Grout Calculator

A 100 sq ft floor in 12x12 inch tile with 1/8 inch joints and a 3/8 inch joint depth needs about 6.4 lb of grout, or about 7.0 lb with a 10% waste allowance. One 25 lb bag covers that job several times over. The number moves fast with tile size. The same 100 sq ft in 2x2 inch mosaic needs about 35.6 lb of grout, about 39.2 lb with waste, which means buying two 25 lb bags.

Run your own tile size through the calculator above before you shop, because guessing from the area alone is what sends people back to the store.

Grout does not cover the floor. It fills the thin network of joints between the tiles, so the amount you need depends on how much joint your layout has, not on the area alone. A grout calculator finds the share of the surface taken by joints from your tile size and joint width, multiplies it by the tiled area and the joint depth to get the joint volume, converts that volume to a weight using the density of your grout type, adds your waste allowance, and rounds the result up into whole bags.

Small tiles pack far more joints into each square foot than large tiles, which is why the same floor can need four or five times more grout in mosaic than in 12x12 tile. The calculator above does the joint geometry for your exact tile, so the answer fits your job instead of a generic one.

1. Pick inches or millimeters. The toggle changes the unit on every field.
2. Enter the tile length and width. Measure one tile rather than trusting the box label if they differ.
3. Enter the grout joint width, the gap your spacers set, and the joint depth, which is normally the tile thickness.
4. Enter the tiled area in square feet or square meters.
5. Choose the grout type, set the waste allowance, and pick your bag size. A 10% waste allowance suits a flat simple floor.
6. Press Calculate. You get the number of bags first, then the weight in pounds and kilograms, the joint volume, the coverage per bag, and the full step-by-step working so you can check the math by hand.

Joint share = 1 - (tile length x tile width) / ((tile length + joint width) x (tile width + joint width)). In this formula, tile length is the long side of one tile, tile width is the short side, and joint width is the gap between tiles. Grout weight = joint share x tiled area x joint depth x grout density.

Picture one tile plus the joint on two of its sides. That repeating cell covers (length + joint) by (width + joint), the tile itself covers length by width, and everything in the cell that is not tile is grout. The calculator turns that joint share into a volume with your area and depth, then into a weight with the density for your grout type. Densities are approximate and vary by manufacturer: sanded grout sits near 1.6 kg per liter, unsanded near 1.5, and epoxy near 1.8.

Worked Example

Here is the full arithmetic for 300 mm square tiles, 8 mm thick, with 3 mm joints over 10 sq m, using sanded grout with a 10% waste allowance.

First the joint share: 1 - (300 x 300) / (303 x 303) = 1 - 0.98030 = 0.019704, so about 1.97% of the surface is joint. Then the joint volume: 0.019704 x 10 sq m x 0.008 m = 0.0015763 cubic m, which is 1.58 liters. Then the grout weight: 1.58 L x 1.6 kg/L = 2.52 kg. With 10% waste: 2.52 x 1.10 = 2.77 kg, or about 2.8 kg. Type these numbers into the calculator above to check the arithmetic yourself. If you are pouring a solid mortar bed rather than filling joints, the concrete calculator handles volume by length, width, and depth.

These numbers assume a 3/8 inch joint depth and sanded grout at 1.6 kg per liter. Each cell shows the grout used per square foot first, then how many square feet one 25 lb bag covers before waste. Scale the pounds per sq ft for your tile thickness. If your tile is 1/4 inch thick instead of 3/8, multiply the pounds per sq ft by 0.67.

Use this chart to plan, then confirm against the coverage table printed on your product packaging before you buy. Density varies by brand, so the bag chart is the final authority for your exact product.

The table below runs the same 100 sq ft floor with 1/8 inch joints at 3/8 inch depth in sanded grout, changing only the tile size. It shows how violently the grout need swings on identical areas.

Halving the tile size roughly doubles the grout on the same floor, and the 2x2 mosaic uses more than five times the grout of the 12x12. Joint width works the same way: doubling the width from 1/16 to 1/8 inch nearly doubles the grout on the same tiles. Because these factors compound, eyeballing the quantity is unreliable and the per-joint calculation is the dependable way to estimate.

The joint volume is identical for all three types. The gaps in your floor do not change because the bag does. What changes is the weight of grout that fills that volume, because each type packs at a different density, and which joints each type is suited to. On the same 100 sq ft floor, unsanded grout runs about 6% lighter than sanded, which can decide whether a single 10 lb bag is enough.

Which Grout for Which Joint Width

The 1/8 inch rule decides it. Use sanded grout for joints 1/8 inch (3 mm) and wider. The sand gives wide joints strength and resists shrinkage and cracking, which is why it suits floors and most tiling. Use unsanded grout for narrower joints and for polished or delicate tile that sand could scratch. Epoxy grout is waterproof and stain resistant, which makes it a good pick for kitchens, bathrooms, and commercial floors, though it costs more and sets fast.

Densities are starting points only. Follow the tile and grout manufacturer guidance for your specific job.

Order the waste-adjusted weight, not the net weight. The net figure is the grout that ends up in the joints on a perfect day. The waste allowance covers what stays in the bucket, what drops and dries on the tile faces, and the part-bag left unmixed when the working time ran out. A 10% allowance suits a flat simple floor laid in a straight grid. Use 15% for diagonal layouts, small or mosaic tile, uneven handmade tile with wider effective joints, or your first tiling job.

Always round bags up. A 39.2 lb result with waste is two 25 lb bags, not one. This calculator already rounds up for you. Grout is only one bag on the shopping list. You also need thinset or mortar to bed the tiles, so plan those materials in the same trip.

Finally, confirm against the coverage table on your product packaging before a large order. Density varies by brand, so the bag chart is the final authority. For full floor material planning beyond grout, the flooring calculator estimates your overall material needs.

These two tools answer different questions about the same project. The tile calculator counts the tiles and boxes your job needs. This grout calculator figures the grout that fills the joints between those tiles, in pounds, kilograms, and bags. Grout fills the gaps, tiles cover the area, and neither tool answers the other's question.

Use them together in this order. Count the tiles and boxes first, then figure the grout for the joints between them, then add thinset or mortar for the bed. That sequence gives you one shopping list and one trip to the store.