About TileGrout
Why grout math matters
Running out of grout halfway through a floor is one of the most frustrating DIY failures, because a second batch mixed on a different day can dry to a slightly different shade and leave a visible patchwork across your tiles. Buying far too much is the opposite waste: opened grout bags harden within weeks, and unopened bags still cost money and cupboard space. A careful calculation before you start removes both risks. This calculator uses the standard volumetric method used by professional tilers. It treats every joint as a small rectangular channel running between the tiles, multiplies the total joint length per square metre by the joint width and depth, and converts that volume into litres, kilograms and whole bags. The result includes a ten percent waste allowance for spills, overfilling and the grout left in the bucket, which is the margin most tradespeople actually work to on real jobs.
How the calculation works
The key insight is that grout volume per square metre depends almost entirely on tile size and joint width, not on the shape of the room. Small mosaic tiles have far more joint length per square metre than large format slabs, so the same floor area can need three or four times as much grout. The formula multiplies the joint cross-section, width times depth, by the total joint length, which is the sum of the reciprocals of the tile length and width. Converting cubic metres to litres gives a number you can compare directly with the coverage printed on a bag. Our weight conversion uses a typical cement grout density of 1.6 kilograms per litre, and the bag count rounds up to whole five kilogram bags, because suppliers do not sell part bags and a shortfall mid-job is far more expensive than one spare bag.
Measuring your floor correctly
Measure the floor area in square metres by multiplying length by width for rectangular rooms, and splitting L-shaped rooms into two rectangles. Subtract large fixed features such as a bath footprint only if you genuinely will not tile under them; for most rooms, tiling wall to wall is simpler and the small overestimate acts as extra margin. Measure tile size from the face of one tile to the face of the next, and use the joint width your spacers actually set, not the width printed on the box, since spacer sizes and tile calibration vary. Depth is usually the full tile thickness for floor tiles, but if your tiles sit in a thick bed of adhesive, measure the visible joint depth instead, because grout only fills the open channel above the adhesive.
Choosing joint width and grout type
Narrow joints of two to three millimetres suit rectified porcelain and small rooms, while wider joints of five millimetres or more suit rustic tiles with uneven edges, because the joint absorbs the size variation and keeps the grid looking straight. Sanded grout is the right choice for joints wider than three millimetres, since the sand prevents shrinkage cracking, while unsanded grout gives a smoother finish for narrow joints and polished stone. Epoxy grout costs more and is harder to work, but it resists stains and chemicals, which makes it worth considering for kitchens, wet rooms and commercial floors. Whatever type you choose, the volume calculation stays the same, though epoxy coverage figures on the pack may differ slightly.
Common mistakes to avoid
The most common mistake is guessing the area by pacing it out instead of measuring with a tape, which routinely produces errors of ten to twenty percent. The second is forgetting waste entirely: grout sticks to the bucket, the float and the sponge, and a perfect theoretical figure will always fall short in practice. The third is mixing the whole bag at once on a hot day, which lets the grout stiffen before you finish spreading it, forcing you to throw away usable material and mix a second batch. Mix in small batches, keep the sponge only damp rather than dripping, and work in sections you can finish within the grout's working time, which the bag usually states as twenty to thirty minutes.
Working efficiently on the day
Start by dry-laying a row of tiles to confirm your joint width and to spot any awkward cuts at the edges. Mix grout to a thick peanut-butter consistency, neither runny nor crumbly, and push it diagonally across the joints with a rubber float so every channel fills completely. After ten to twenty minutes, when the surface turns hazy, wipe diagonally with a damp sponge, rinsing often, and then buff the haze with a dry cloth once the joints have firmed. Avoid walking on the floor for at least twenty-four hours and keep it dry for seventy-two hours where possible, because early water exposure weakens cement grout and can cause patchy colour. Sealing the joints after curing helps in wet areas and makes future cleaning much easier.
Saving money without cutting corners
Buying the right number of bags is the single biggest saving, since a spare unopened bag can often be returned, while a shortfall costs a second trip and risks a shade mismatch. Compare the price per kilogram rather than per bag, because bag sizes range from two to twenty kilograms and the cheapest bag is not always the cheapest grout. Check whether your supplier sells the same shade in a larger trade bag, and buy a single spare bag only if the shade is a special order that might be hard to match later. Keep the receipt and the batch number until the job is finished, so a top-up bag comes from the same production run if you need one.
When to call a professional
Large floors over forty square metres, heated screeds, and natural stone that needs sealing before grouting are all cases where a professional tiler earns their fee, because the material cost of a mistake outweighs the labour saving. Movement joints also matter: long floors need flexible sealant joints at regular intervals and around the perimeter, and grouting over a movement joint guarantees a crack. If your substrate is cracked, springy or uneven, fix that first, because no grout calculation can compensate for a floor that moves. For straightforward rooms, however, this calculator plus careful measuring gives a DIYer the same quantities a professional would order.
Aftercare and maintenance
Cement grout stays clean longest when it is sealed and when spills are wiped promptly rather than left to stain. Use a pH-neutral floor cleaner for routine mopping, and avoid bleach and strong acids, which erode grout and lighten its colour unevenly. For stained joints, a paste of baking soda and water left for fifteen minutes before scrubbing usually lifts everyday grime, while a dedicated grout cleaner handles deeper discolouration. Recolouring products can refresh faded joints without regrouting, but they need a clean, dry surface to bond, so deep-clean first and test in a corner. Re-seal shower and wet-room joints once a year to keep water out of the substrate.
Estimating for patterned and mosaic layouts
Patterned layouts change grout demand in ways a plain area figure misses, so adjust the calculation before ordering. Herringbone and basketweave patterns use the same tile size but create more cut joints at edges and around obstacles, adding roughly five to ten percent to joint length compared with a straight grid. Mosaic sheets are the extreme case: dozens of tiny tiles per sheet mean joint length per square metre can be ten times higher than large format tiles, and the grout figure rises proportionally, which surprises first-time mosaic buyers who expected a single bag to suffice. Border strips and feature panels also add joints where two tile sizes meet, since the smaller border tiles contribute extra joint length along their edges. For any patterned floor, run the calculator with the dominant tile size first, then add ten percent for pattern waste on top of the standard ten percent, giving a combined margin that covers both cutting waste and joint length variation. Keep a spare bag of the exact shade for future repairs, because grout colour varies between production batches and a repair with a mismatched shade stands out on a patterned floor far more than on a plain one. Photograph the bag label and batch number, store the spare bag dry and sealed, and note the shade name in your household records so a future repair matches.