How the grout calculation works
Every joint is a long, thin channel: as wide as the joint and as deep as the tile is thick. The calculator works out how many inches of joint each square foot of tile has, turns that into a volume, then converts the volume to weight.
- Joint length per square foot. For a tile L × W inches, there are (L + W) ÷ (L × W) inches of joint per square inch of floor, or 144 times that per square foot.
- Joint volume. Multiply by the joint width and the tile thickness, both in inches.
- Weight. Multiply the volume by the density of the grout powder. We use 1.7 g/cm³, about 0.061 lb per cubic inch, a typical figure for sanded cementitious grout.
- Extra. Add about 10 percent for grout left in the bucket, on the float and in the sponge.
lb = area (sq ft) × 144 × (L + W) ÷ (L × W) × joint width × thickness × 0.061 × (1 + extra)
Worked example
A 100 sq ft floor of 12 × 12 in tile, 3/8 in thick, with 1/8 in joints: 144 × 24 ÷ 144 × 0.125 × 0.375 = 1.125 cu in of joint per square foot, so about 113 cu in in total. With 10 percent extra that is 124 cu in, or about 7.6 lb (3.4 kg) of grout. One 10 lb bag is enough. At the same sizes, one 25 lb bag would cover about 362 sq ft.
How far a 25 lb bag goes
These figures come from the same formula, with no extra added. They show how much tile size and joint width matter.
| Tile | Thickness | Joint | One 25 lb bag covers about |
|---|---|---|---|
| 2 × 2 in mosaic | 1/4 in | 1/16 in | 181 sq ft |
| 3 × 6 in subway | 5/16 in | 1/8 in | 145 sq ft |
| 6 × 6 in | 5/16 in | 1/4 in | 109 sq ft |
| 12 × 12 in | 3/8 in | 1/8 in | 362 sq ft |
| 12 × 24 in | 3/8 in | 1/8 in | 482 sq ft |
| 24 × 24 in | 3/8 in | 1/8 in | 724 sq ft |
Halving the joint width halves the grout. Doubling the tile size roughly halves it too, because there are fewer joints.
Measuring the inputs
- Tile size: use the actual size, not the nominal size on the box. Measure one tile with a tape.
- Thickness: grout normally fills the joint to near the tile surface, so the tile thickness is a good stand-in for joint depth. For tiles with rounded (cushioned) edges, the real depth is a little less.
- Joint width: the spacer size you plan to use. Rectified porcelain is often laid with 1/16 to 1/8 in joints; handmade or uneven tile needs wider joints.
Why the density varies
The weight of grout needed to fill a cubic inch depends on the sand content and the product. Sanded cementitious grouts are roughly 1.6 to 1.9 g/cm³ of dry powder per volume filled, unsanded grouts are usually a little lighter, and premixed and epoxy grouts are sold by volume or by kit. Treat this calculator as a planning estimate and compare it with the coverage chart printed on the bag, which is specific to that product.
Practical tips
- Buy all the grout from the same batch so the color matches.
- Mix only what you can use within the working time on the bag, especially in warm rooms.
- Keep a little dry grout for repairs, stored sealed and dry.
- Use a flexible sealant, not grout, at inside corners, where tile meets a tub or counter, and at perimeter movement joints.
- Wear gloves and eye protection. Cement-based grout is alkaline and can irritate or burn skin, and mixing the powder raises dust, so avoid breathing it.
Planning the tile itself? Use the tile calculator, and the thinset calculator for the mortar underneath.
Frequently asked questions
How much grout do I need for 100 square feet?
For 12 × 12 in tile, 3/8 in thick, with 1/8 in joints, about 7.6 lb including 10 percent extra, so one 10 lb bag. Small mosaics or wide joints need much more.
How many square feet does a 25 lb bag of grout cover?
It depends on tile size and joint width. With 12 × 12 in tile and 1/8 in joints it covers about 362 sq ft; with 6 × 6 in tile and 1/4 in joints, about 109 sq ft.
Does tile thickness affect how much grout I need?
Yes. Joint depth is about the tile thickness, so a tile twice as thick needs about twice as much grout for the same joint width.
What grout density does this calculator assume?
About 1.7 g/cm³, or roughly 106 lb per cubic foot of joint, a typical value for sanded cementitious grout. Products vary, so compare with the bag's chart.