How the Grout Calculator Works

Enter your tile length and width, joint width, joint depth, and total area. The calculator computes the joint volume per square metre, converts it to weight using the grout's density, and multiplies across your area.

The formula the whole industry uses is:

where L is tile length, W is tile width, J_width and J_depth are joint width and depth respectively (all dimensions in millimetres), and 1.6 is the typical density in kg per litre for standard cementitious grout. The first bracket is the clever part: it measures how many linear metres of joint run through each square metre of tiling, which is why tile size changes grout consumption so dramatically. If you are ordering adhesive as well, you can estimate both together with our Grout and Adhesive Calculator.

Worked Example

A 5 m² bathroom floor in 300 × 300mm tiles, 3mm joints, 6mm deep:

• Joint ratio: (300 + 300) ÷ (300 × 300) = 0.00667
• Grout per m²: 0.00667 × 3 × 6 × 1.6 = 0.19 kg/m²
• Total: 0.19 × 5 = 0.96kg, plus 10% waste = about 1.1kg
• Order: One 5kg bag covers it with plenty spare for future repairs.

Now consider the same 5 m² in 100 × 100mm mosaics with 2mm joints at 4mm depth: the joint ratio triples to 0.02, and the total rises to about 1.4kg despite the narrower, shallower joints. Small tiles always outdrink large ones, which is the single most useful intuition this formula teaches.

Choosing the Joint Width

The joint width in the formula is a design decision with rules behind it. Rectified tiles with machine-cut edges take tight 1 to 2mm joints for a near-seamless look. Standard ceramic and porcelain want 2 to 3mm to absorb the small size variations of manufacturing. Natural stone needs 3 to 5mm because its edges are less uniform, and handmade or rustic tiles can carry 8 to 10mm joints as a deliberate feature.

Joint depth should fill the tile thickness, typically 6 to 8mm on walls and 8 to 12mm on floors. This is where quantity estimates quietly fail: adhesive squeezed up into joints during tiling steals depth from the grout, so rake joints clean while the adhesive is soft and measure the real remaining depth rather than assuming tile thickness.

Picking the Right Grout Type

Joint width selects the grout as well as the quantity. Fine unsanded grout suits joints under 3mm and polished tiles that sanded grout would scratch. Sanded cementitious grout handles 3mm to 10mm joints with extra strength. Both cementitious types are porous, wanting a sealer after curing to resist staining. Epoxy grout changes the rules: waterproof without sealing, stain-proof, ideal for showers and worktops, at a higher price, a denser 1.8 kg per litre in the formula, and an application technique unforgiving enough that many DIYers hire it out.

Two buying rules apply whatever the type: add 10% for mixing losses and cleanup, and buy the whole job's grout from one batch, since colour varies between batches and a joint colour change halfway up a wall cannot be fixed without raking out.

Application in Brief

Grouting starts at least 24 hours after tiling so the adhesive cures. Mix powder grout to a thick toothpaste consistency, let it slake for five minutes, then work it diagonally across joints with a rubber float, pressing firmly to fill the full depth. Clean the tile faces with a barely damp sponge once the grout firms, polish off the haze the next day, and hold off wet cleaning for 24 hours.