Grout Calculator

Grout needs depend less on the room size than on the tiles: small mosaics with wide joints can use ten times as much grout per square foot as large-format tiles with tight joints. This calculator uses the standard joint-volume formula to estimate grout by weight and bags, and the guide explains joint widths, sanded and unsanded grout, and how to avoid running short mid-job.

The grout formula

Grout fills the joints between tiles, so the amount depends on the total length of joint and its cross-section. Per unit area, the joint length is (tile length + tile width) ÷ (tile length × tile width). Multiplying by joint width and depth gives the volume, and multiplying by the grout’s density gives the weight. In metric units: kg per m² = (L + W) ÷ (L × W) × joint width × joint depth × density, with tile sizes and joints in millimetres and a density of about 1.6 kg per litre for cement grout. Epoxy grouts are a little denser.

Coverage for common tiles

These figures come from the formula with a cement grout density of 1.6 kg/L. Mosaics need far more grout per square foot because there are many more joints. Add 10–20% for waste, spills and grout left in the bucket.

Choosing joint width

Tile manufacturers often recommend a minimum joint based on the tile’s size variation. Very tight joints on non-rectified tiles make uneven edges more noticeable.

Sanded, unsanded and epoxy grout

Worked example

A 100 sq ft bathroom floor uses 12 × 12 in porcelain tiles 3/8 in thick with 1/8 in joints. In millimetres: (304.8 + 304.8) ÷ (304.8 × 304.8) × 3.175 × 9.525 × 1.6 ≈ 0.318 kg/m². The floor is 9.29 m², so it needs about 2.95 kg, or 6.5 lb, before waste. With 15% waste that’s about 7.5 lb — a single 10 lb bag. The same floor in 2 × 2 in mosaics with 1/8 in joints and 1/4 in thickness would need around 26 lb before waste — two 25 lb bags once waste is added.

Buying and mixing tips

• Buy all the grout from the same batch (lot number) so the colour is consistent.
• Mix only as much as you can use within the working time on the bag, often 20–30 minutes.
• Use the water ratio on the bag; extra water weakens grout and can cause colour variation.
• Keep a small amount for future repairs, labelled with the colour and lot.
• Wait for tile adhesive to cure (often 24 hours) before grouting.

Grout colour choices

• A grout close to the tile colour makes joints disappear and the surface look larger.
• A contrasting grout emphasises the tile shape — popular with white subway tiles and dark grout.
• Mid-grey and beige hide dirt better than white on floors.
• Colours can dry lighter or darker than they look wet; make a sample board first.
• Colour-consistent products and epoxy grouts show less variation than traditional cement grout.

Sealing and aftercare

Most cement grouts are porous and benefit from a penetrating sealer once fully cured — often after 48–72 hours, but follow the product instructions. Epoxy grout doesn’t need sealing. Clean with pH-neutral cleaners; acidic and bleach-based cleaners can etch grout and stone.

The estimating pack

The optional pack includes an Excel tile and grout estimator for several rooms at once (tiles, adhesive and grout with waste), a printable tiling checklist, a grout colour and batch record sheet and a project cost log. The calculator on this page is free.

AI-assisted content

This page was drafted with AI assistance and reviewed by Kedop. Always check the grout manufacturer’s coverage chart for your product.