Grout Width Chart: Pick the Right Grout Joint Size

What does a 1/8-inch grout joint at 3.18 mm mean?

On the default — a 1/8-inch grout joint — the joint width is 3.18 mm and the class is standard. The width is the gap between two tiles that the grout fills, and 1/8 in is the standard grout joint for a home — the joint that hides small tile-size variation without swallowing the tile, strong enough to hold a floor tile without cracking the way a 1/16-in hairline joint does under foot traffic. The millimetres are the inches times 25.4: 0.125 × 25.4 = 3.18 mm. The classes are CLOSED at both ends: hairline 1/16 in only, standard 1/8–1/4 in, wide 5/16–1/2 in, rustic 5/8–1 in (grout JOINT WIDTH). The 1/4-in joint is a standard joint, the 1/2-in joint is a wide joint and the 1-in joint is a rustic joint. Widths below 1/16 in and above 1 in match NO standard class — too narrow for grout, or a stone joint sized by the tile maker.

Three things about the model are worth stating plainly.

First, the conversion is 0.125 × 25.4 = 3.18 mm. Second, the meaning is a 1/8-inch grout joint for most floor tile — the standard joint for a home, the joint that hides small tile-size variation without swallowing the tile. And third, the millimetre figure is displayed to two decimals, half-up, and a figure that displays as 3.18 mm can sit a hair below the line — 1/8 in gives 3.1750 mm, which displays as 3.18 mm — and one that displays as 4.76 mm can sit a hair below it — 3/16 in gives 4.7625 mm, which displays as 4.76 mm. The page prints the unrounded value so the rounding does not mislead you.

The worked example, line by line

Setup: grout joint 1/8 inch wide.

Line 1 — the millimetres. 0.125 × 25.4 = 3.18 mm (unrounded 3.1750 mm).

Line 2 — the class. 1/8 inch sits in the middle of the 1/8–1/4 in standard class, so the class is standard.

Line 3 — the check. At the 1/4-in row the width is 6.35 mm, above 5.00 mm; and at the 3/16-in row it is 4.76 mm — below 5.00 mm.

Every grout width in the table, checked

The table runs from 1/16 to 1 inch. Every width in the table sits inside one of the four classes: 1/16 in is a hairline joint, 1/8–1/4 in are standard joints, 5/16–1/2 in are wide joints and 5/8–1 in are rustic joints. The classes are closed at both ends — 1/4 in is a standard joint, 1/2 in is a wide joint and 1 in is a rustic joint. Widths below 1/16 in and above 1 in match NO standard class — too narrow for grout, or a stone joint sized by the tile maker. The table is a synthesis of published tile-setting guidance for household use — a 1/8-in joint for standard tile is the published standard, and the joint roughly doubles as the tile grows. Joint width varies by tile size, edge (rectified vs cushion) and substrate — size up one row for large-format or handmade tile, and always check the tile maker's own table for the exact product.

1/16 in: 1/16 in = 1.59 mm; hairline — The bottom of the table, a hairline joint.

1/8 in: 1/8 in = 3.18 mm; standard — The household standard, the middle of the standard zone.

3/16 in: 3/16 in = 4.76 mm; standard — A standard joint, one row above the household standard.

1/4 in: 1/4 in = 6.35 mm; standard — A standard joint, the top of the standard zone.

5/16 in: 5/16 in = 7.94 mm; wide — A wide joint, the bottom of the wide zone.

3/8 in: 3/8 in = 9.53 mm; wide — A wide joint, the middle of the wide zone.

Grout widths — 7/16 in, 1/2 in, 5/8 in, 3/4 in, 7/8 in, 1 in: 7/16 in: 11.11 mm — wide; 1/2 in: 12.70 mm — wide; 5/8 in: 15.88 mm — rustic; 3/4 in: 19.05 mm — rustic; 7/8 in: 22.23 mm — rustic; 1 in: 25.40 mm — rustic — One width from each of the wide and rustic zones: the rest of the table, wide turning rustic.

The reverses, worked backwards

Smallest table row whose width reaches 5.00 mm: At the displayed 1/4-in row the figures are 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard. At the 3/16-in row the figures are 3/16 in = 4.76 mm (unrounded 4.7625 mm); standard — below 5.00 mm — so the 1/4-in row is the smallest table row whose width reaches 5.00 mm.

Largest table row whose width STAYS at or below 5.00 mm: At the displayed 3/16-in row the figures are 3/16 in = 4.76 mm (unrounded 4.7625 mm); standard. At the 1/4-in row the figures are 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard — above 5.00 mm — so the 3/16-in row is the largest table row whose width stays at or below 5.00 mm.

Smallest table row that needs a wide joint: At the displayed 5/16-in row the figures are 5/16 in = 7.94 mm (unrounded 7.9375 mm); wide. At the 1/4-in row the figures are 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard — a standard joint — so the 5/16-in row is the smallest table row that needs a wide joint.

Smallest table row whose width reaches 10.00 mm: At the displayed 7/16-in row the figures are 7/16 in = 11.11 mm (unrounded 11.1125 mm); wide. At the 3/8-in row the figures are 3/8 in = 9.53 mm (unrounded 9.5250 mm); wide — below 10.00 mm — so the 7/16-in row is the smallest table row whose width reaches 10.00 mm.

Largest table row whose width STAYS at or below 10.00 mm: At the displayed 3/8-in row the figures are 3/8 in = 9.53 mm (unrounded 9.5250 mm); wide. At the 7/16-in row the figures are 7/16 in = 11.11 mm (unrounded 11.1125 mm); wide — above 10.00 mm — so the 3/8-in row is the largest table row whose width stays at or below 10.00 mm.

Smallest table row that needs a rustic joint: At the displayed 5/8-in row the figures are 5/8 in = 15.88 mm (unrounded 15.8750 mm); rustic. At the 1/2-in row the figures are 1/2 in = 12.70 mm (unrounded 12.7000 mm); wide — a wide joint — so the 5/8-in row is the smallest table row that needs a rustic joint.

A warning about the neighbours: every neighbour above sits one table row from the star, and every neighbour millimetre figure is a hair below or above the target even though it displays as the target — a one-row move changes the millimetres by more than a millimetre, but the unrounded figure still sits a hair from the round displayed figure, so the displayed two-decimal figure cannot tell the star from its neighbour. The page prints the unrounded four-decimal value in full so the rounding does not mislead you: 6.3500, 4.7625, 7.9375, 11.1125, 9.5250, 12.7000, 15.8750, 19.0500, 22.2250, 25.4000, 1.5875, 3.1750.

What the defaults assume

The defaults are a grout joint 1/8 inch wide — 1/8 in = 3.18 mm; standard. With 1/16 in, 1/16 in = 1.59 mm; hairline. With 1/8 in, 1/8 in = 3.18 mm; standard. With 3/16 in, 3/16 in = 4.76 mm; standard. With 1/4 in, 1/4 in = 6.35 mm; standard. With 5/16 in, 5/16 in = 7.94 mm; wide. With 3/8 in, 3/8 in = 9.53 mm; wide. With Grout widths — 7/16 in, 1/2 in, 5/8 in, 3/4 in, 7/8 in, 1 in, 7/16 in: 11.11 mm — wide; 1/2 in: 12.70 mm — wide; 5/8 in: 15.88 mm — rustic; 3/4 in: 19.05 mm — rustic; 7/8 in: 22.23 mm — rustic; 1 in: 25.40 mm — rustic. With Width 0 in, 0 in = 0.00 mm; matches no standard class — no width at all, and no matching class. With Width -0.5 in, 0 in = 0.00 mm; matches no standard class — a negative width is floored to 0.00 mm, never a negative number. With Width 1/8 in, 1/8 in = 3.18 mm (unrounded 3.1750 mm); standard — the millimetre figure displays as 3.18 mm. With Width 3/16 in, 3/16 in = 4.76 mm (unrounded 4.7625 mm); standard — the millimetre figure displays as 4.76 mm. With Width 1 in, 1 in = 25.40 mm (unrounded 25.4000 mm); rustic — the top of the table. With Width 0.30 in, 0.3 in = 7.94 mm; wide — between rows, so it sizes up to the 5/16-in row. With Width 1.5 in, 1.5 in = 25.40 mm; rustic — past the rustic class, so the core clamps to the 1-in row. With Width 0.05 in, 0.05 in = 1.59 mm; matches no standard class — below the hairline class, so it matches no standard class. With Width 1/16 in, 1/16 in = 1.59 mm (unrounded 1.5875 mm); hairline — the millimetre figure displays as 1.59 mm. With Width 5/16 in, 5/16 in = 7.94 mm (unrounded 7.9375 mm); wide — the millimetre figure displays as 7.94 mm. With Width 7/8 in, 7/8 in = 22.23 mm (unrounded 22.2250 mm); rustic — the millimetre figure displays as 22.23 mm. With Width 1/4 in, 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard — the millimetre figure displays as 6.35 mm.

A note on the rounding: the millimetre figure is displayed to two decimals, half-up. With a 1/8-in grout joint the unrounded figure is 3.1750 mm — it displays as 3.18 mm. With a 3/16-in joint the unrounded figure is 4.7625 mm — it displays as 4.76 mm. With a 1/16-in joint the unrounded figure is 1.5875 mm — it displays as 1.59 mm. With a 5/16-in joint the unrounded figure is 7.9375 mm — it displays as 7.94 mm. With a 7/8-in joint the unrounded figure is 22.2250 mm — it displays as 22.23 mm. With a 1/4-in joint the unrounded figure is 6.3500 mm — it displays as 6.35 mm. The page prints the unrounded value so the rounding does not mislead you.

Output · formula · meaning

Who uses this

• The homeowner tiling a bathroom floor: they learn a 1/8-inch grout joint is 3.18 mm — standard — and they check the 1/4-in row to see the smallest row whose width reaches 5.00 mm.
• The DIYer laying rectified tile: they learn below 1/8 in the joint is a hairline for rectified tile that shows every lippage — and they check the 5/16-in row to see the smallest row that needs a wide joint.
• The renovator laying large-format tile: they learn above 1/4 in the joint needs sanded grout and the tile needs a spacer — and they check the 7/16-in row to see the smallest row whose width reaches 10.00 mm.
• The student converting inches to millimetres: they learn the millimetres are the inches times 25.4 — and they check the 5/8-in row to see the smallest row that needs a rustic joint.

Accuracy & limitations

The math is first-principles: the millimetres are the width in inches times 25.4, and the class comes from the closed bands — hairline 1/16 in only, standard 1/8–1/4 in, wide 5/16–1/2 in, rustic 5/8–1 in (grout JOINT WIDTH) — all in inches and millimetres, the millimetre figure displayed to two decimals half-up. There is no division in this calculator — nothing is ever undefined. A negative width is floored to 0.00 mm, never a negative number. With a 1/8-in grout joint the unrounded figure is 3.1750 mm — it displays as 3.18 mm. With a 3/16-in joint the unrounded figure is 4.7625 mm — it displays as 4.76 mm. With a 1/16-in joint the unrounded figure is 1.5875 mm — it displays as 1.59 mm. With a 5/16-in joint the unrounded figure is 7.9375 mm — it displays as 7.94 mm. With a 7/8-in joint the unrounded figure is 22.2250 mm — it displays as 22.23 mm. With a 1/4-in joint the unrounded figure is 6.3500 mm — it displays as 6.35 mm. 1/8 in is the standard grout joint for a home — the joint that hides small tile-size variation without swallowing the tile, strong enough to hold a floor tile without cracking the way a 1/16-in hairline joint does under foot traffic; below 1/8 in the joint is a hairline for rectified tile that shows every lippage; above 1/4 in the joint needs sanded grout and the tile needs a spacer. 1/16 in is the hairline floor — below it there is no joint to grout, the tiles butt together; 1 in is the practical ceiling — above it the joint is a stone mason's job, sized by the tile maker, not a chart. This is why the standard zone runs 1/8–1/4 in while the hairline zone is a single row — the standard joint is the home's joint, and 1/8 in is its middle. What the model cannot do is pick the grout joint for you: it says nothing about the tile size, the edge or the substrate — a 1/8-in joint that suits a standard tile can crack under a large-format tile, and a handmade tile needs a wider joint than the chart says. Size up one row for large-format or handmade tile, and always check the tile maker's own table for the exact product. Use it to check the size before you buy — then check the tile-size, caulk-coverage and paint-coverage charts before you set the rest of the job.

No grout-width page of any kind exists on the site — this is the first. The closest pages are the tile size chart (TILE sizes — how big the tiles are, not how wide the joint between them is) and the drywall size chart (drywall sheet sizes and thickness — not grout at all). This one is the GROUT JOINT width's own 12-row width → millimetres reference (1/16–1 in, inches and millimetres) with its four classes (hairline 1/16 in, standard 1/8–1/4 in, wide 5/16–1/2 in, rustic 5/8–1 in) and class logic.

Frequently asked questions