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.
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. Other products differ — check the box.
Common questions
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 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.
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?
No. These figures are for one cement-based product, cited below with its data sheet. Epoxy grouts have quite different coverage and are usually sold by a different unit entirely. Read the coverage printed on whatever you actually buy — it always beats a calculator, including this one.
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.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures