Construction Calculator
Grout Calculator
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Work out the grout a tile job takes — from the area, tile size, joint width and depth — with the coverage rate behind the figure, the bags it comes to, and why small tile matters far more than a wide joint.
Grout Calculator
Grout weight, coverage and bags
Grout type
Silica sand in the mix resists shrinkage, which is what makes a wide joint hold. Denser than unsanded, so the same joint volume weighs more. Can scratch polished stone and some glazes.
Results
Grout needed
7.3 lb
3.3 kg · Sanded
Bags to buy
1 × 8.5 lb
1.2 lb spare
Tile Size Comparison — 0.125 in Joint
| Tile | Joint % | lb/sq ft | This job |
|---|---|---|---|
| 1 × 1 in mosaic | 20.99% | 0.6821 | 75.0 lb |
| 2 × 2 in mosaic | 11.42% | 0.3711 | 40.8 lb |
| 4 × 4 in | 5.97% | 0.1940 | 21.3 lb |
| 6 × 6 in | 4.04% | 0.1313 | 14.4 lb |
| 6 × 24 in plank | 2.55% | 0.0828 | 9.1 lb |
| 12 × 12 in ← | 2.05% | 0.0667 | 7.3 lb |
| 12 × 24 in | 1.54% | 0.0502 | 5.5 lb |
| 18 × 18 in | 1.37% | 0.0447 | 4.9 lb |
| 24 × 24 in | 1.03% | 0.0336 | 3.7 lb |
Same area and joint, different tile. A 1 × 1 in mosaic takes over ten times the grout of a 12 × 12 in tile, because the joint fraction scales with the perimeter per unit area.
Bag Size Comparison — 7.33 lb Needed
| Bag | Bags | Supplied | Spare |
|---|---|---|---|
| 8.5 lb ← | 1 | 8.5 lb | 1.17 lb |
| 10 lb | 1 | 10.0 lb | 2.67 lb |
| 12.5 lb | 1 | 12.5 lb | 5.17 lb |
| 25 lb | 1 | 25.0 lb | 17.67 lb |
Grout has a shelf life once the bag is opened, and matching the colour of a later batch is difficult — so a bag that fits the job beats a big bag half used.
Joints are 2.05% of the tiled surface here, which works out to 1.108 cubic inches of grout per square foot, or 0.0667 lb. One 8.5 lb bag covers about 128 square feet at that rate.
The joint fraction is worked out from one grid cell: the cell is (tile length + joint) by (tile width + joint), and whatever the tile does not fill is joint. That counts each joint once, since every cell owns one vertical and one horizontal joint. A formula in circulation uses twice the tile perimeter without halving it, which double-counts the shared joints and overstates grout by nearly 2×.
Tile size drives this far more than joint width does. Halving the tile roughly doubles the grout: a 1 × 1 in mosaic at a 1/8 in joint puts 20.99% of the surface in joint, against 1.03% for a 24 × 24 in tile — a factor of 20 for the same joint.
That is roughly 98 tiles across 100 square feet, at 147.02 square inches per tile including its joint.
Densities used are about 104 lb/cu ft for sanded grout and 86 lb/cu ft for unsanded, the difference being the silica sand. Figures such as "0.1875 lb per cubic inch" circulate for sanded grout, which comes to 324 lb/cu ft — a unit error of roughly three times.
Checked against a published figure: Laticrete lists 388 square feet of coverage for a 25 lb bag of PERMACOLOR grout, and this model gives 375 square feet for a 12 × 12 in tile with a 1/8 in joint at 3/8 in deep, which agrees to within 3.3%.
Joint depth is normally the tile thickness, since a joint is filled flush. It is a separate input because the grout may not reach full depth over a trowel ridge, and because mosaic sheets are much thinner than floor tile.
A uniform rectangular grid is assumed. Herringbone, basketweave and other patterns carry more joint length per unit area, as do cushion-edge and wave-edge tiles, and all need adjusting by hand. This estimates quantity and does not replace a manufacturer coverage chart for a specific product.
Step-by-Step Calculation
Step 1 — Joint area fraction
one grid cell = (12 + 0.125) × (12 + 0.125) = 147.0156 sq in
the tile fills 12 × 12 = 144.0000 sq in
joint = 147.0156 − 144.0000 = 3.0156 sq in
fraction = 3.0156 ÷ 147.0156 = 0.020512 (2.05%)
Step 2 — Grout volume per square foot
0.020512 × 144 sq in × 0.375 in deep = 1.1077 cu in/sq ft
Step 3 — Total volume
1.1077 × 100 sq ft = 110.77 cu in
110.77 ÷ 1,728 = 0.06410 cu ft
Step 4 — Weight
Sanded grout is about 104 lb/cu ft
0.06410 × 104 = 6.666 lb
that is 0.06666 lb per square foot
Step 5 — Add 10% waste
6.666 × 1.1 = 7.333 lb
Step 6 — Bags
7.333 ÷ 8.5 lb = 0.8627 → 1 bag
one 8.5 lb bag covers about 128 sq ft at this rate
Result — 7.33 lb (3.33 kg), 1 × 8.5 lb bag
How Much of the Surface Is Grout
Picture the floor as a repeating cell: one tile plus half a joint on every side. Whatever part of that (tile + joint) square the tile does not cover is grout. Multiply that fraction by the joint depth to get grout volume per square foot.
One Grid Cell Tells You Everything
Grout fills the gaps in a grid, so the whole calculation comes down to one repeating cell. A cell is (tile length + joint) by (tile width + joint), and whatever the tile does not fill is joint.
For a 12 × 12 in tile with a 1/8 in joint, the cell is 12.125 × 12.125 = 147.0156 square inches. The tile occupies 144, so 3.0156 is joint — 2.05% of the surface. Multiply that by 144 and the joint depth to get cubic inches per square foot, then by the grout density for pounds.
That cell owns exactly one vertical and one horizontal joint, which is why no halving is needed. It matters because a formula in wide circulation uses twice the tile perimeter without halving it, double-counting the joints shared between neighbouring tiles and overstating grout by nearly 2×.
Grout Formulas
1. Joint Area Fraction
2. Volume per Square Foot
3. Weight
4. Waste and Bags
Why Quoted Grout Figures Are Often Three Times Too High
For 100 square feet of 12 × 12 in tile with a 1/8 in joint at 3/8 in deep, you will commonly see 15–25 lb quoted. The geometry gives 6.67 lb.
The gap is not a matter of being conservative. To reach 20 lb from that joint volume you would need a grout density of 312 lb per cubic foot, three times that of real cementitious grout, or a joint 1.125 in deep — three times the stated 3/8 in. Even the double-counted perimeter formula only reaches 13.3 lb, so that is not the explanation either.
Two independent checks put the correct figure near 6.5 lb. Laticrete publishes 388 square feet of coverage for a 25 lb bag of PERMACOLOR grout; this model gives 375 square feet for exactly that tile and joint, agreeing to 3.3%. Working backwards from the published figure, 100 square feet needs 100 ÷ 388 × 25 = 6.44 lb. The geometry says 6.67.
The reason the error survives is that the bag count hides it. Both 6.7 lb and 20 lb fit inside one 25 lb bag, so the shopping conclusion is right even when the weight is badly wrong. It only shows up when you pick a bag that fits the job.
Tile Size Drives Grout, Not Joint Width
All at a 1/8 in joint, 3/8 in deep, sanded grout:
| Tile | Joint % | lb/sq ft | 25 lb covers | vs 12 × 12 |
|---|---|---|---|---|
| 1 × 1 mosaic | 20.99% | 0.6821 | 37 sq ft | 10.23× |
| 2 × 2 mosaic | 11.42% | 0.3711 | 67 sq ft | 5.57× |
| 4 × 4 | 5.97% | 0.1940 | 129 sq ft | 2.91× |
| 6 × 6 | 4.04% | 0.1313 | 190 sq ft | 1.97× |
| 6 × 24 plank | 2.55% | 0.0828 | 302 sq ft | 1.24× |
| 12 × 12 | 2.05% | 0.0667 | 375 sq ft | 1.00× |
| 12 × 24 | 1.54% | 0.0502 | 498 sq ft | 0.75× |
| 18 × 18 | 1.37% | 0.0447 | 560 sq ft | 0.67× |
| 24 × 24 | 1.03% | 0.0336 | 744 sq ft | 0.50× |
A 1 × 1 in mosaic takes 20.3 times the grout of a 24 × 24 in tile for the same joint and the same floor. That is why a bag's single coverage figure is almost meaningless without the tile size it assumes — the same 25 lb bag does 37 square feet of mosaic or 744 square feet of large-format tile.
Joint Width, and the Grout Type It Forces
On a 12 × 12 in tile at 3/8 in deep, across 100 square feet:
| Joint | Joint % | lb/sq ft | 100 sq ft | Type required |
|---|---|---|---|---|
| 1/16" | 1.034% | 0.0278 | 2.78 lb | unsanded |
| 3/32" | 1.544% | 0.0415 | 4.15 lb | unsanded |
| 1/8" | 2.051% | 0.0667 | 6.67 lb | sanded |
| 3/16" | 3.053% | 0.0992 | 9.92 lb | sanded |
| 1/4" | 4.040% | 0.1313 | 13.13 lb | sanded |
| 3/8" | 5.969% | 0.1940 | 19.40 lb | sanded |
| 1/2" | 7.840% | 0.2548 | 25.48 lb | sanded |
The last column is not a preference. Unsanded grout suits joints up to 1/8 in; sanded is for 1/8 in and wider. Unsanded shrinks and cracks in a wide joint, while sand will not pack into a fine one and can scratch polished stone or a delicate glaze. The two also differ in density — about 104 lb per cubic foot against 86 — so switching type changes the weight by roughly 21% for the same joint volume.
The Corner Term, and Why It Only Matters on Mosaic
A common shortcut computes joint area as j × (L + W), which misses the little square where two joints cross. The exact cell-based figure includes it:
| Tile | Exact | Shortcut | Corner adds |
|---|---|---|---|
| 24 × 24 | 1.0336% | 1.0309% | 0.26% |
| 12 × 12 | 2.0512% | 2.0406% | 0.52% |
| 6 × 6 | 4.0400% | 3.9983% | 1.04% |
| 4 × 4 | 5.9688% | 5.8770% | 1.56% |
| 2 × 2 | 11.4187% | 11.0727% | 3.13% |
| 1 × 1 | 20.9877% | 19.7531% | 6.25% |
The corner is worth a quarter of a percent on a 24 × 24 in tile and 6.25% on a 1 × 1 in mosaic, because the number of joint intersections per square foot rises as the tile shrinks. Negligible on large format, worth having on mosaic — which is exactly where grout quantities are largest and a shortfall is most annoying.
Example Calculations
A standard floor, a plank with a wide joint, and a mosaic:
Example 1 — 100 sq ft of 12 × 12 in Tile, 1/8 in Joint
The common case, and the one most often quoted wrongly.
Grid cell = 12.125 × 12.125 = 147.0156 sq in
Tile fills 144 sq in, so the joint is 3.0156 sq in
Joint fraction = 3.0156 ÷ 147.0156 = 2.05%
Volume = 2.05% × 144 × 0.375 deep = 1.108 cu in per sq ft
100 sq ft = 0.0641 cu ft × 104 lb/cu ft = 6.67 lb
One 8.5 lb bag covers it, with 1.17 lb spare after 10% waste
Example 2 — 200 sq ft of 6 × 24 in Plank, 1/4 in Joint
A narrow plank with a wide joint: both push the quantity up.
Cell = 6.25 × 24.25 = 151.5625 sq in; tile fills 144
Joint fraction = 7.5625 ÷ 151.5625 = 4.99%
That is 2.43× the grout per square foot of Example 1
Volume = 2.694 cu in per sq ft
200 sq ft = 32.43 lb, or 35.68 lb with 10% waste
Two 25 lb bags — the joint width alone nearly doubles the rate
Example 3 — 100 sq ft of 1 × 1 in Mosaic, 1/16 in Joint
Small tile is what really drives grout, even on a narrow joint.
Cell = 1.0625 × 1.0625 = 1.1289 sq in; tile fills 1
Joint fraction = 0.1289 ÷ 1.1289 = 11.42%
That is 5.6× the joint fraction of a 12 × 12 tile
At 1/4 in deep and unsanded: 0.2046 lb per sq ft
100 sq ft = 22.50 lb with 10% waste — one 25 lb bag
About 12,756 tiles, and 3.07× the grout of the 12 × 12 case
Before You Buy
Check the coverage chart for the exact product you are buying, since this is a geometric estimate and a specific grout may differ — and check which tile size any printed coverage figure assumes, because the same bag covers 37 square feet of mosaic or 744 of large-format tile. Measure the tile rather than trusting the nominal size: a tile sold as 12 inch is often 11⅞, and that changes the joint fraction. Buy all of it in one lot with the same batch number, because colour varies between batches and a second bag bought later rarely matches. Get the joint depth from the actual tile rather than assuming 3/8 in, as mosaic sheets are far thinner and the grout will not reach full depth where it bridges a trowel ridge. Prefer a bag size that fits the job over a big bag half used, since opened grout has a limited shelf life. And if the pattern is herringbone, basketweave or anything other than a straight grid — or the tile has a cushioned or wave edge — add to the figure by hand, because those carry more joint per square foot than a rectangular grid does.
Frequently Asked Questions
- How do you calculate how much grout you need?
- Work out what fraction of the surface is joint, then turn that into a volume and a weight. One grid cell is (tile length + joint) by (tile width + joint), and whatever the tile does not fill is joint. For a 12 × 12 in tile with a 1/8 in joint that is 3.0156 of 147.0156 square inches, or 2.05%. Multiply by 144 and the joint depth for cubic inches per square foot, then by the grout density — about 104 lb per cubic foot for sanded. A 100 square foot floor comes to 6.67 lb.
- How much grout for 100 square feet of 12 × 12 tile?
- About 6.7 lb with a 1/8 in joint at 3/8 in deep, or 7.3 lb once you add the usual 10% waste. That fits in a single 8.5 lb bag. You will see figures of 15–25 lb quoted for this exact case, and they are roughly three times too high — reaching 20 lb would need a grout density of 312 lb per cubic foot, three times that of real cementitious grout.
- Does tile size or joint width matter more for grout quantity?
- Tile size, by a wide margin. At a 1/8 in joint, a 1 × 1 in mosaic puts 20.99% of the surface in grout against 1.03% for a 24 × 24 in tile — a factor of twenty for the same joint. Widening the joint on a 12 × 12 tile from 1/16 to 1/4 in only moves it from 1.03% to 4.04%, a factor of 3.9. Halving the tile size roughly doubles the grout.
- What is the formula for grout coverage?
- Joint fraction = ((L + j)(W + j) − L × W) ÷ ((L + j)(W + j)), where L and W are tile dimensions and j is the joint width. Multiply by 144 and the joint depth for cubic inches per square foot, then by the density for pounds. Be careful with a version in circulation that uses twice the tile perimeter without halving it: that double-counts the joints shared between neighbouring tiles and overstates grout by very nearly 2×.
- How many square feet does a 25 lb bag of grout cover?
- It depends entirely on the tile and joint. For a 12 × 12 in tile with a 1/8 in joint at 3/8 in deep, about 375 square feet. For a 1 × 1 in mosaic at the same joint it drops to 37 square feet, and for a 24 × 24 in tile it rises to 744. Any single coverage figure on a bag is for one assumed tile size, so check which.
- How is epoxy grout like Spectralock estimated?
- Not by weight. Epoxy grout is sold as complete multi-part kits with their own published coverage per unit, so a pounds-per-bag calculation does not apply. This calculator reports the joint volume and an equivalent weight for comparison but deliberately gives no unit count for epoxy — take that from the product coverage chart.
- What waste factor should I use for grout?
- 10% is the manufacturer-stated figure, covering what stays in the bucket and on the float plus clean-up. Laticrete adds a further 10–15% for unglazed quarry tile, whose porous surface absorbs grout during installation. On a job this small the waste rarely changes the bag count, but it matters on a large floor.
- Should joint depth equal the tile thickness?
- Usually yes, since a joint is filled flush to the tile face. It is a separate input here for two reasons: the grout may not reach full depth where it bridges a trowel ridge of thinset, and tile thickness varies a great deal — a mosaic sheet is far thinner than a floor tile, so assuming 3/8 in across the board overstates mosaic jobs.
- Does this work for herringbone or other patterns?
- Not directly. The calculation assumes a uniform rectangular grid. Herringbone, basketweave and similar patterns carry more joint length per unit area, as do cushion-edge and wave-edge tiles where the joint widens toward the surface. Treat the figure as a floor and add by hand for the pattern. The result is an estimate and does not replace a manufacturer coverage chart for a specific product.