Grout fills joints, not floors
Area is the input everyone reaches for and it is the least important
variable here. What actually consumes grout is the total length of joint,
and that is driven by how small the tiles are.
One 17 lb box, at a 1/8 in joint:
2 × 2 in mosaic 96 ft²
6 × 6 in 266 ft²
12 × 12 in 348 ft²
24 × 24 in 688 ft²
Same area. Seven times the grout at the small end.
Why there is no formula here
You can derive joint length from tile size easily enough. What you cannot
derive without the manufacturer is how much grout fills a given length of
joint at a given depth, and the depth is the tile thickness — which is
exactly the term the popular formulas drop. So this reads the published
chart instead, and the sizes and widths on offer are the ones the chart
contains rather than anything typed in.
Common mistakes
- Scaling from a different tile size. Coverage is not linear in area — it is driven by joint length.
- Ignoring thickness. A 3/8 in tile has a joint half again as deep as a 1/4 in one.
- Buying one box exactly. A second box from a later batch can shade differently.
- Using a cement chart for epoxy grout. Different product, different figures entirely — switch the grout at the top and the chart switches with it.
Before the grout
The tile calculator gives the
tile count and boxes with the pattern waste counted, and the
square footage calculator
works out the area to feed both.
Square feet covered by one 17 lb box
CUSTOM Prism Ultimate Performance Grout, per the published data sheet. Switch the grout in the calculator for MAPEI Kerapoxy CQ, which is sold by the gallon and covers rather less. Other products differ again — check the box.
Which grout?
The guide to cement grout or
epoxy puts both published charts side by side, and explains the joint
width at which one of the two stops being an option at all.
The rest of the job
Grout follows tile. The
tile calculator works out the
tiles and boxes and hands the area straight over, the
square footage calculator
handles a floor that is not a plain rectangle, and the
drywall calculator covers the
board behind a tiled wall. Painting the rest of the room? The
paint calculator takes the same
dimensions.
Common questions
My tile size is not on the chart. What now?
Switch to "My own size" and enter the width, length and thickness. The chart stops at fifteen sizes and fixes a thickness to each, which left anyone laying a 4 by 16 subway tile or a 10 mm porcelain with nowhere to go. Rather than invent a coverage formula, the calculator scales the nearest published row by exact geometry: grout volume follows joint length, joint width and joint depth together, and only the first and last of those differ from the anchor row. Scaling any published row to any other reproduces the printed figure to within about two percent. The ticket says which row it came from, because a scaled figure and a published one should not look alike — and you should buy an extra box.
How much grout do I need for 100 square feet of tile?
It depends far more on the tile than on the area. A hundred square feet of 12 by 12 inch tile with a 1/8 inch joint takes a single 17 pound box, because one box covers 348 square feet at that size. The same hundred square feet in 2 by 2 inch mosaic with a 1/4 inch joint takes two boxes, because coverage collapses to 54 square feet. Small tiles mean more joints, and grout fills joints, not floors.
Why does this ask me to pick a tile size instead of typing one?
Because grout coverage is a published chart, not a formula. Quantity depends on the total linear footage of joint, which is a function of tile size, joint width and tile thickness together — and almost every calculation circulating online drops the thickness. Choosing from the chart means the number you get is one the manufacturer printed, rather than one this page interpolated between two of theirs.
Does tile thickness really change the grout?
Yes. The joint is as deep as the tile is thick, so a 3/8 inch tile holds half again as much grout in the same joint as a 1/4 inch one at the same face size. That is why the chart fixes a thickness for each row — 1/4 inch up to 6 inch tiles, 3/8 inch above — and why changing it would invalidate the figure.
What joint width should I use?
That is a tile question rather than a grout one, and it is largely set by the tile itself: rectified porcelain can take a 1/16 inch joint, while a handmade or variable-sized tile needs a wider one to absorb the variation. The cement chart publishes six widths from 1/16 to 1/2 inch and the difference between the narrowest and widest is a factor of four in grout. The epoxy chart stops at 3/8 inch, which is the widest joint that product is rated not to sag in, so those two widths disappear from the list when you pick it.
Should I buy an extra box?
If your figure lands anywhere near a box boundary, yes. Grout is tinted per batch and a box bought a week later can shade differently — on a floor that is a visible line. An unopened box goes back; a mismatched batch does not come out.
Does this work for epoxy grout?
Yes — pick MAPEI Kerapoxy CQ from the Grout list and the whole chart changes with it, including which tile sizes and joint widths are on offer, because the two manufacturers do not publish the same ones. It is worth doing rather than assuming the figures are close: at a 1/8 inch joint under 12 by 12 inch tile, one 17 pound box of the cement grout covers 348 square feet and one gallon kit of the epoxy covers 156. Three hundred square feet of floor is a single box either way you look at it — or two kits. Both figures come from the manufacturers' own data sheets, cited below.
Why does the tile size list change when I change the grout?
Because each manufacturer publishes their own chart and they do not cover the same ground. The cement chart runs to fifteen sizes at two thicknesses and six joint widths up to half an inch; the epoxy chart has fifteen rows too but stops at a 3/8 inch joint, which is also the widest joint that product is rated not to sag in. The epoxy chart is the more interesting of the two on thickness — it lists 4 by 8 inch tile four separate times at 1/2, 3/4, 1-1/8 and 1-3/8 inches, and the coverage falls from 139 square feet to 51. That is the clearest published proof that what fills a joint is its depth, and the reason this calculator will not let you type a size the chart does not have without saying so.
Sources
- Default waste allowance
- 0%
A published chart, not a derived formula. Grout quantity depends on the total linear footage of joint, which is a function of tile size, joint width and tile thickness together — the implementation plan explicitly forbids deriving it and requires a manufacturer chart. The thickness is not a free variable here: it is fixed per row by the chart, 1/4 in for tiles up to 6 in and 3/8 in above, and changing it would invalidate the figure. Sizes and joint widths are therefore offered as a choice from the chart rather than as free numbers, which is how the manufacturer intends it to be read. Other grout products differ; check the box.
- Default waste allowance
- 0%
A second published chart, alongside the cement-based one, because the audit asked for a brand people can actually check against and epoxy is where the two products diverge most: at a 1/8 in joint under 12 x 12 in tile the cement grout covers 348 ft2 per 17 lb box and this covers 156 ft2 per gallon kit. Sold by volume rather than by weight, so no weight is reported. Thickness is fixed per row by the manufacturer exactly as in the cement chart, and this one publishes several thicknesses at the same face size — 4 x 8 in appears at 1/2, 3/4, 1-1/8 and 1-3/8 in, which is the clearest published demonstration that joint depth is what drives grout. Four joint widths rather than six: the chart stops at 3/8 in, which is also the widest joint this product is rated non-sagging in.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures
