Adhesive and grout
Grout quantity (manufacturer formula, kg/m²)
- where
- grout per m² (kg/m²)
- tile length and width (mm)
- tile thickness, i.e. joint depth (mm)
- joint width (mm)
- grout density factor (g/cm³)
- grout needed (kg)
- area to grout (m²)
- allowance for losses (%)
- bags to buy
- bag size (kg)
Explain this equation
Estimates the weight of grout to fill the joints, using the formula printed on manufacturers' grout data sheets.
L and W are the tile length and width, T the tile thickness (which sets the joint depth) and j the joint width, all in millimetres. K is the density factor for the grout from its data sheet. q is the grout per square metre, M the total with a loss allowance, and B the number of bags of size Bs.
(L + W)/(L × W) is the length of joint per unit area: each tile brings one long and one short joint, shared over its own face area. Multiplying by joint width and depth gives joint volume per square metre, and the density turns volume into kilograms. With millimetres throughout and K in g/cm³ the answer comes out in kg/m².
Sense check: 100 × 100 × 6 mm tiles with 3 mm joints take about 0.58 kg/m²; 600 × 300 × 10 mm tiles with 3 mm joints only about 0.24 kg/m². Doubling the joint width doubles the grout.
Watch out: use the K value for your grout; Mapei use 1.5 to 1.7 for their cement grouts and different values for epoxies.
Weight of grout for a tiled area using the joint-volume formula printed on Mapei grout data sheets (similar formulas are used by BAL and others).
Enter your values to see the result.
Good to know
(L + W)/(L × W) is the joint length per unit area for each tile module; times joint width and depth gives joint volume per m², and the density factor converts it to kg. Use the factor from your grout’s data sheet: Mapei quote 1.6 for Ultracolor Plus and Kerapoxy, 1.5 for Keracolor FF/SF, 1.7 for Maxifuga. Joint depth is normally the tile thickness. Small tiles with wide joints use a lot more grout than large tiles.
Common mistake
Using the tile size in metres and the joint in millimetres. Keep L, W, T and j all in millimetres.
Limits
Assumes joints are filled to the full tile thickness. Bevelled-edge or textured tiles use slightly less; rustic tiles with irregular edges more. Movement joints filled with sealant are not deducted.
Sources
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.