Caulk vs. Grout: Where to Use Each

Prepared and maintained by Murat Yilmaz · Calculations checked against the cited sources and reference tests · Updated September 7, 2026

A tiled room holds two different kinds of joint, and they fail for opposite reasons. Cement grout is a rigid filler that locks tiles into one continuous, washable plane. Flexible sealant, which almost everyone calls caulk, is a soft bead that stretches and squeezes while the two surfaces beside it drift apart and come back. Put grout where the building moves and it cracks within a season. Run sealant across an open tile field instead of grout and it collects grime, sags, and never matches the rest of the floor.

The choice has nothing to do with brand, color, or price. It comes down to one question: are both edges of this joint bonded to the same piece of the building? If the answer is yes, the joint is a grout joint. If the answer is no, it is a movement joint and it takes sealant. This page maps those joints for a normal bathroom, lists the published dimension rules from the tile standards, and runs the arithmetic for a real room so you can see how little of each material a small job actually consumes.

What each material is doing in the joint

Cement grout is portland cement, graded sand, and polymer additives. Once it cures it behaves like a narrow strip of mortar: hard, scrubbable, dimensionally stable, and almost completely unable to stretch. That rigidity is the point. It keeps tile edges from chipping against each other and it gives the floor one surface that a mop can cross.

Sealant is a cured elastomer, usually silicone or polyurethane, sold in North America against ASTM C920. The label does not really describe a color, it describes a movement budget: the class number on the cartridge states how far the cured bead can stretch and compress as a percentage of the joint it was tooled into. The Tile Council of North America states the boundary plainly, noting that anywhere there is a change in substrate or backing surface, such as the joint between the floor and the wall, caulk should be used in place of grout because those surfaces move independently of each other.

The joint map for a tiled bathroom

Walk the room once and label every joint before you open anything. The perimeter where the floor meets the wall, the inside corner behind the tub, the line where tile meets a threshold or a different flooring, and the seam where a wall panel meets the ceiling are all movement joints. Everything inside the tile field, where one continuous substrate carries both tiles, is a grout joint. If you are still sizing that field, the tile calculator converts room dimensions into boxes, and the grout calculator turns joint length, width, and depth into kilograms.

The 1/8 inch product-selection line in the TCNA grout FAQ helps distinguish unsanded from sanded grout, so measure the field joint before choosing a bag. Movement-joint dimensions are different: TCNA explicitly says its public FAQ cannot prescribe exact location or frequency because site conditions control them. Use the current EJ171 detail and the selected sealant manufacturer's instructions rather than copying a perimeter number from a secondary summary.

Reading the sealant label before you buy

A cartridge that says only bathroom on the front tells you nothing measurable. The ASTM C920 designation does. It is written as four parts, and each part answers a question you already have while standing in the aisle.

For a tub surround or a floor perimeter, a nonsag single component sealant in a mid or high movement class is the ordinary answer. A traffic rated product matters wherever the bead sits in the walking surface rather than up a wall.

Replacing a failed tub joint step by step

1. Cut the old bead out completely along both faces with a sharp blade, then lift it away in one piece if you can. A partial removal leaves a strip of dead sealant that the new bead cannot bond through, which is the most common reason a repair fails again inside a year.
2. Scrape the residue off both faces with a plastic blade or a purpose made joint scraper. Metal blades leave scratches in acrylic tubs and in glaze, and those scratches hold soap film that will keep the next bead from wetting the surface properly.
3. Clean and dry the joint. Wipe both faces with denatured alcohol, then leave the joint open for several hours so any water trapped behind the tub flange can evaporate. The same discipline that the paint prep checklist applies before a first coat applies here, because adhesion is decided before anything is applied.
4. Fill the tub with water before you tool the bead if the tub is acrylic or pressed steel. A loaded tub sits lower, so the joint is at its widest while you seal it, and the cured bead is then compressed rather than stretched every time the tub is used.
5. Cut the nozzle to the joint detailed for the project, run one continuous bead in a single pass, and tool it immediately with the profile required by the sealant manufacturer. Backing material, depth, and adhesion surfaces must match that product data rather than a generic ratio copied from another system.
6. Leave the joint dry for the water exposure time printed on the cartridge before anyone showers. That number varies by chemistry and by humidity, so read the tube rather than assuming, and drain the tub slowly the first time.

A worked example for a 3.0 by 2.4 meter bathroom

Take a floor that measures 3.0 m by 2.4 m, tiled in 300 mm squares with a 3 mm joint that is 8 mm deep. Along the 3.0 m direction there are 10 tiles and therefore 9 internal joints, each 2.4 m long, which is 21.6 m. Along the 2.4 m direction there are 8 tiles and 7 internal joints, each 3.0 m long, which is 21.0 m. Total grout joint length is 42.6 m. Measure the room the same way you would for a finish schedule, and the walkthrough in how to measure a room for paint transfers directly.

Grout mass is joint length times joint width times joint depth times density, plus a waste allowance. At a density of 1600 kg per cubic meter and a 10 percent allowance the arithmetic runs 42.6 by 0.003 by 0.008 by 1600, which gives 1.636 kg, and 1.10 times that is 1.80 kg. One small bag covers it. Note that the 3 mm joint sits just under the 1/8 inch threshold in the table above, so this floor takes unsanded grout rather than sanded. The grout calculator runs the same formula, and the waste factor calculator lets you test what a 15 percent allowance would add.

The sealant side is much smaller than most people expect. The floor perimeter is 2 times 3.0 plus 2 times 2.4, which is 10.8 m. A bead 6 mm wide and 3 mm deep has a cross-section of 18 square millimeters, so 10.8 m of joint holds 10800 by 18, or 194400 cubic millimeters, which is about 194 milliliters. One standard cartridge covers that whole perimeter with margin left for the tub corner. Every other estimator on the site is listed on the tools page if you need to size the rest of the job.

Mistakes that put grout where sealant belongs

• Grouting the wall to floor corner because it is easier to keep one color across the room. That corner is a change of plane, it moves, and the grout in it will crack and then let water into the wall cavity.
• Sealing over an old bead instead of removing it. New sealant bonds to the old surface film rather than to the tile and the tub, so the repair looks right for a few weeks and then peels away as one long strip.
• Choosing sanded or unsanded by habit rather than by measurement. A 3 mm joint filled with sanded grout will not compact properly, and a 6 mm joint filled with unsanded grout is far more likely to shrink and crack as it cures.
• Sizing or tooling a bead without the selected sealant's joint-design instructions. Depth, backing material, and bond surfaces work as a system, so a generic ratio cannot replace the product data and current EJ171 detail.
• Painting trim next to a fresh bead before it has cured. Wait until the sealant has skinned over and cured, then plan the coats the way how many coats of paint you need describes, so you are not brushing wet paint into a soft joint.

Questions about caulk and grout

Can I use sealant for every joint in a shower?

You can, but you should not. Sealant across an open tile field stays slightly soft, so it holds soap film and body oils that no amount of scrubbing fully removes, and it discolors long before the grout around it would. It also shifts under foot traffic on a floor. Keep sealant to the corners, perimeters, and transitions where movement actually happens, and let grout do the flat work.

Why does the corner keep cracking even after I regrout it?

Because regrouting treats a symptom. A corner where a wall meets a floor joins two assemblies that expand, contract, and deflect at different rates, so any rigid filler placed in that corner is loaded in tension every time the building moves. The crack returns because grout has effectively no movement capability. Cut the grout out of that corner, clean both faces, and fill it with a sealant rated for movement instead.

How do I know whether my joint takes sanded or unsanded grout?

Measure it with a rule rather than estimating by eye. The Tile Council of North America places the line at 1/8 inch: joints smaller than 1/8 inch should generally be filled with unsanded grout, and joints of 1/8 inch and larger should be filled with sanded grout. Above roughly 3/8 inch a more heavily sanded product is normally required so the joint does not shrink back as it cures.

Does the color ever match between the two materials?

Manufacturers sell sealant in colors keyed to their own grout lines, and the match is usually close enough that nobody notices from standing height. It is rarely exact, because a cured elastomer reflects light differently than a cement surface does. Buy both from the same brand and the same color name, and tool the bead cleanly, because a ragged edge draws far more attention than a slight shade difference.

How far apart do movement joints need to be in a larger floor?

TCNA's official placement FAQ says it cannot specify an exact location or frequency because site conditions must be addressed. It notes that a perimeter gap is often sufficient in a small room, while moisture, direct sunlight, and exterior temperature range can require more frequent movement joints. Confirm the current EJ171 detail, project specification, and sealant manufacturer instructions; do not reuse a fixed spacing from an undated web summary.

Sources

• Tile Council of North America, Grout FAQ. Source of the 1/8 inch sanded and unsanded threshold, the 3/8 inch heavily sanded note, and the statement that caulk belongs anywhere the substrate or backing surface changes.
• Tile Council of North America, movement-joint placement FAQ. The standards owner explains why movement joints are needed, why exact location and frequency are site-specific, why a perimeter gap can be sufficient in a small room, and why heat or moisture may require closer placement.
• ASTM C920-18, Standard Specification for Elastomeric Joint Sealants. Source of the type, grade, class, and use designation system, including movement classes 12.5, 25, 35, 50, and 100/50.