Grout is ordered last and measured least. Tile is settled by floor area, mortar is settled by trowel notch, and then somebody has to say how many boxes of grout go on the truck. That number never comes from floor area. It comes from the empty space the layout leaves between the pieces, which is a volume: total joint length multiplied by joint width multiplied by joint depth. Multiply that volume by the density printed on the product data sheet and you have a mass in kilograms that can be compared against a published coverage chart while the order is still changeable.
Getting total joint length out of a tile layout
- Count the joint runs, not the floor area. Divide each room dimension by the tile module to get the tile count in that direction, subtract one for the joints between them, and multiply by the opposite dimension. A 4.8 by 6.0 m floor in 600 mm tile is 8 tiles by 10 tiles: 7 joints spanning 6.0 m gives 42 m, and 9 joints spanning 4.8 m gives 43.2 m, for 85.2 m in total. Tool: Tape measure.
- Measure the width after two rows are set. Set the first two rows with spacers, then slide a spare spacer or a feeler gauge into five separate joints across the run and record the widest reading rather than the nominal spacer size. Warped or calibrated tile opens joints beyond the spacer, and the widest joint is what drains the bag first. Tool: Feeler gauge or spare spacer.
- Probe the open depth of a cleaned joint. Rake one joint clean, stand a steel rule or a depth gauge in it, and read the distance from the tile face down to the set mortar. Record it in millimeters. Do not substitute the tile thickness stamped on the carton, because squeezed mortar routinely takes 2 to 3 mm of that channel away. Tool: Depth gauge or steel rule.
- Read density from the product data sheet. Open the technical data sheet for the exact grout you intend to buy and look for a density or a coverage chart before you accept the default. Convert any pounds per cubic foot figure by multiplying it by 16.0185, and note whether the product is described as lightweight, because that can move the mass by a third. Tool: Manufacturer data sheet.
- Apply the waste allowance one time only. Enter the allowance in the waste field and nowhere else. Ten percent covers float loss, the material that stays in the bucket, and joints that are wider than measured. Doubling it by also rounding every intermediate figure upward is how a 5 kg job turns into three boxes on the invoice.
A 120 meter joint run at 3 mm by 8 mm
| Field | Value |
|---|---|
| jointLength | 120 |
| jointWidth | 0.003 |
| depth | 0.008 |
| density | 1600 |
| waste | 10 |
Start with the volume. A 120 m run of joint at 0.003 m wide and 0.008 m deep encloses 0.00288 cubic meters of empty channel. At 1600 kg per cubic meter that channel holds 4.608 kg of grout, and the 10 percent allowance lifts the order to 5.0688 kg. Width, depth, and length each enter the formula once, so each moves the answer in a straight line. Widening the joint from 3 mm to 5 mm raises the order to 8.448 kg. Deepening it from 8 mm to 10 mm raises it to 6.336 kg. That is why a miscounted joint run costs more than a mismeasured room. Nothing is rounded until the end. Set against a 17 lb (7.71 kg) box, 5.0688 kg is one box with about 2.6 kg left sealed for repairs.
Result: 5.0688 kilograms.
Why the specified joint width rarely survives the floor
A drawing that calls for 3 mm joints is a starting position, not a promise. The Tile Council of North America identifies the current ANSI A108 installation standards as the governing documents for tile workmanship, including joint planning, but the detailed standard text is licensed and edition-specific. Do not copy a numeric rule from an undated summary. Confirm the current edition required by the project, follow the tile and grout manufacturers' instructions, and measure the actual joint after the first two rows are set. Facial-size variation and edge warpage can move the installed width away from the spacer size, so the measured channel is the number this volume calculation needs.
| Tile size and thickness | 1/16 in joint | 1/8 in joint | 1/4 in joint | 1/2 in joint |
|---|---|---|---|---|
| 1 x 1 x 1/4 in | 96 sq ft | 54 sq ft | 33 sq ft | 24 sq ft |
| 2 x 2 x 1/4 in | 181 sq ft | 96 sq ft | 54 sq ft | 33 sq ft |
| 4.25 x 4.25 x 1/4 in | 372.4 sq ft | 192 sq ft | 101 sq ft | 56 sq ft |
| 6 x 6 x 1/4 in | 521 sq ft | 266 sq ft | 139 sq ft | 75 sq ft |
| 12 x 12 x 3/8 in | 688 sq ft | 348 sq ft | 177 sq ft | 92 sq ft |
| 12 x 24 x 3/8 in | 915 sq ft | 461 sq ft | 234 sq ft | 121 sq ft |
| 24 x 24 x 3/8 in | 1369 sq ft | 688 sq ft | 348 sq ft | 177 sq ft |
Source: Custom Building Products, Prism Ultimate Performance Grout, Technical Data Sheet TDS-128.
Published for estimating. Coverage varies with installation practice and site conditions, and the same product is rated only for joints up to 1/2 inch (13 mm).
Joint depth is not the same number as tile thickness
The depth field asks how deep the open channel is, measured from the tile face down to whatever stops the grout. On a floor set in a thin bed, mortar usually squeezes part way up the joint, so a 3/8 inch (1 cm) porcelain plank may leave only 6 to 7 mm of open channel instead of the full 9.5 mm. On a wall built with 1/4 inch (0.64 cm) tile the joint is often open for its whole depth because nothing sits behind it. Manufacturer coverage charts treat this as a real variable rather than a detail: the Custom Building Products chart names a thickness in every single row, 1/4 inch for the small formats and 3/8 inch from 8 inch tile upward. The arithmetic is unforgiving. A 6 mm channel entered as 9.5 mm overstates the order by about 58 percent. Scrape one joint clean with a screwdriver blade and measure the channel before you type a number.
Grout density is the input most people guess
The tool defaults to 1600 kg per cubic meter, a fair stand in for conventional sanded portland cement grout. Lightweight grouts are nowhere near it. Custom Building Products states that Prism is 32 percent lighter than typical grouts, and applying that 32 percent to 1600 gives roughly 1090 kg per cubic meter, which cuts the mass answer by about a third for exactly the same joint volume. Unit systems bite here too: a data sheet quoting pounds per cubic foot must be multiplied by 16.0185 to reach kilograms per cubic meter. When a data sheet publishes no density at all, work backward from the coverage chart instead of inventing a figure. One 17 lb (7.71 kg) box covering 348 square feet of 12 by 12 inch tile at a 1/8 inch joint describes the product you are buying far better than any generic textbook value.
Common mistakes
- Estimating grout from tile area instead of from total joint length
- Floor area says nothing about how the tile is cut up. The same 30 square meters holds far more joint in 100 mm mosaic than in 600 mm porcelain, and the coverage chart shows the gap clearly: a 17 lb box reaches 96 square feet of 1 inch mosaic but 688 square feet of 12 by 12 inch tile at the same 1/16 inch joint. Count the joints in both directions and add the two lengths before touching the tool. Area belongs in the tile order and the mortar order, not in the grout order, and the two numbers diverge by a factor of seven across common tile sizes.
- Entering the tile thickness in the joint depth field
- Thin bed mortar squeezes into the bottom of the joint during setting, so the open channel is routinely 2 to 3 mm shallower than the tile. Entering 9.5 mm where the channel measures 6 mm overstates the order by about 58 percent and leaves unopened boxes that cannot be returned once the seal is broken. Rake one joint clean, stand a steel rule in it, and read the actual open depth from tile face to mortar. Record that reading in millimeters and use it for every joint of the same construction.
- Reusing 1600 kg per cubic meter for a lightweight grout
- Lightweight formulas replace part of the sand with recycled glass and other low density aggregate. Custom Building Products describes Prism as 32 percent lighter than typical grouts, which puts it near 1090 kg per cubic meter and makes the default density wrong by roughly a third on every joint. Open the technical data sheet for the product you are actually buying, take the density or the coverage chart from it, and convert any pounds per cubic foot value by multiplying by 16.0185 before entering it.
- Including movement joints and changes of plane in the grout run
- Cement grout is rigid and cracks where the assembly is designed to move. Custom Building Products states plainly that its grout is not for use in movement joints or changes of plane, and directs those locations to a caulk or sealant instead, which is priced and ordered separately. Subtract perimeter joints, inside corners, and every designed movement joint from the total joint length, then list them as linear meters of sealant so both materials appear on the same order.
- Adding water to grout that has started to stiffen in the bucket
- Retempering weakens the cured grout and causes color variation, shrinkage, cracks, and pinholes, which is why the data sheet fixes the water at 4 to 5 pints per 17 lb box and gives a pot life of 60 to 90 minutes rather than leaving it to feel. Mix only what the crew can place inside the pot life, measure the water rather than pouring by eye, and discard any batch that has gone stiff instead of thinning it back to a workable state.
Buying grout in boxes, bags, and color lots
Grout sells by mass, and the pack size decides how much of the answer you actually pay for. Prism ships in a 17 lb (7.71 kg) box holding two sealed 8.5 lb (3.85 kg) bags, so a 5 kg result can be mixed in two sittings rather than one, and a 6.8 lb (3.1 kg) single bag box exists in Canada only across 15 colors. The line runs to 40 standard colors, which is the practical reason to buy the whole job in one order instead of topping up weeks later from another lot. Water is fixed by the box rather than by judgment: 4 to 5 pints (1.89 to 2.36 liters) of cool potable water per 17 lb box, mixed under 300 rpm, with a pot life of 60 to 90 minutes. Round up to the next whole box and leave the spare bag sealed.
Frequently asked questions
How do I turn a room measurement into total joint length?
Work one direction at a time. Divide the room dimension by the tile module, subtract one to get the number of joints between the tiles, then multiply that count by the length each joint runs. Do the same in the other direction and add the two figures. A 4.8 by 6.0 m floor in 600 mm tile gives 7 joints at 6.0 m plus 9 joints at 4.8 m, which is 85.2 m of joint before any perimeter allowance.
Why does my result differ from the coverage chart on the bag?
Coverage charts are built for one tile size, one thickness, and one joint width at a time, and they assume the joint is filled to the stated depth. Your floor probably differs on at least one of those. Compare the two properly by taking the chart row closest to your tile, converting its square feet to joint length for your layout, and then checking which input is responsible for the gap. Depth and density explain most disagreements.
Can I use the same density for sanded and unsanded grout?
No, and the difference is large enough to change what you buy. Sanded grout carries silica aggregate and sits near the 1600 kg per cubic meter default, while lightweight and unsanded products are lighter. Custom Building Products rates its unsanded grout for joints up to 1/8 inch and its sanded lightweight grout for joints up to 1/2 inch (13 mm), so the joint width usually decides the product before density even enters the discussion.
Should grout fill the full thickness of the tile?
It should fill the joint completely, but the joint is rarely as deep as the tile. On a thin bed floor the mortar rises into the bottom of the channel, leaving perhaps 6 to 7 mm of a 9.5 mm plank open. On a wall the joint is often open for its full depth. Fill whatever channel exists, then enter that measured depth rather than the carton thickness, because the tool multiplies depth directly into the answer.
How much grout should I order beyond the calculated mass?
The 10 percent default covers float loss, material left in the bucket, and small measuring variation. Do not raise it as a substitute for the real joint width: set two rows, measure the installed channel, and update that input first. If the product data sheet or project specification calls for a larger job allowance, enter that documented percentage once. Then compare the calculated mass with the manufacturer's pack size and keep any unopened remainder from the same color lot for repairs.
Sources
- ANSI Standards Tile Council of North America. Standards developer page identifying the current ANSI A108 installation standards and their edition-controlled scope; the page deliberately does not reproduce licensed numeric clauses. Accessed 2026-09-06.
- Prism Ultimate Performance Grout, Technical Data Sheet TDS-128 Custom Building Products. Source of the coverage table, the 17 lb (7.71 kg) box with two 8.5 lb bags, the 6.8 lb Canada only box, 40 colors, the 4 to 5 pint water ratio, 300 rpm mixing limit, 60 to 90 minute pot life, the 1/2 inch joint limit, and the 32 percent lighter claim. Accessed 2026-09-06.
Related calculators
This is a planning result, not a supplier quote; sellers use different pack, bag, board, roll, and bundle conventions, so confirm the exact product label and selling unit before you order.