Grout and Thinset Calculator

What Drives Grout Quantity

Grout fills the space between tiles, so the amount needed has nothing to do with the price of the tile and everything to do with geometry. Three numbers control it: the tile size, the joint width and the joint depth. Small tiles have far more joint per square foot than large tiles, which is why a mosaic floor can take five times the grout of a large format floor over the same area.

A worked example

Take a 12 in by 12 in floor tile with a 1/8 in joint and a 3/8 in joint depth. The tile factor is (12 + 12) divided by 144, which is 0.1667. Multiply by 0.125 for the joint width, by 0.375 for the depth, and by 14.5, and the result is 0.113 pounds per square foot. A 200 square foot floor needs about 22.6 pounds, so a single 25 lb bag covers it with a little spare.

Change the tile to a 2 in by 2 in mosaic on the same floor with a 1/8 in joint and a 1/4 in depth and the tile factor becomes (2 + 2) divided by 4, which is 1.0. That gives 1.0 x 0.125 x 0.25 x 14.5, or 0.453 pounds per square foot, which is 90 pounds over 200 square feet. Same room, four times the grout.

Joint depth is not the tile thickness

Joint depth is how deep the grout actually sits, measured from the tile face down to the top of the thinset that has squeezed between the tiles. On a properly set floor that is usually a bit less than the tile thickness, commonly 1/4 in to 3/8 in. Using the full tile thickness overstates the order, which is safer than understating it but wasteful on a large job.

Sanded, Unsanded and Epoxy

Unsanded grout

Unsanded grout is used in joints under 1/8 in. It is smoother, it packs into narrow joints without bridging, and it will not scratch polished marble, glass tile or metal tile. Its weakness is shrinkage: pushed into a wide joint it cracks as it cures, because there is no aggregate holding the volume.

Sanded grout

Sanded grout carries fine silica sand and is the choice for joints of 1/8 in and wider, which covers most floors. The sand resists shrinkage and gives the joint compressive strength under foot traffic. It can scratch soft polished stone and glass, so test it on a spare tile first, or use a non sanded high performance product on those materials.

Epoxy grout

Epoxy grout is a resin and hardener system rather than a cement. It is effectively stain proof and waterproof, needs no sealing, and stands up to acids, which makes it the standard in commercial kitchens, on countertops, in steam showers and behind ranges. The trade offs are real: it costs several times more, it has a short working window, it is stiff and physically demanding to spread, and any haze left on the tile face sets permanently. Clean it as it goes, not at the end.

Thinset and the Trowel Notch

Thinset mortar coverage is set almost entirely by the size of the trowel notch, since the notch decides how thick a ridge of mortar is combed onto the substrate. A 50 lb bag of standard modified thinset covers about 95 square feet with a 1/4 in notch and about 60 square feet with a 1/2 in notch. Larger notches and back buttering push that lower still.

The notch is a starting point, not a rule. The real test is coverage on the back of the tile. Set a tile, pull it straight back up and look: residential floors want at least 80 percent mortar contact, and wet areas and exterior work want 95 percent with no voids at the corners. If the check fails, go up a notch size or back butter the tile.

Mixing and Pot Life

Mix grout and thinset with a paddle on a slow speed drill, around 300 rpm, to avoid whipping air into the mix. Add powder to water, not water to powder. Mix to a stiff peanut butter consistency, then let it slake, meaning stand undisturbed, for 5 to 10 minutes so the polymers hydrate fully. Remix briefly and do not add more water after slaking, because extra water weakens the cured strength and lightens the grout color unevenly.

Pot life for cement grout is roughly 1 to 2 hours depending on temperature, and for thinset it is typically 2 to 4 hours. Epoxy is far shorter, often 45 to 90 minutes from the moment the parts are combined, and it cannot be revived. Mix epoxy in small batches sized to what can be spread and cleaned in that window.

Consistent water measurement across batches is what keeps grout color consistent across a floor. Use the same container and the same fill line every time. Color variation between a wet batch and a dry batch shows up plainly once the floor dries, and it cannot be corrected without regrouting.

Sealing

Cement grout is porous and absorbs water, oil and dirt. Seal it once it has cured, which is generally 48 to 72 hours after grouting for a penetrating sealer, and check the manufacturer's stated cure time because it varies. A penetrating or impregnating sealer soaks in and leaves no film, which is the right choice for floors. Topical sealers form a surface layer that looks glossy and eventually wears in traffic patterns.

Reseal floor grout every 1 to 2 years in wet or high traffic areas, and every 3 to 5 years elsewhere. Test by dripping water on a joint: if it darkens quickly, the sealer has worn through. Epoxy grout needs no sealer at all.

Practical Points

• Buy grout from one lot number. Cement grout color varies between production lots, and a floor grouted from two lots can show a visible line.
• Do not grout movement joints. Perimeters, changes of plane such as a wall to floor corner, and any joint over a structural break get flexible sealant in a matching color, not grout.
• Clean with a barely damp sponge and rinse it constantly. A wet sponge pulls cement out of the joint and leaves a soft, pale, uneven surface.
• Haze comes off within 24 hours with a dry cloth or a haze remover. Left for a week it needs acid, which can damage the joints and some stone.