Understanding your result
The headline is the number of bags, with the pounds or kilograms of dry grout under it and the coverage of one bag for your tile beside it. The breakdown shows the chain: the joint volume per square foot of floor, the grout that volume weighs, the total for your area, the allowance and the bags.
The row that explains most surprises is joint volume per square foot. It depends on three things at once: how much joint there is per square foot (small tiles have more), how wide the joint is, and how deep it is, which is the tile thickness. Double the joint width and the volume roughly doubles; halve the tile size and it roughly doubles again; move from 1/4 in wall tile to 3/8 in floor tile and it rises by half. This is why a 4 × 4 floor with 1/4 in joints takes six times the grout of a 12 × 24 floor with 1/8 in joints.
One bag covers is the figure to compare with the chart on the bag. If the two disagree by more than 15%, check that you entered the thickness and the joint correctly; charts assume full-depth joints. The notes under the result tell you when the grout type does not match the joint: unsanded grout in a joint wider than 1/8 in will crack, and sanded grout in a joint narrower than 1/16 in will not pack. For the mortar under the tile, which is a separate bag, use the thinset calculator; for the tile itself, the floor tile calculator works from the same tile size and joint.
How we calculate this
The joint volume is exact geometry: each tile owns one joint along its length and one along its width, plus the small square where they cross, and that area times the tile thickness is the volume of grout around each tile. Divided by the pitch and scaled to a square foot, it gives the volume per square foot of floor. The density is what turns volume into pounds of dry powder. It is calibrated so that the common case, 12 × 12 × 3/8 in tile with a 1/8 in joint, gives the coverage on Custom Building Products’ grout card, about 348 sq ft per 25 lb for unsanded grout; sanded grout is slightly denser. The grout and thinset coverage reference tabulates the result for common tiles and joints next to the manufacturers’ figures, and the guide to tile size and grout joint explains why installers care so much about a sixteenth of an inch.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Dry grout density | 0.070 lb/cu in sanded, 0.065 unsanded | Calibrated to Custom Building Products’ coverage card (12 × 12 × 3/8 in, 1/8 in joint, about 348 sq ft per 25 lb bag, unsanded); Mapei and Laticrete charts agree within about 15% |
| Tile thickness | 3/8 in (10 mm) | Typical porcelain floor tile; wall tile is 1/4 to 5/16 in (manufacturer specifications) |
| Joint width | 1/8 in (3 mm) | TCNA Handbook and ANSI A108.02: not less than 1/8 in for non-rectified tile |
| Sanded / unsanded limit | 1/8 in | Manufacturer labelling: unsanded for joints up to 1/8 in, sanded for 1/8 to 1/2 in (Custom, Mapei, Laticrete product data) |
| Bag size | 25 lb (11.3 kg) | The standard large bag in the US; 10 and 7 lb bags are common for small jobs |
| Allowance | 10% | Typical mixing and clean-up loss quoted in manufacturer installation guides |
Assumptions last reviewed October 7, 2026.
The calculator assumes rectangular tiles set in a regular grid. Hexagons, penny rounds and irregular stone have more joint per square foot than a rectangle of the same face area; treat a hexagon as a square of the same width and add 10% to the joint. Pre-mixed and epoxy grouts are sold by volume (quarts and gallons) rather than weight, so use the joint volume row and the maker’s yield per container instead of the bag count. The thinset coverage guide covers the other bag you will be carrying.
Two worked examples
A 200 sq ft floor of 12 × 24 porcelain
Tile 3/8 in thick, 1/8 in joint, sanded grout in 25 lb bags, 10% allowance.
- Pitch: 24.125 × 12.125 = 292.5 sq in; joint per tile 292.5 − 288 = 4.52 sq in
- Joint volume per sq ft: 144 × 4.52 × 0.375 ÷ 292.5 = 0.83 cu in
- Grout per sq ft: 0.83 × 0.070 = 0.058 lb
- For 200 sq ft: 0.058 × 200 = 11.7 lb; with 10%: 12.8 lb
- Bags: 12.8 ÷ 25 = 1 bag, which covers 428 sq ft of this tile
A whole floor on half a bag. Large tiles with narrow joints use very little grout, which is one reason the 12 × 24 format is popular with installers. The second half of the bag is worth keeping sealed for repairs.
A 48 sq ft subway backsplash
3 × 6 in tile 5/16 in thick, 1/16 in joint, unsanded grout in 10 lb bags.
- Pitch: 6.0625 × 3.0625 = 18.57 sq in; joint per tile 0.57 sq in
- Joint volume per sq ft: 144 × 0.57 × 0.3125 ÷ 18.57 = 1.37 cu in
- Grout per sq ft: 1.37 × 0.065 = 0.089 lb
- For 48 sq ft: 4.3 lb; with 10%: 4.7 lb
- Bags: 4.7 ÷ 10 = 1 bag, which covers 112 sq ft
Per square foot the backsplash takes half again as much grout as the porcelain floor, despite a joint half as wide, because a 3 × 6 tile has eight times the joint length per square foot. The 10 lb bag is the right size; a 25 lb bag would leave 20 lb on the shelf.
Where to find your inputs
Tile thickness. On the specification sheet, or measure the edge of a tile with a ruler. Porcelain floor tile is 3/8 in (9 to 10 mm); ceramic wall tile 1/4 to 5/16 in; quarry tile and pavers 1/2 in and up.
Joint width. The spacer size you will use. If the tile is non-rectified (pressed edges), 1/8 in is the minimum the standards allow; rectified tile can go to 1/16 in.
Area. The area of tile, not of the room; the floor tile calculator gives it, and for a shower the shower calculator gives the wall and floor areas separately.
Bag size. On the bag. The common sizes are 25, 10 and 7 lb; metric bags are 5, 10 and 20 kg, with 11.3 kg being a relabelled 25 lb.
Common mistakes
- Using a chart for a different thickness. Most charts assume 3/8 in tile. Wall tile at 1/4 in takes a third less; pavers take far more.
- Unsanded grout in a wide joint. Above 1/8 in it shrinks and cracks. Use sanded or a high-performance grout.
- Sanded grout on polished stone or glass. The sand scratches the surface. Check the tile maker’s instructions.
- Buying the exact weight. Grout is lost to the bucket and the sponge. Keep the 10% allowance, and more for a first job.
- Mixing batches. Colour varies between production lots. Buy all the grout for one floor at once and check the lot numbers.
- Counting the room instead of the tile. Grout follows the tiled area, which for a bathroom is the floor plus the walls plus the shower.
Questions people ask
- How much grout do I need for 100 square feet?
- It depends entirely on the tile. 12 × 24 inch porcelain 3/8 inch thick with a 1/8 inch joint takes about 6 lb per 100 sq ft; 12 × 12 takes about 8 lb; 4 × 4 ceramic with a 1/4 inch joint takes 36 lb. A 25 lb bag covers the first two jobs with plenty over and is not enough for the third.
- What is the difference between sanded and unsanded grout?
- Sanded grout contains fine sand that stops it shrinking and cracking in joints 1/8 inch and wider; most makers rate it for 1/8 to 1/2 inch. Unsanded grout is smoother and is used for joints up to 1/8 inch and on polished stone, glass and soft tile that sand would scratch. The joint width decides which you need.
- How many square feet does a 25 lb bag of grout cover?
- Anywhere from about 50 sq ft for a small tile with a wide joint to over 400 sq ft for a large tile with a narrow one. For the common case of 12 × 12 inch tile 3/8 inch thick with a 1/8 inch joint, the makers' charts put a 25 lb bag at roughly 300 to 350 sq ft, and this calculator at 322 sq ft for sanded grout.
- Does tile thickness matter for grout?
- Yes, directly. The joint is filled to the depth of the tile, so a 3/8 inch porcelain floor tile takes 50% more grout than a 1/4 inch wall tile with the same joint. Pavers an inch thick take nearly three times the grout of standard floor tile.
- Why does my grout usage never match the chart?
- Because the charts assume a joint filled to full depth with nothing wasted, and real jobs lose grout to the bucket, the float, the sponge and the haze. The 10% allowance here covers typical loss; a first-time grouter may lose 20%. Buying one bag more than the bare figure is cheap insurance against a colour mismatch from a later batch.
- Should I buy grout before or after the tile?
- After the joint width is fixed, and in the same order as the tile if you can. The joint width decides sanded against unsanded, and the quantity depends on the tile's thickness, which you can measure once the tile is in hand. Buy all the grout for a floor from one batch; colour varies between batches.