Grout Calculator

The four numbers behind a grout estimate

Two of them are settled before you start: the tile size and the area. Together they fix the metres of joint, because in a single-size layout every tile carries half of its own perimeter and its neighbours carry the other half. That is arithmetic, not a feature — it works out the same on any calculator worth using. What it means in practice is that format matters more than square metres. A floor of narrow plank tile holds noticeably more joint per square metre than the same floor in 600x600, because a long, thin tile puts more edge into the same coverage.

Depth is the input people guess at, and it isn't a design choice: the joint is filled the full thickness of the tile, from the bed up to the face. A 10mm porcelain floor tile therefore takes roughly twice the grout of a 5mm wall tile at the same joint width over the same area. Rather than ask for a decimal, the tool offers the thicknesses that are actually sold — that figure is on the box, and nobody reaches for calipers to find it.

The fourth is the grout itself. Sanded, unsanded and epoxy are different materials with different densities — 1.60, 1.45 and 1.85 kg per litre respectively — so an identical volume of joint comes out at a different weight depending which one goes in the bucket. That is why the answer is given as a weight: weight is what the label states, whatever bag size your merchant stocks.

Joint width is a spacer size, not a decimal

Most calculators ask for a gap width to two decimal places. Nobody measures a grout joint. You buy spacers — 1.5mm, 2mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm — and the joint is whatever the spacer is. So the width here is an optional input offering those sizes, not a free-text field, and leaving it alone costs you nothing.

Left alone, the page shows two regimes side by side, because the tile decides before the tiler does. A rectified tile, its edges ground square after firing, sits on a tight joint, commonly 2mm to 3mm; a calibrated tile, sorted into size ranges as it comes out of the kiln, needs a wider one, typically 3mm and up, to keep the joint lines running straight. That split is manufacturer guidance rather than a fixed rule, and it is worth checking against the batch you are actually holding rather than the box it came out of.

Width is also where the quantity moves fastest. Going from 2mm to 4mm on the same floor roughly doubles the grout, and going again to 8mm doubles it once more. It is the number least worth approximating, which is precisely why it is offered as a spacer size you can point to rather than a value you invent.

Sanded, unsanded, or epoxy — the joint decides

Sanded and unsanded are not grades of the same product, and the dividing line sits at about 3mm. Unsanded grout shrinks as it cures; much beyond that width it pulls away from the tile edges and cracks. Sanded grout takes its body from the aggregate and will not pack down into a joint much under 3mm without leaving voids. Getting this pair the wrong way round is a failure no amount of estimating precision repairs.

The tile has a say too. Sand scratches: on polished marble, honed limestone or glass, sanded grout can leave the faces dulled after the first pass across them, so unsanded or epoxy is the usual answer even where the joint looks wide enough for sand. Epoxy sits outside the pair altogether — non-porous, stain-resistant, unforgiving about working time, and heavier per unit of joint than either cement-based grout, which is why the type is an input rather than an afterthought.

Sized on the tile you lay, not the tile you buy

The area behind the estimate is the area you lay. Offcuts go in the bin and cracked tiles never reach the floor; neither takes any grout. That is also why nothing here adds a flat percentage to the answer. A padding figure applied to every job regardless of the tile, the joint and the room is a guess wearing the clothes of a calculation, and it is the habit this page exists to replace.

Two details run the other way and are worth knowing before you order. The joint where the field meets a wall, a bath or a change of plane is a movement joint, filled with sealant rather than grout, so it makes no call on the bag. And a certain amount is always lost in the bucket and off the float. Both are calls to make with a real figure in front of you, rather than ones already folded into it.

Mesh-mounted sheets don't follow the same maths

A mesh-mounted sheet is not a big tile. Across a sheet of small mosaic chips, the joint is not a width anyone chooses — it is the pitch of the mesh, fixed at the factory, and it runs between every chip on the sheet as well as around the outside of it. A square metre of mosaic can hold several times the length of joint that a square metre of field tile holds.

This is where a plain rectangle formula goes badly wrong: given the sheet's outside dimensions, it counts the perimeter and misses the entire interior network, understating the quantity by a wide margin. Setting penny round mosaic sheets or any other mesh-backed pattern is a case for choosing the pattern rather than typing in the sheet size, because the joint then comes out of the mesh geometry instead of the sheet outline.

When the layout stops being a straight grid

On one tile size, the layout does not change the grout. A half-offset running bond, a straight stack and a diagonal all put the same length of joint into the same room, since each tile still carries half its perimeter whichever way it is turned. What the layout changes is the cutting — a chevron or herringbone floor meets every wall on the diagonal, so the perimeter is cut tile from end to end, and that is a tile-count question the pattern pages answer directly.

Mixed-format layouts are the real exception. The four-size French pattern draws its joint from four different tiles inside a single repeating module, and no one-size formula reaches it: the length of joint depends on how many of each size the room takes. If your floor is one of these, start from the pattern instead — the tile count and the grout come out of the same layout, with nothing to key in twice.