Grout Calculator

Grout fills the joints between tiles, so the amount you need follows the volume of those joints, not the size of the room. Small tiles, wide joints and thick tiles all put more joint into every square foot. That is why a small mosaic shower floor can use almost as much grout as a kitchen six times its size. Below you will find the formula the grout calculator uses, two worked jobs, reference tables computed from the same formula, and the thinset figures for each job.

If you still need the tile count, start with the Tile Calculator, which handles boxes and the cut allowance. For an L-shaped kitchen or a bathroom with a tub alcove, the Square Footage Calculator adds up the separate rectangles before you enter the total here.

The grout formula

Every tile owns one long edge and one short edge of joint. For one tile, the joint volume is (L + W) times the joint width times the joint depth, and the tile covers L × W of floor. Divide one by the other and you get the joint volume per unit of floor, which is the industry formula printed on grout charts:

L and W are the tile length and width, J is the joint width and T is the tile thickness, which sets the joint depth, all in inches. The 144 turns square feet into square inches and the 1,728 turns cubic inches into cubic feet. The last factor, 110 lb per cubic foot, is the weight of dry sanded grout powder it takes to fill one cubic foot of joint once mixed. Folded together, the shortcut is lb per sq ft = 9.17 × J × T × (L + W) ÷ (L × W).

The 110 figure was chosen because it matches what grout makers publish. For 12 × 12 × 3/8 in tile with 1/8 in joints the formula gives 349 sq ft per 25 lb bag, and the coverage chart for Custom Building Products' Polyblend Plus sanded grout lists 348. For 6 × 24 in planks it gives 279 sq ft, and the chart lists 279.

Worked example: 180 sq ft of 12 × 24 porcelain

A kitchen measures 12 × 15 ft, so the floor is 180 sq ft. The tile is 12 × 24 in porcelain, 3/8 in thick, set with 1/8 in joints:

• Tile factor: (12 + 24) ÷ (12 × 24) = 36 ÷ 288 = 0.125
• Joint volume per sq ft: 144 × 0.125 × 0.375 × 0.125 = 0.84375 cu in
• Whole floor: 180 × 0.84375 = 151.9 cu in, or 0.0879 cu ft
• Weight: 0.0879 × 110 = 9.67 lb of sanded grout
• With 10% waste: 10.6 lb, so one 25 lb bag or two 10 lb bags

At this tile size one 25 lb bag covers about 465 sq ft, so a leftover half bag is normal and worth keeping for repairs. For the thinset, 12 × 24 in counts as large format (a side of 15 in or more), so the calculator starts with a 1/2 × 1/2 in square notch. The floor plus 10% is 198 sq ft, and at 42 to 47 sq ft per 50 lb bag that is 4.2 to 4.7 bags, so buy 5 bags. A smaller 1/4 × 3/8 in notch would stretch the same floor to 3 to 4 bags, but large tile set with too small a notch rarely reaches the required mortar contact.

Worked example: a 2 × 2 mosaic shower floor

Now a 5 × 6 ft shower floor, 30 sq ft, in 2 × 2 in mosaic that is 1/4 in thick, again with 1/8 in joints:

• Tile factor: (2 + 2) ÷ (2 × 2) = 1
• Joint volume per sq ft: 144 × 1 × 0.25 × 0.125 = 4.5 cu in
• Whole floor: 30 × 4.5 = 135 cu in, or 0.0781 cu ft
• Weight: 0.0781 × 110 = 8.59 lb; with 10% waste 9.45 lb, so one 10 lb bag

That shower floor needs almost as much grout (8.6 lb) as the 180 sq ft kitchen (9.7 lb). Per 100 sq ft the mosaic takes 28.6 lb against 5.4 lb, about 5.3 times as much, because each square foot of 2 in mosaic carries 12 ft of joint while 12 × 24 tile carries 1.5 ft. The mosaic is a third thinner, which trims that 8-to-1 joint ratio to about 5.3 to 1.

For mosaics the formula is deliberately generous. It divides by the tile face alone instead of tile plus joint, which overstates the joint volume by under 2% for 12 in tile but by about 9% for 2 in mosaic. The Polyblend Plus chart lists 96 sq ft per 25 lb bag for 2 × 2 × 1/4 in tile at 1/8 in joints, against 87 sq ft here, so the calculator leaves a cushion on mosaic jobs. The thinset is simple: 33 sq ft at 80 to 100 sq ft per bag is one bag with a 1/4 × 1/4 in notch. A shower is a wet area, so the tile needs 95% mortar contact.

Pounds of grout per 100 sq ft

Every figure in this table comes from the formula above at 110 lb per cubic foot, before any waste allowance. The thickness is the usual one for each format: 1/4 in for mosaics and small wall tile, 5/16 in for subway tile and 3/8 in for porcelain floor tile. Multiply by your area in hundreds of square feet, then add your waste percentage.

Read across a row and the grout scales exactly with the joint: going from 1/8 to 1/4 in doubles it. Read down a column and it falls as the tile grows. Notice that 24 × 24 tile at a 1/4 in joint needs the same 7.2 lb per 100 sq ft as 12 × 12 tile at 1/8 in, because doubling both tile sides halves the joint length per square foot.

How many square feet one 25 lb bag covers

The same numbers turned around answer the question you have in the store aisle: how far does a bag go? These are the square feet of floor or wall one 25 lb bag of sanded grout fills, before waste. A 10 lb bag covers 40% of each figure.

Thinset calculator: bags by trowel notch

Thinset is figured from the area and the trowel, not from the joints. The notch sets how much mortar each pass leaves behind, and manufacturers publish coverage per 50 lb bag for each notch. The ranges below combine two current data sheets (Custom Building Products VersaBond and LATICRETE 253 Gold), with the bag counts including a 10% allowance:

The notch is a starting point. The tile installation standards judge the result, not the tool: ceramic and porcelain tile need at least 80% mortar contact in dry areas and 95% in wet areas and outdoors. Set a tile, lift it and look at the back; if the ridges have not collapsed into a solid bed, move up a notch or back-butter the tile. Standard thinsets are meant for a finished bed between 3/32 and 1/4 in, and deeper beds need a mortar rated for large and heavy tile. Small mosaics and wall tile are often set with a 3/16 in V-notch, which spreads much farther: one unsanded thinset lists 150 to 170 sq ft per 40 lb bag with that trowel.

Sanded, unsanded or epoxy grout

Sanded grout is the default for joints from 1/8 in up to about 1/2 in. The sand gives the joint body, so it shrinks and cracks less as it cures, and it is what the 110 lb per cubic foot figure describes. Unsanded grout is for joints narrower than 1/8 in and for surfaces that sand can scratch, such as polished stone and glass. It is lighter per cubic foot, so the calculator's pounds run a little high for it, which only means a little left over. Both are cement grouts covered by ANSI A118.6 (standard cement grouts) and A118.7 (high-performance cement grouts).

Epoxy and urethane grouts are sold in kits by volume rather than by weight. For those, use the joint volume in cubic inches from the calculator and compare it with the yield printed on the kit; 1 gallon is 231 cubic inches.

How to use this calculator

1. Enter the area. Type the room length and width in feet, or switch to total square feet and add floors, walls and backsplashes together.
2. Pick the tile size. A preset fills a typical thickness too; overwrite either value with the numbers on your box.
3. Set the thickness and the joint width. Use the joint your spacers or leveling clips actually produce, not the one you hope for.
4. Choose the waste allowance. 10% suits most jobs; 15% suits mosaics, textured tile and first-timers.
5. Check the trowel. Leave the notch on Auto for a common starting point, or pick the trowel you own. The table shows all four notches at once.
6. Read the result. The headline is grout to buy with waste, the chip gives bag counts, the tiles show the joint volume and coverage, and the steps card shows the arithmetic. If you already have the bags, enter the coverage printed on them and the bag counts use your label instead.

Who this calculator is for

Homeowners planning a bathroom or kitchen use it to buy the right number of bags in one trip and to see why a mosaic floor needs so much more grout than its size suggests. Tile setters and remodelers use the per-100 sq ft figures to price bids quickly and to sanity-check a supplier's suggestion. Designers comparing a 12 × 24 field tile with a 2 × 2 accent can see the grout and mortar cost of each choice before the tile is ordered, and property managers can stock repair grout for a whole building from the same numbers.

Key terms

• Joint width: the gap between two tiles, set by spacers or leveling clips.
• Joint depth: how deep the grout goes, taken here as the tile thickness.
• Sanded grout: cement grout with fine sand, for joints from 1/8 in to about 1/2 in.
• Unsanded grout: cement grout without sand, for joints under 1/8 in and scratch-prone surfaces.
• Thinset: dry-set or latex-modified cement mortar that bonds tile to the substrate in a thin bed.
• Square notch and U-notch: trowel teeth shapes; a deeper notch leaves taller ridges and more mortar.
• Mortar contact: the share of the tile back that ends up bonded, checked by lifting a freshly set tile.
• Large format tile: tile with at least one side of 15 in or more, which needs flatter substrates and more mortar.
• Back-buttering: skimming mortar onto the back of the tile as well as the floor to improve contact.
• Rectified tile: tile ground to exact size after firing, which allows narrower joints than pressed tile.

What changes the amount, and tips for buying

• Tile size: the biggest lever. Halving both sides of the tile doubles the joint length per square foot and doubles the grout.
• Joint width: grout scales one to one with it. A 3/16 in joint takes 50% more grout than a 1/8 in joint.
• Tile thickness: also one to one. Moving from 1/4 in wall tile to 1/2 in quarry tile of the same size doubles the depth and the grout.
• Edge profile: cushioned or tumbled edges open the top of the joint and add grout the flat-edge formula does not count.
• Mix in batches: mix only what you can grout and clean before it stiffens, following the pot life on the label. Most waste comes from batches that set in the bucket.
• Buy one extra bag of the same lot: it covers a surprise and stays on the shelf for repairs that match.

Limitations

The calculator assumes straight, uniform joints that are filled for the full tile thickness. Real joints vary with tile tolerances, and hand-cut edges along walls hold a little more grout. The 110 lb per cubic foot density describes typical sanded cement grout; premium and lightweight formulas differ, and the bag's own coverage chart is the final word. The thinset ranges are typical data-sheet values, and actual use depends on the substrate's flatness, how the trowel is held and how much back-buttering the tile needs. Mortar beds, crack isolation membranes, waterproofing and leveling compounds are separate materials with their own coverage figures.

Related calculators

Work through a tile job in order: measure with the Square Footage Calculator, count tile and boxes with the Tile Calculator, and finish here with grout and thinset. If you are still deciding between tile and another surface, the Flooring Calculator sizes laminate and vinyl plank orders. For brick and block mortar, use the Brick Calculator, and for outdoor pavers set on sand rather than mortar, the Paver Calculator covers base gravel, bedding sand and polymeric joint sand.

Sources

• Custom Building Products, Polyblend Plus Sanded Grout technical data sheet (TDS-360): coverage per 25 lb bag by tile size and joint width, rated joints of 1/8 to 1/2 in, ANSI A118.6 and A118.7.
• Custom Building Products, VersaBond Professional Thin-Set Mortar: coverage per 50 lb bag by trowel notch, mortar contact of 80% in dry areas and 95% in wet areas and exteriors, bed thickness of 3/32 to 1/4 in.
• LATICRETE, 253 Gold data sheet (DS-167.6): coverage per 50 lb bag with 1/4 × 1/4 in and 1/4 × 3/8 in notched trowels.
• Custom Building Products, SureBond Unsanded Thin-Set Mortar: V-notch coverage per 40 lb bag.
• U.S. Occupational Safety and Health Administration, 29 CFR 1926.1153, Respirable crystalline silica: Table 1 controls for stationary masonry saws and handheld power saws.
• National Institute of Standards and Technology (NIST), Handbook 44 (2026), Appendix C: General Tables of Units of Measurement: 1 cubic foot = 1,728 cubic inches and 1 gallon = 231 cubic inches.
• American National Standards Institute, ANSI A108.02 (general requirements, including grout joint width), A108.5 (setting tile with dry-set and modified mortars), A108.10 (installing grout), and A118.1, A118.4, A118.6, A118.7 and A118.15 (mortar and grout material standards).