How Much Grout Do I Need?

100 sq ft of 12×12 tile at a ⅛" joint takes 4.5 lb. The same 100 sq ft in 2-inch mosaic takes 27 — because grout fills joints, not floors.

Grout is about joints, not floors

100 sq ft of 12×12 inch tile, ¼" thick, with ⅛" joints: 4.5 lb of grout.

The same 100 sq ft in 2×2 inch mosaic: 27.1 lb.

Six times as much, same room, same joint width. Because grout fills the gaps between tiles, and small tiles have far more gap.

Buying grout "for a bathroom" without knowing the tile size is how people end up back at the shop.

Your details

Area to grout(sq ft)

Tile length(in)

Tile width(in)

Joint width(in)

1/16 = 0.0625, 1/8 = 0.125, 3/16 = 0.1875, 1/4 = 0.25.

Tile thickness(in)

This is the joint DEPTH. Measure the edge of a real tile — wall tile is often 1/4, porcelain floor 3/8.

Bag size(lb)

Result

Bags to buy

Rounded up. Buy one more than this — see the note about colour matching.

Grout neededPounds, for the whole area.: 4.5
Per 100 sq ftPounds. This is the figure published coverage tables quote, so you can check it against your bag.: 4.5
If the tile were half that sizePounds. Roughly DOUBLE — the tile size drives this far more than the room does.: 9
If the joint were twice as widePounds. Exactly double — joint width scales linearly.: 9
Joint to fillLinear feet of joint. This is what you are actually filling, and why small tiles are so much more work.: 200
Sanded grout needed?1 = yes, use sanded. Unsanded grout is for joints under 1/8 in; it shrinks and cracks in a wide joint.: 1

Open the Grout Calculator on its own page to bookmark or share it.

The formula, and why it's (L + W)

lb per sq ft = (L + W) × joint × thickness ÷ (L × W) × 144 ÷ 1728 × 104

All dimensions in inches. Since 144 ÷ 1728 is exactly 1/12, that reduces to ÷ 12 × 104.

144 converts square inches to square feet
1728 converts cubic inches to cubic feet
104 lb/cu ft is the material density of sanded grout

The interesting bit is why it's (L + W) rather than the tile's full perimeter.

Each tile shares every joint with the tile next to it. So a tile only owns half of each of its four edges — half of two lengths plus half of two widths, which is exactly one length plus one width.

I checked it three times before publishing it

A formula that looks right is not a formula that is right, so this one was tested against published coverage tables it wasn't derived from:

Case	Published	Calculator
12×12, ¼" thick, ⅛" joint	4.5 lb / 100 sq ft	4.5
12×12, ¼" thick, ⅜" joint	~13 lb / 100 sq ft	13.5
3×6 subway, 1/16" joint	~7.0 lb / 100 sq ft	6.8

Agreement at three points, across two tile shapes and three joint widths. That's reasonable evidence the model is right rather than tuned to one example.

(The first version of this calculator was wrong by a factor of 12 — I'd dropped the square-inches-to-square-feet conversion. It gave 0.03 lb where the table said 4.5. Checking against a published number is what caught it, which is rather the point of doing it.)

Halve the tile, double the grout

Because the term is (L + W) ÷ (L × W), shrinking the tile has an outsized effect.

The calculator shows if the tile were half that size right next to your figure: at the default, 4.5 becomes 9.0. Exactly double.

Another way to see it — linear feet of joint per 100 sq ft:

12×12 tile: 200 ft
2×2 mosaic: 1,200 ft

Six times the joint to fill. Which also explains why mosaic takes so much longer to grout, not just so much more grout.

Joint width scales exactly linearly

Double the joint, double the grout. Precisely.

That's easy to overlook, because the difference between a ⅛" and a ¼" spacer looks like nothing when you're holding one. The calculator shows the doubled-joint figure beside yours so it's visible rather than theoretical.

Worth knowing before you change spacers halfway through a job because the tiles weren't quite square.

The input everyone guesses: thickness

Tile thickness is the joint depth. It's what you're filling.

Wall tile is frequently ¼"
Porcelain floor tile is often ⅜"

That's a 50% difference in grout with everything else identical — and it's the field most people leave at whatever the default was.

Measure the edge of an actual tile. Especially with natural stone, which varies within a single box.

Sanded or unsanded isn't a preference

It's decided by the joint width:

Under ⅛" → unsanded
⅛" and wider → sanded

The reason is shrinkage. Unsanded grout has nothing in it to hold volume, so in a wide joint it shrinks back and cracks. The calculator flags which one your joint calls for.

The 104 lb/cu ft density used here is for sanded grout. Unsanded is close enough for estimating, but check what your bag claims.

Buy a bag more than the arithmetic

Two reasons, and the second is the important one.

Waste: coverage figures assume perfectly packed joints and no losses. A real job leaves grout in the bucket, in the bag, and on the float.

Colour: grout colour depends on how much water went into the mix and how it cured. A second batch mixed a week later can be visibly different — and a patch that doesn't match is permanent without regrouting the whole area.

One spare bag is a great deal cheaper than that. And buy every bag from the same batch number while you're at it.

On the day

Thinset fully cured — check the bag, usually 24 hours
Every spacer removed
Joints raked clean of thinset — grout needs the depth
Water measured, not eyeballed. It sets the colour
Mixed, slaked for the stated time, then remixed
Worked diagonally across the joints with the float
Haze removed at the right moment, not the next morning