How much grout do I need per square meter of tile?

It depends heavily on tile size and joint width, not just area — smaller tile and wider joints both increase grout use per square meter since there is more joint length relative to tile face. As a rough guide, 300 x 300mm tile with a 3mm joint and 8mm depth uses roughly 0.14-0.16 kg of dry sanded grout per square meter; this calculator gives you an exact figure for your specific tile size and joint.

Should I use sanded or unsanded grout?

Use sanded grout for joints of roughly 3mm (1/8in) or wider, since the sand reduces shrinkage cracking as the joint dries. Use unsanded grout for narrower joints, and for tile with a polished or easily scratched surface regardless of joint width, since sand grains can mark soft glazes during application.

Why does the calculator ask for joint depth as well as joint width?

Grout is a volume, not just an area, so depth matters as much as width. Two floors with identical tile and joint width can need meaningfully different amounts of grout if one has a 6mm joint depth and the other has a 10mm depth — always check (or measure) your actual tile thickness rather than assuming a default.

Do I need waterproof or flexible grout for a UK bathroom or wet room?

For shower areas and wet rooms, use a grout rated for continuous water exposure (most cementitious grouts are suitable for splash zones but check the manufacturer's wet-area rating), and make sure the substrate is properly tanked before tiling — grout is not a substitute for a waterproof membrane underneath.

Grout Calculator — How Much Grout Do I Need?

Grout Calculator: How Much Grout Do I Need?

Grout is the last material you buy on a tiling job, but running short mid-mix is still a hassle you can avoid. This calculator takes the same tile size and joint width you'd already know from tiling the area, adds the joint depth, and converts the total gap volume into a dry-mix weight and a bag count — so you know exactly how many bags to bring home before you crack the first one open.

The math looks more complex than it is because grout doesn't fill a simple rectangle — it fills only the thin lattice of joints between tiles, which is a small fraction of the total surface. The calculator works out that fraction from your tile size and joint width, applies your joint depth to turn it into a volume, then uses your grout type's density and dry-material percentage to turn that volume into a weight you can match against a bag size. Enter your numbers below for a volume, weight, and bag count in seconds.

How to use the grout calculator

Enter the length and width of the tiled area (the calculator multiplies these into a total surface area).
Enter your tile size — length and width — the same figures you used when working out how many tiles to buy.
Enter the joint (gap) width between tiles and the joint depth, which is usually close to the tile thickness.
Choose a grout type — unsanded, sanded, or epoxy — each has a different default density and dry-material percentage, since epoxy grout cures without losing water weight the way cementitious grout does.
Adjust the waste percentage if needed (5-15% covers mixing losses, tooling-out overfill, and spillage).
Enter the weight of a single bag from your supplier to get a bag count, and a price per bag for an instant materials cost estimate.

The grout calculation formula, explained

The calculator first works out what fraction of the surface each tile actually covers, since grout only fills the rest. That fraction is R = (tile length x tile width) / ((tile length + joint width) x (tile width + joint width)) — the tile's face area divided by its footprint including half the joint on each side. The remaining fraction, 1 - R, is the share of the surface taken up by grout joints.

Grout area = total area x (1 - R). Grout volume = grout area x joint depth, then increased by your waste percentage. Dry-mix weight = grout volume x grout density x dry-material percentage (the fraction of the wet mix that is actually dry powder rather than water, since some of that water evaporates or gets sponged off during cleanup). Bags needed = dry-mix weight / bag weight, always rounded up to the next whole bag.

Worked example: a 4m x 3m floor (12 m² total) with 300 x 300mm tile, a 3mm joint width, and an 8mm joint depth, using sanded grout (1,600 kg/m³ density, 50% dry-material fraction) and a 10% waste allowance.

Coverage ratio R = (0.3 x 0.3) / (0.303 x 0.303) = 0.09 / 0.091809 = 0.9803 (98.03% tile, 1.97% joint).
Grout area = 12 m² x (1 - 0.9803) = 12 x 0.0197 = 0.236 m².
Grout volume = 0.236 m² x 0.008m depth = 0.00189 m³ (about 1.89 liters).
With 10% waste: 0.00189 x 1.10 = 0.00208 m³ (about 2.08 liters).
Dry-mix weight = 0.00208 m³ x 1,600 kg/m³ x 0.50 = 1.66 kg.
With an 11.3 kg (25 lb) bag, that rounds up to 1 bag — small-format tile with a narrow joint uses surprisingly little grout across a room-sized area.

Pro tips and common mistakes

Grout looks like it should scale linearly with area, and it mostly does — but joint width matters more than people expect. Doubling the joint width roughly doubles the grout volume for the same tile size and area, since it's the joint that grout fills, not the tile.

Worked example at UK prices: for a 12 m² bathroom floor using 300 x 300mm tile with a 3mm joint and 8mm depth, sanded grout and 10% waste, you'd need about 1.66 kg of dry mix — which rounds up to just 1 bag. At a typical UK price of around £15 per bag of grout, that's roughly £15 in grout for the whole floor; tile, adhesive and a tiler's labour are the real cost drivers, not the grout itself.

Sanded vs unsanded: use sanded grout for joints of about 3mm (1/8in) or wider — the sand aggregate reduces shrinkage and cracking in a wider joint. Use unsanded grout for joints narrower than that, and always for polished or easily scratched tile, since sand grains can scuff soft glazes.
Epoxy grout is entered as 100% dry material since it cures by chemical reaction rather than by water evaporating out of the mix — it typically has no water-shrinkage loss, is more stain- and chemical-resistant than cementitious grout, and costs noticeably more per bag.
Joint depth is usually close to the tile thickness (commonly 6-10mm for standard tile) rather than the full joint width — check your tile spec sheet if you are unsure, since guessing too deep overstates how much grout you need.
Buy grout from the same production batch where possible — like tile, grout mix color can vary slightly between batches, and a visible seam of mismatched color is hard to fix once it has cured.
Keep a small amount of leftover dry mix (sealed, dry storage) for future repairs — pre-mixed leftover grout does not store well, but an unopened bag from the same batch will still match years later.