Grout Calculator: How Much Grout for Your Tile

Grout is a volume problem, not an area problem. Two floors of the same size can need triple the grout if one is mosaic and the other is large-format.

Your tile and joints

Area

Use the net tiled area from your tile calculator.

Joint geometry

This is the tile thickness. Measure the edge of a tile — it is rarely what the box says.

Product

Cementitious grout is roughly 100–115. Epoxy is lighter per unit volume.

What stays in the bucket, on the float and in the sponge. 20% is not pessimistic.

What to buy

  • Bags to buy — bagsCross-check against the coverage chart on your bag before you buy — see the note below.
  • Grout needed — lb
  • Joint as a share of the surface — %
  • Joint volume per square foot — cu in
  • Grout by geometry alone — lbBefore waste. Nobody achieves this in practice.
  • Coverage you are assuming — sq ft per bagCompare this with the manufacturer's chart. If theirs is much lower, use theirs.

Next step in your project

Grout is a volume problem

Almost every grout calculator asks for your floor area and gives you a bag count. That is the wrong shape of question, and it is why people end up either with eleven spare bags or halfway across a floor at closing time.

Grout fills joints. The amount you need depends on how much joint there is, and two floors of identical area can differ by a factor of ten. A square foot of 12 × 24 porcelain with 1/8 inch joints is about 1.5% joint by area. A square foot of 1 inch mosaic with the same joint is over 20%. Same room, same tile thickness, fifteen times the grout.

That is why this calculator asks for the tile size and the joint depth, and why it shows you the joint percentage in the results. Once you can see that number, the bag count stops being surprising.

The arithmetic, in full

Nothing here is hidden:

module area   = (tile width + joint) × (tile length + joint)
joint fraction = 1 − (tile face area ÷ module area)
volume per sq ft (cu in) = 144 × joint depth × joint fraction
total volume  = volume per sq ft × area
weight (lb)   = total volume (cu in) × density (lb/cu ft) ÷ 1728

Joint depth is the tile thickness — the joint is filled from the substrate to the surface. Measure the edge of an actual tile rather than trusting the box; nominal thicknesses and real ones drift apart, especially with porcelain.

Read this before you buy anything

This is a geometric calculation, and the manufacturer's chart is more conservative. When they disagree, buy to the chart.

The geometry above is correct: it tells you how much grout ends up in your joints. What it does not include is everything that never gets there — what stays in the bucket, what the float pushes off the edge, what the sponge lifts back out, and the batch you mixed too stiff. Manufacturers' coverage charts fold all of that in, plus a margin, because they would much rather you over-bought.

So the results here include a waste allowance, defaulting to 20%, and they show you the implied coverage in square feet per bag. Take that number to the bag in your hand. If the chart on the back says fewer square feet than the calculator implies, the chart wins. Use this page to understand why your particular job needs what it needs, and use their number to decide what goes in the cart.

That is a less satisfying answer than a confident bag count. It is also the true one.

Sanded or unsanded

The traditional rule is straightforward: sanded grout for joints 1/8 inch and wider, unsanded for anything narrower.

The reasoning behind it still holds. Unsanded grout is essentially cement and pigment, and in a wide joint it shrinks as it cures and cracks. Sanded grout has aggregate that resists that shrinkage — but the same aggregate makes it impossible to pack properly into a very narrow joint, and it can scratch soft polished surfaces like marble and some glass.

Modern high-performance grouts blur this. Many are rated across the whole range from 1/16 up to 1/2 inch, and some are single-component products that behave differently again. Read what the bag says before you apply a rule of thumb from 1995.

Colour, and the water you cannot see

Cementitious grout colour is decided by the water ratio, and the difference between a batch mixed to a stiff paste and one mixed slightly loose is visible on the finished floor as a lighter patch.

Three habits that avoid it:

  1. Measure the water, do not judge it. A jug and a note of the ratio beats experience here.
  2. Mix whole bags where you can. Partial bags are where ratios drift, and if you must split one, weigh the powder.
  3. Let it slake. Most grouts want five to ten minutes of rest after mixing, then a quick remix. Skipping it changes both the working properties and the final colour.

Then there is efflorescence — the white haze that appears days later when water carries salts to the surface. It is a moisture problem more than a grout problem, and it is far more common on floors where the substrate was damp when the grout went in.

Epoxy is a different product

If you are using an epoxy grout, do not use this calculator's weight figure. Epoxy comes in fixed kits, is sold by volume rather than by the bag, has its own coverage tables and a pot life measured in tens of minutes.

It is also considerably harder to work with, considerably more expensive, and genuinely worth it in a commercial kitchen or a shower floor where staining is the enemy. For a typical residential wall or floor, a good cementitious grout with a decent sealer is the usual answer.

A worked example

A 120 square foot floor, 12 × 24 porcelain, 1/8 inch joints, 5/16 inch thick, 25 lb bags.

  • Module: 12.125 × 24.125 = 292.5 sq in, against a tile face of 288 sq in
  • Joint fraction: 1 − (288 ÷ 292.5) = 1.54%
  • Volume per square foot: 144 × 0.3125 × 0.0154 = 0.69 cu in
  • Total: 0.69 × 120 = 83 cu in
  • Weight at 105 lb/cu ft: 83 × 105 ÷ 1728 = 5.1 lb
  • Plus 20% waste: 6.1 lb — comfortably one bag

Now change nothing but the tile, to 2 inch mosaic:

  • Module: 2.125 × 2.125 = 4.52 sq in, against a face of 4 sq in
  • Joint fraction: 11.5%
  • Weight, same floor, same joints: around 38 lb before waste

Same room. Same 120 square feet. Seven times the grout, and the difference between one bag and three.

What this calculator assumes

  • This is a geometric calculation, not a manufacturer specification. It works out the actual volume of the joints and converts it to weight using the density you entered.
  • Manufacturers' coverage charts are usually more conservative than the geometry, because they build in mixing losses, uneven joint depth and the grout that never leaves the bucket. If the chart on your bag gives a lower coverage than the figure shown here, trust the chart.
  • Joint depth is assumed to equal the tile thickness. Where thinset has squeezed up into the joint — which happens constantly with mosaic — real depth is less and you will use less grout.
  • The 20% default waste covers the float, the sponge, the bucket and the batch you mix too stiff. On a small job the percentage effectively rises, because a partial bag is still a whole bag.
  • Epoxy grout behaves differently: it is sold by volume in fixed kits and has its own coverage tables. Use those, not this.

Common questions

Why does mosaic need so much more grout?

Because grout volume follows the total length of joint, and small tiles have an enormous amount of it. A square foot of 12×24 tile with 1/8 inch joints is around 2% joint. A square foot of 1 inch mosaic with the same joint is over 20%. Same floor area, ten times the grout — which is exactly why the joint percentage is shown in the results.

Should I buy sanded or unsanded grout?

The traditional rule is sanded for joints of 1/8 inch and wider, unsanded for narrower, because unsanded grout shrinks and cracks in a wide joint while sanded grout will not pack into a narrow one. Modern high-performance grouts blur the line and many are rated across the whole range — check what the bag says before applying the old rule.

My bag's chart says far fewer square feet than this calculator. Who is right?

The bag. The geometry here is correct, but it describes the grout that ends up in the joints, not the grout you buy. The manufacturer's chart includes everything you lose along the way, and it is written by people who would rather you over-bought than ran out. Use this to understand why your job needs what it needs, and use their chart to decide what to put in the cart.

Can I mix a partial bag?

You can, but weigh both the powder and the water rather than eyeballing it. Grout colour is sensitive to the water ratio, and a batch mixed wetter than the last one will dry visibly lighter. On a floor that is the kind of mistake you look at every day.

Where these numbers come from

  • The joint fraction and volume calculation is straightforward geometry, shown in full on this page so you can check it.
  • Density of mixed cementitious grout varies by product; the 105 lb/cu ft default is a mid-range figure and is editable for that reason.
  • Manufacturer coverage charts remain the authority for purchasing decisions.

Last checked 2026-08-11.