Grout calculator

How much grout to buy, from the size of the tiles and the joints between them. Uses the same formula tile-product makers print on their coverage charts, and shows how much the answer depends on the product's density.

To buy

1 bag

6.0 lb (2.7 kg) including 10% waste, in 25 lb bags

100.0 sq ft (9.29 m²) of tiling has 46 m (150 ft) of joint. At 1/8 in (3.2 mm) wide and 3/8 in (9.5 mm) deep that is 1.38 litres (1.46 US quarts) of grout, which at 1.80 kg/L weighs 5.5 lb (2.5 kg) before waste. Rate: 5.5 lb per 100 sq ft (2.68 kg per 10 m²).

How much density changes the answer

The joint volume is fixed by geometry; the weight is not. These rows are the same joints at the low and high ends of published cement-grout densities.

Density	Rate	To buy	Bags
Low (1.5 kg/L)	4.6 lb per 100 sq ft (2.23 kg per 10 m²)	5.0 lb (2.3 kg)	1
Yours (1.80 kg/L)	5.5 lb per 100 sq ft (2.68 kg per 10 m²)	6.0 lb (2.7 kg)	1
High (1.9 kg/L)	5.8 lb per 100 sq ft (2.83 kg per 10 m²)	6.4 lb (2.9 kg)	1

How to use it

Enter the tiled area (the floor and wall tile calculators both give it), the tile's actual length and width, the joint width you will use, and the joint depth. Depth is normally the tile's thickness. Leave the density at its default unless your grout's technical data sheet gives one. Enter a bag size to get a bag count; bags are commonly sold in several sizes, so check the one on the shelf.

The formula

This is the coverage formula used on grout makers' charts, with every length in millimetres:

litres per m² = (L + W) ÷ (L × W) × joint width × joint depth

kg per m² = litres per m² × density (kg/L)

The first term is the length of joint per unit of area. A square metre of 300 × 300 mm tiles contains (300 + 300) ÷ (300 × 300) = 0.00667 mm of joint per square millimetre, or 6.67 m of joint per square metre. Multiply by the joint's cross-section and you have its volume, and because a one-millimetre layer over one square metre is exactly one litre, the result comes out directly in litres per square metre.

The tile size is in the denominator, which is the most useful thing to understand about grout. Halve the tile size and you double the joint length, and the grout. Mosaics use a lot; large-format porcelain uses very little.

A worked example

The default: 100 sq ft (9.29 m²) of 12 × 24 in tile, 3/8 in thick, with 1/8 in joints. In millimetres the tile is 304.8 × 609.6 and the joint 3.175 wide and 9.525 deep. (304.8 + 609.6) ÷ (304.8 × 609.6) = 0.00492 per mm, times 3.175 × 9.525 gives 0.149 litres per square metre. Over 9.29 m² that is 1.38 litres. At 1.8 kg/L it weighs 2.49 kg, or 5.5 lb; with 10% waste, 6.0 lb (2.7 kg). One 25 lb bag is plenty, and a smaller bag would do. At the low and high ends of the density range the figure to buy is 5.0 lb and 6.4 lb.

Pounds per 100 sq ft by tile size and joint

At 1.8 kg/L with no waste allowance, joint depth equal to the thickness shown. Multiply by 0.49 for kilograms per 10 m².

Tile	1/16 in joint	1/8 in joint	3/16 in joint	1/4 in joint
2 × 2 in mosaic, 1/4 in thick	14.6	29.3	43.9	58.5
4 × 4 in, 1/4 in thick	7.3	14.6	21.9	29.3
3 × 6 in subway, 5/16 in thick	9.1	18.3	27.4	36.6
12 × 12 in, 3/8 in thick	3.7	7.3	11.0	14.6
12 × 24 in, 3/8 in thick	2.7	5.5	8.2	11.0
24 × 24 in, 3/8 in thick	1.8	3.7	5.5	7.3

Where this stops being reliable

Density. The biggest uncertainty, as the table above shows. If your grout's data sheet gives a coverage chart, trust the chart over this page.
Tiles with rounded or bevelled edges. Pillowed edges, tumbled stone and handmade tiles leave a V- or U-shaped joint that holds more grout than a square-edged one of the same width. Add to the waste figure.
Joints not cleaned out. Adhesive left in the joints reduces the depth available and so the grout needed, but it also weakens the grout. Do not plan on it.
Irregular patterns. The formula assumes a regular grid of identical rectangles. Hexagons, multi-size patterns and irregular stone need the maker's chart or a larger allowance.
Waste in practice. Grout left in the bucket, in the float and on the sponge is real. Small jobs lose a higher share to it, which is why 10% is a floor, not a ceiling.

Common questions

How deep is a grout joint?

For a normal installation, the full thickness of the tile, less a small amount if the grout is tooled slightly below the surface. The adhesive underneath should have been cleaned out of the joints before it set, leaving at least two thirds of the tile depth open for grout, and more is better. Use the tile thickness unless you know otherwise.

Sanded or unsanded grout?

The usual guidance is unsanded for joints under 1/8 in (3 mm) and sanded for 1/8 in and wider, because sand lets cement grout fill a wide joint without shrinking and cracking but will not pack into a very narrow one. Some polished stones and glass tiles scratch under sanded grout; check the tile maker’s instructions. Epoxy and urethane grouts follow their own rules.

Why do different grout calculators give different answers?

Mostly density. The joint volume is geometry and every calculator agrees on it; turning volume into weight needs a density, and published figures run from about 1.5 to 1.9 kg per litre depending on the product. That alone is a 25% spread. Some calculators also quietly add waste or assume a joint depth. This page shows the low and high ends so you can see the range.

Does epoxy grout need a different amount?

By volume, no: the joints are the same size. By weight, epoxy grouts are generally lighter per litre than cement grouts, and they are sold in kits by weight or by volume. Use the manufacturer’s coverage chart or density where it is published, and buy close to the amount needed, because mixed epoxy cannot be kept.

Is one bag always enough for a small bathroom floor?

Often, with large tiles and narrow joints; much less often with mosaics. A 2 × 2 in mosaic at 1/8 in joints needs roughly eight times the grout of a 12 × 24 in tile at the same joint, because it has that much more joint per square foot. The reference table below shows how fast it adds up.