) × 0.125 × 0.3125 = 8.75 × 0.5 × 0.0391 = 0.171 lb
Total with 10 percent waste: 105 × 0.171 × 1.1 = 19.7 lb, so one 25 lb bag.
Thinset: 105 ÷ 85 = 1.24, so 2 bags of 50 lb.

Note how much grout a small tile takes. The same 105 square feet in 12 by 24 tile, at the same thickness and joint, would need about 4.9 pounds, roughly a quarter as much.

Common mistakes

Quoting coverage without the tile size. "A bag covers 200 square feet" is meaningless on its own. The same bag covers 122 square feet of subway tile or 731 of 24 by 24.

Forgetting joint depth. Depth is the tile thickness, and a 1/2 inch tile takes a third more grout than a 3/8 inch one at the same joint width.

Changing the joint width after ordering. Going from 1/8 to 1/4 inch doubles the grout.

Ignoring porous tile. Unglazed quarry and saltillo absorb grout through the face. Add 10 to 15 percent beyond the usual waste.

Estimating thinset by tile size. Thinset goes by trowel notch, and the notch goes by tile size. Two steps, not one.

Skipping back-butter on large format. It can add 30 to 50 percent to the mortar, and it is not optional on big panels.

Frequently asked questions

How much grout do I need?

Multiply the area by the grout per square foot, which is 8.75 × (tile length + width) ÷ (length × width) × joint width × tile thickness, all in inches. For 12 by 12 tile at 3/8 inch thick with 1/4 inch joints, that is 0.137 pounds per square foot, so a 25 pound bag covers about 183 square feet. Add 10 percent for waste.

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

It depends entirely on the tile. At a 1/8 inch joint and 3/8 inch thickness, one 25 pound bag covers about 122 square feet of 3 by 6 subway tile, 366 square feet of 12 by 12, and 731 square feet of 24 by 24. Any coverage figure quoted without a tile size and joint width is not usable.

Does tile size change how much grout I need?

Enormously. Grout fills joints, and small tiles have far more joint length per square foot. Subway tile takes roughly six times the grout of 24 by 24 tile over the same floor.

Should I use sanded or unsanded grout?

Unsanded for joints 1/8 inch and narrower, and on polished or soft surfaces like marble and glass that sand would scratch. Sanded for joints 1/8 inch and wider, because the sand stops the grout shrinking and cracking across a broader gap.

How much thinset do I need?

Thinset goes by trowel notch rather than tile size. A 50 pound bag covers roughly 85 to 110 square feet with a 1/4 by 1/4 inch notch, 60 to 100 with a 1/4 by 3/8, and 42 to 55 with a 1/2 by 1/2. Large format tile also needs back-buttering, which can add 30 to 50 percent.

What size grout joint should I use?

An eighth of an inch is the general-purpose default. Go to 1/16 inch only on rectified porcelain with precision edges, and up to 1/4 inch or more on handmade, quarry, saltillo or natural stone where the pieces vary in size. The joint absorbs that variation, so a tile with irregular edges needs a wider one.

Do manufacturers publish a grout formula?

No. They publish coverage tables from testing, because grout density varies by product and no single constant fits them all. The formula on this page is an estimating tool, and it lands within about 6 percent of LATICRETE's published figures on the conservative side.

How much extra grout should I buy?

Ten percent over the calculation, which the calculator includes. Add another 10 to 15 percent on unglazed quarry or saltillo tile, which absorbs grout through the face, and buy the whole quantity in one go so the colour matches across the job.

For the tile itself, the tile square footage calculator handles area, waste factor and box counts. For the cost side, the price per square foot calculator runs materials either direction.