Grout formula, assumptions and a worked example
What the formula calculates
For a repeated rectangular tile grid, the grout-joint area fraction is 1 − (tile width × tile length) ÷ ((tile width + joint width) × (tile length + joint width)). This models joints in both directions without counting a crossing twice.
From joint volume to bags
Multiply that fraction by the area in square meters, filled joint depth in millimeters and the grout density in kg per liter. Add an allowance and divide by the actual grout-bag weight. Always round up to a whole bag.
Worked example
For 120 sq ft using 300 × 600 mm tiles, 3 mm wide and 6 mm deep joints, density 1.6 kg/L and 10% allowance, the estimate is roughly 1.75 kg. A 4.54 kg bag rounds to one bag.
Important installation limits
This is a simplified geometric estimate for rectangular tiles and evenly filled joints, not a product-specific grout coverage guarantee. Joint cleaning, tile edges, surface texture, grout chemistry and manufacturer instructions change consumption. Check the product’s official coverage table before ordering.
Frequently asked questions
How much grout do I need for 100 square feet?
There is no fixed answer. Smaller tiles and wider or deeper joints use more grout. Enter your exact tile and joint dimensions, and cross-check with the grout manufacturer.
Is grout coverage the same for sanded and unsanded products?
No. Product density, approved joint widths and usage guidelines vary. Follow the specific product technical data sheet.
Why are the inputs metric on a US-focused site?
A 1 mm layer over 1 m² equals 1 liter, so millimeter-based tile and joint inputs make the volume formula easy to verify. Your project area is still entered in square feet.
Reference: manufacturer grout calculators and product data sheets: LATICRETE grout guidance. Manufacturer references are for checking coverage and installation; no endorsement or partnership is implied. Updated 2026-11-03.