Grout Calculator
Find out how many pounds (or kilograms) of grout to buy for your tile job.
Tile, joint, and area
Results
For 100 sq ft of 12 × 12 in tile with 0.13 in joints, 0.38 in deep, you'll need about 8 lb of dry grout (including 10% waste).
Dry grout needed
8 lb
That's a small amount — a backsplash, a bathroom floor, or a tub surround.
One small box (8–10 lb) covers it. Mix only what you can place and tool in about 30–45 minutes — small jobs fail by mixing the whole box at once, not by running out.
Raw computed amount: 7.60 lb
Joint volume (before waste): 1.8 L (0.07 cu ft)
How this works
The calculator uses the joint-volume method that grout manufacturers use to build their own coverage charts. Step one computes the length of grout joint per unit of tiled area: (tile length + tile width) ÷ (tile length × tile width). Each tile contributes half of the joint on each of its four sides, and this expression is exactly that shared-perimeter length normalized by tile area — it's why small and elongated tiles need disproportionately more grout. Step two multiplies by joint width and joint depth (use your tile's thickness) to convert joint length into joint volume, and by your total tiled area to get the whole job's volume. Step three converts volume to purchasable weight using the dry grout's effective placed density — defaulted to 1,700 kg/m³, the mid-range figure obtained by back-computing major manufacturers' published coverage tables for sanded cementitious grout, and adjustable in the advanced field for unsanded (slightly lighter) or epoxy (heavier) products. A fixed, disclosed 10% waste factor covers mixing loss, overfill, and cleanup haze, which manufacturer charts typically exclude. The result is shown in pounds or kilograms; divide by your product's bag size (commonly 8, 10, or 25 lb) and round up to whole bags. Thinset mortar, sealer, and movement-joint sealant are separate products outside this model. The unit toggle switches all inputs and outputs between imperial and metric; internally every calculation runs in canonical SI units, so switching never introduces rounding drift Joint-volume coverage methodology: Custom Building Products grout coverage calculator (Polyblend), consistent with Mapei's published grout coverage method.
Worked example
A 100 sq ft floor of 12×12-in ceramic tile, 3/8 in thick, with 1/8-in joints: joint length per square foot is (12 + 12) ÷ (12 × 12) = 0.167 linear units per unit area — about 2 ft of joint per square foot of floor. The joint network's volume works out to about 0.00184 m³ (0.065 cu ft), and at the default sanded-grout density of 1,700 kg/m³ plus 10% waste, that's about 3.4 kg — roughly 7.6 lb of dry grout. One 8- or 10-lb box covers the job with a sensible margin.
A second example: the same floor in 2×2-in mosaic
The same 100 sq ft with 2×2-in mosaic chips of the same 3/8-in thickness and the same 1/8-in joints: joint length per square foot jumps to (2 + 2) ÷ (2 × 2) = 1.0 per inch — six times the 12×12 tile's figure. Grout needed scales identically: about 20.7 kg ≈ 45.6 lb, versus 7.6 lb for the 12-in tile. That's two 25-lb bags instead of one small box, for the identical floor area. Tile size, not floor size, is the variable that dominates a grout estimate — always recompute when the tile changes.
Common mistakes
Guessing joint depth instead of using tile thickness
Entering a token 1/8-in depth for a 3/8-in-thick floor tile cuts the estimate to a third of the real need. The joint is as deep as the tile is thick — measure the tile edge or read the box, and enter that.
Using a large-format coverage figure for mosaics
Grout demand is dominated by tile size. A chart line or a remembered per-bag coverage from a 12×12 job is off by a factor of four to six for a 2×2 mosaic. Always recompute when tile size changes — it's the strongest variable in the formula.
Buying exactly the chart amount with no waste
Coverage charts describe joint volume, not job reality: some grout cures in the bucket, some overfills and gets tooled off, some washes away as haze. This calculator adds a disclosed 10% for that; buying to the bare chart figure is the classic way to run out with one wall left.
Mixing the whole batch at once
Cementitious grout has a working time of 30–60 minutes after mixing. Mixing all 20 lb for a big floor guarantees the last third stiffens in the bucket and gets thrown away (and re-tempering with extra water weakens and lightens it). Mix in batches you can place and tool within the working time.
Adding water to revive stiffening grout
Extra water beyond the specified ratio weakens cured grout, causes shrinkage cracking, and lightens the color unevenly. If a batch stiffens, discard it and mix fresh — the quantity buffer in this estimate exists partly so you can afford to.
Grouting too soon after setting tile
Most thinsets want 24 hours (check the bag) before grouting; trapping moisture early can weaken the bond and cause efflorescence in the joints. This doesn't change quantity, but it's the most common sequencing error on first tile jobs.
Ignoring lot numbers across multiple bags
Pigment varies slightly between production lots. On a multi-bag job, buy matching lots and dry-blend bags before mixing — banding between areas grouted from different lots is visible and permanent.
Sealing (or not sealing) at the wrong time
Cementitious grout should cure 48–72 hours before sealing, and sealer is a separate product not included in this weight estimate. Epoxy grout needs no sealer at all. Budget sealer by joint area for cement grout in wet areas — an item people discover at the end of the job.
When this calculator's model stops applying
- Mosaic sheets with mixed chip sizes: enter the individual chip dimensions, not the 12×12 sheet size — the formula needs the size of the actual grouted unit. For blended mosaics, use the smallest common chip size for a conservative estimate.
- Deeply structured or cushioned-edge tiles: bevelled or cushioned edges enlarge the visible joint at the surface beyond the spacer width. For heavily cushioned tile, add roughly one bevel-width to the joint width you enter, or the joints will run hungrier than computed.
- Ungauged natural stone and terracotta of varying thickness: joint depth varies piece to piece. Enter the maximum thickness — the overestimate is the safe direction, and porous stone also loses more grout to surface texture during cleanup.
- Very wide joints (over ~1/2 in) with brick pavers or saltillo: the linear joint-volume model still holds geometrically, but products change — you may need a sanded grout rated for wide joints or a mortar-type joint filler with its own density; adjust the density field to the product's data sheet.
- Epoxy and urethane grouts: geometry identical, density and packaging different (see the epoxy FAQ). Use the density field, and round up to whole kits rather than whole pounds.
- Tile patterns with unequal joint networks (herringbone with tight side joints and wider ends, deliberately variable 'rustic' joints): the single joint-width input models a uniform grid. Enter the average joint width, and expect a few percent of extra uncertainty.
- Grouting only part of an installation (a repair or re-grout): raked-out joints for re-grouting are rarely cleaned to full tile depth. Measure the actual raked depth and use it in place of tile thickness, or the estimate will run roughly double the real need.
- Large-format tile with 1/16-in joints on a perfectly flat substrate: quantities become tiny (a few pounds per hundred square feet) and the 10% waste plus whole-bag purchasing dominates — the smallest retail box will almost always be the purchase regardless of decimal precision.
- Structural movement joints: expansion joints at perimeters and field intervals must be filled with flexible sealant, not grout — subtract their length mentally, and note they're one more reason real consumption lands slightly under the uniform-grid model.
FAQ
How does the calculator know how much grout the joints hold?
It uses the standard formula grout manufacturers themselves use for their coverage charts: ((tile length + tile width) ÷ (tile length × tile width)) × joint width × joint depth × tiled area. The first term is the linear length of joint per unit of tiled area — smaller tiles pack more joint into every square foot — and multiplying by the joint's width and depth turns that into a volume. Multiply by the grout's density and you get weight, which is how grout is sold.
What should I enter for joint depth?
Use your tile's thickness — grout fills the joint from the substrate (or thinset line) to the tile surface, so joint depth is effectively tile thickness. Typical values: 5/16–3/8 in (8–10 mm) for standard ceramic floor tile, 1/4 in (6 mm) for wall tile, 3/8–1/2 in (10–12 mm) for porcelain pavers and natural stone. Entering the tile's nominal thickness slightly overestimates (the joint sits on a bed of thinset that squeezes up into it), which is the safer direction.
Sanded or unsanded grout — and does the choice change the amount?
Sanded grout is for joints 1/8 in and wider (the sand prevents shrinkage cracking in wide joints); unsanded is for joints under 1/8 in and for polished stone that sand would scratch. The quantity formula is the same for both, but their effective densities differ slightly — unsanded is a bit lighter. The default density here is calibrated to sanded cementitious grout; for unsanded, reduce the density modestly using the advanced field or simply accept a small overestimate.
Why do small tiles need so much more grout than large ones?
Joint length per square foot scales with (L + W) ÷ (L × W), which explodes as tiles shrink. A 2×2-in mosaic has six times the joint length per square foot of a 12×12-in tile — so at the same joint width and depth it needs six times the grout. This is why mosaic jobs routinely consume a full 25-lb bag where a large-format floor of the same area needs a fraction of one.
Does the calculator include waste?
Yes — a fixed, disclosed 10% is added to the computed joint volume for mixing loss, the grout that cures in the bucket, joint overfill before tooling, and haze washed away during cleanup. Manufacturer coverage charts are typically quoted without waste, which is one reason a chart's optimistic per-bag coverage can leave you a little short on a real job.
Why is my result different from the grout manufacturer's coverage chart?
Three usual reasons: charts assume specific joint depths that may not match your tile's thickness; charts quote coverage without a waste allowance; and charts round to friendly numbers for common tile sizes. The underlying formula is the same one this calculator uses — Custom Building Products and Mapei both publish calculators built on the joint-volume method — so entering identical assumptions reproduces their figures within rounding.
How many bags should I buy?
Cementitious grout is commonly sold in 8 or 10 lb boxes and 25 lb bags. Divide this calculator's pounds result by your product's bag size and round up. For results just over a bag boundary (say 26 lb), check whether a small joint-width reduction is acceptable before buying a second 25-lb bag — joint width is the input you control most freely, and it's linear in the result.
Does grout color affect quantity?
No — pigment load differences between colors are negligible for quantity. But color matters for purchasing discipline: mix all your grout from bags of the same lot where possible, and if a job needs multiple bags, dry-blend them before mixing water in. Slight lot-to-lot pigment variation shows as banding between areas grouted from different bags.
Is epoxy grout calculated the same way?
The joint-volume geometry is identical, but epoxy grout is denser (roughly 1,900–2,100 kg/m³ effective) and sold in kits by combined weight, often with very different unit sizes. Enter a higher density in the advanced field to approximate epoxy quantities, then convert to your kit size — and note that epoxy's working time and cleanup are unforgiving enough that overbuying one kit is cheap insurance.
What joint width should I actually use?
Common practice: 1/16–1/8 in for rectified porcelain and polished stone, 1/8–3/16 in for standard ceramic floor tile, up to 1/4 in or more for handmade, terracotta, or size-irregular tile (wider joints absorb tile-to-tile size variation). Tile industry standards also require the joint to be at least three times any tile-to-tile facial dimension variation. Wider joints are more forgiving to install and proportionally hungrier for grout — the relationship is exactly linear in this calculator.
Do rectangular (plank) tiles change the math?
No — enter the two sides as they are. A 6×24-in plank has more joint per square foot than a 12×12 of the same area, because (L + W) ÷ (L × W) grows as a shape gets more elongated. This is why wood-look plank layouts take noticeably more grout than square tiles of similar size, and the formula captures it automatically.
Can I switch between imperial and metric partway through?
Yes — the unit toggle converts every field you've already entered (tile sizes in inches or millimetres, area in sq ft or m², results in pounds or kilograms). Internally the tool always computes from the same canonical SI values regardless of the displayed unit, so switching back and forth doesn't introduce rounding error.
Does this calculate thinset mortar too?
No — thinset (the adhesive bed under the tile) is estimated per area by trowel notch size, a completely different model from joint volume. A 1/4×1/4-in square-notch trowel typically runs 60–90 sq ft per 50-lb bag. Estimate it separately from your thinset bag's chart; this page covers only the grout between tiles.
Related tools
Tile Calculatorif you haven't bought the tile yet, work out how many tiles (and boxes) the same area needs first — the tile size you settle on there is exactly what this grout estimate depends on.More flooring tools (thinset and sealer estimators) are planned but not yet published.