Grout calculator

How much grout a tile job takes, from the tile size, the joint width, and the tile thickness — the joint depth most grout calculators leave out. Volume, weight, and bag count, imperial or metric.

01 How the math works

Grout fills the joints between tiles. So the amount you need is the joint cross-section — the joint width times the joint depth — run along the total length of joint in the floor or wall. The joint depth is the tile thickness. That is the whole reason thickness is an input here and not a hidden assumption.

Written per square foot of tile, with every dimension in inches, it is the estimating formula the trade uses:

The (L + W) / (L × W) term is the joint length packed into each square foot of tile face. It grows as tiles get smaller, which is why a mosaic drinks grout and a large-format tile barely sips. Multiply the whole thing by the area, add a mixing allowance, and convert to weight at a grout density of 100 lb/ft³ to get bags. There is no honest single "square feet per bag" number for grout, because coverage moves with tile size, joint, and thickness all at once — which has never stopped the coverage charts from printing one.

02 Worked example

Take the bathroom floor: 6 ft × 8 ft (1.83 m × 2.44 m), which is 48 ft² (4.46 m²), laid in 12 × 12 in (305 × 305 mm) tile with 1/8 in (3 mm) joints. Measure the tile edge and it is 5/16 in (7.9 mm) thick.

Per square foot: 144 × 0.125 × 0.3125 × (24 / 144) = 0.9375 in³. Across 48 ft²: 45.0 in³ (737 cm³ / 0.74 L) raw, 49.5 in³ (811 cm³ / 0.81 L) with 10% for mixing loss. At 100 lb/ft³ that is 2.86 lb (1.30 kg) of grout.

One 7 lb bag covers it, with room to spare — the smallest bag on the shelf is more grout than this floor needs. That is the normal shape of a small-tile job: the question is not how many bags but whether one bag is enough. It flips the other way on mosaics and big rooms, which is the next section.

03 When this calculator is wrong

The formula assumes joints filled flush to the tile face and packed the full tile thickness deep. Four things move the real number:

• Tile thickness is the input other calculators drop. Grout scales directly with joint depth, and joint depth is the tile thickness. Keep a 12×12 tile and a 1/4 in joint fixed, and the grout goes from 1.875 in³/ft² at 5/16 in thick to 3.0 in³/ft² at 1/2 in thick — 60% more for a thicker tile of the same face size. A table keyed only to "12×12" cannot see that.
• Mosaic sheets follow the chip, not the sheet. A 12×12 mounted sheet of 1 × 1 in chips has twelve times the joint length of a single 12×12 tile, so at the same joint and depth it needs 12× the grout — 9.0 in³/ft² against 0.75 in³/ft². Enter the individual chip size, not the sheet.
• Grout is linear in joint width. A 1/8 in joint installed at 3/16 in is 50% more grout than planned. Hand-set joints drift wider than the spacer, so estimate from the joint you will actually float, not the one on the box.
• The setting bed can steal joint depth. If thinset squeezes up between tiles, or the tile has a thick cushioned edge, the grout is shallower than the full tile thickness and the estimate runs high. That is the safe direction — over-ordering grout costs a few dollars, running out mid-float costs a day.

04 What to do with the result

Buy the rounded-up number, and never less. Grout is cheap and returns are easy; stopping a float halfway to drive back for one more bag is neither. The real reason to buy it all at once is colour: cement grout is made in batches, and shade drifts between lots. Buy every bag you need in one lot, and mix each batch to the same water ratio — too much water lightens the cured colour, so an inconsistent mix reads as blotchy joints even from one bag.

The exception is the same as for any material: on a large job where one extra bag is real money, weigh it against the return trip. For grout that break-even almost never arrives, because a bag is a few dollars and a mismatched joint colour cannot be fixed without regrouting. When in doubt, one more bag from the same lot.

05 Common questions

How much grout do I need per square foot?

It depends on three things, not one: tile size, joint width, and tile thickness. A 12×12 in tile 5/16 in thick with a 1/8 in joint takes about 0.94 in³ of grout per ft². Drop to a 2×2 in mosaic and it is several times that; go to a large-format 24×24 and it is a fraction. Enter your own numbers above rather than trusting a flat per-foot figure.

How many square feet does a bag of grout cover?

There is no fixed answer, which is why this page asks for the tile. The same 25 lb bag that grouts a few hundred square feet of thin large-format tile at a fine joint covers a small fraction of that in thick tile with wide joints. Coverage is the grout volume the tile needs divided into the bag — compute the volume first.

Sanded or unsanded grout?

Unsanded for joints 1/16 to 1/8 in (1.5–3 mm); sanded above 1/8 in, because the sand keeps a wider joint from cracking as it cures. Sanded grout carries up to 3/8 in (9 mm) on a standard cement grout. The calculator flags which one your joint calls for.

Does tile thickness really change how much grout I need?

Directly. The joint is as deep as the tile is thick, so grout is proportional to thickness. A 1/2 in tile needs 60% more grout than a 5/16 in tile of the same face size and joint. Grout calculators that skip thickness are guessing at it — usually low, on the thin tiles they were built around.

How much extra grout should I buy?

Add about 10% for the grout left in the bucket, on the float, and wiped off the faces, then round up to whole bags. Buy every bag from the same colour lot so the joints cure to one shade.