Grout is the part of tiling that people forget to buy, and then the finished floor waits days for a bag that costs very little. It is also easy to overbuy, because grout bags look small and the fear of running out pushes people to grab three when one would do. This calculator gives the honest number. Enter the tiled area, the tile size, the joint width and depth, and the bag size, and it returns kilograms of grout and bags to buy, with 10 percent waste included.
The formula comes from the shape of the joints. Small tiles have far more joint length per square metre than large tiles, so they need far more grout for the same floor. A 300 mm square tile has about 6.7 metres of joint per square metre, while a 600 mm square tile has only about 3.3 metres. Wider joints and deeper joints both increase the volume in direct proportion. The tool multiplies joint length by width and depth, applies a density factor of 1.6, and adds 10 percent waste for spills and mixing losses.
Here is a worked example. A floor of 20 square metres with 300 mm square tiles, 3 mm joints, and 6 mm joint depth needs about 0.192 kilograms per square metre. Multiply by 20 square metres and you get about 3.84 kilograms, and with 10 percent waste about 4.22 kilograms. That is one 5 kilogram bag, with a little left over. The same floor with 100 mm mosaic tiles would need several times more grout, which surprises many first-time tilers.
Joint width is usually chosen for looks, but it changes the grout quantity a lot. A 2 mm joint uses a third less grout than a 3 mm joint, and a 5 mm joint uses two thirds more. Do not narrow the joints just to save grout: narrow joints suit flat, calibrated tiles on stable floors, while wider joints forgive small size differences and movement. Follow the tile maker guidance for the minimum joint width, and measure the spacers you actually used rather than guessing.
Joint depth is the number people guess worst. It is not the tile thickness: grout fills the joint down to the adhesive bed, which usually leaves a depth around two thirds of the tile thickness. If you set the depth too high, you will overbuy; too low, and you will run out mid-wall. After laying a few tiles, push a spacer or a thin stick into a joint and measure the real depth before you buy the grout.
Colour and type matter as much as quantity. Buy all your grout from one batch if you can, because shades can vary slightly between batches, and mix the full amount for one room in a single batch where possible. Cement grout suits most floors and walls; epoxy grout resists stains and water but costs more and sets faster, so it suits showers and kitchens rather than a first tiling job. Keep the bag dry and use it within its shelf life.
Remember that this is a planning estimate, not a professional quote. It assumes even joints and a flat bed; uneven tiles, deep adhesive beds, and very rough tile edges all use more grout. If your joints vary in width, estimate with the widest width you see. For a large commercial floor, show your numbers to the supplier and ask them to confirm the bag count, and always buy one spare bag for repairs if the grout colour is unusual. Also check the grout colour against a dried sample, not the wet mix, before you buy, since wet grout always looks darker than the finished joint.