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Prism Grout Calculator: How Many 17 lb Boxes Do You Need?

Enter your area, tile size and joint width to see how many 17 lb (7.71 kg) boxes of Prism Ultimate Performance Grout to buy. The calculator uses a joint-volume formula calibrated against the coverage chart in Custom Building Products TDS-128 (published 6/20/2023), set so it never claims more coverage than the chart. For 12 x 12 in tile with 1/8 in joints, Custom's chart gives 348 sq ft per box; the calculator gives about 346.

Custom Prism coverage

Source: Custom Building Products Technical Data Sheet TDS-128: Prism Ultimate Performance Grout, Published 6/20/2023, read 2026-10-17.

How the calculation works

grout per sq ft = 144 × ((L + W) × J + J²) × T ÷ ((L + J) × (W + J))
units = area × (1 + waste ÷ 100) × grout per sq ft ÷ joint volume one unit fills, rounded up

L and W are the tile length and width, T the tile thickness (the joint is assumed to be filled to full tile depth) and J the joint width, all in inches; the result is in cubic inches. The volume one box fills is worked back from the manufacturer's coverage chart and set at the lowest value any cell of the chart implies, so for every tile size in the chart this calculator gives the same or less coverage than the data sheet, never more. Tiles not in the chart use the same formula.

Worked example

A 400 sq ft floor of 12 x 12 x 3/8 in tile with 1/8 in joints and 10% extra: 440 sq ft to grout. Each tile plus its share of joint is a 12.125 × 12.125 in square, and the joint part of it holds (12 + 12) × 1/8 + 1/8² = 3.016 sq in, 3/8 in deep. That works out to 1.11 cubic inches of grout per sq ft of floor. One 17 lb (7.71 kg) box fills about 383 cubic inches of joint, so it covers 383 ÷ 1.11 = 346 sq ft. 440 ÷ 346 = 1.27, so buy 2 boxes.

Custom Prism key specs

What Prism is

Prism Ultimate Performance Grout from Custom Building Products is a powdered, cement-based grout that you mix with water. Unlike ordinary sanded grout, it is built on calcium aluminate cement with a latex polymer modifier, and it uses a blend of lightweight recycled glass and fine sand as the aggregate. Custom's selling points are color consistency and speed: the data sheet says Prism will not mottle or shade regardless of tile type, temperature or humidity, will not effloresce, and is walkable in about four hours. It meets ANSI A118.6 and A118.7 and contains 15 percent post-consumer recycled material by weight.

It is rated for joints up to 1/2 inch on floors and walls, interior and exterior, residential and commercial work, including countertops, tub surrounds, showers and high-traffic areas. Unusually for a cement grout, TDS-128 also lists submerged conditions such as swimming pools, spas, water features and fountains. Suitable tiles include vitreous, semi-vitreous and non-vitreous ceramic, porcelain, glass, brick and thin brick, precast terrazzo and natural stone.

Why a 17 lb box is not a small bag

The most common mistake with Prism is comparing it to other grouts by weight. Because the glass aggregate is light, Prism is about 32 percent lighter than typical grout for the same volume. Custom states that one 17 lb (7.71 kg) box covers the same area as a typical 25 lb bag of conventional grout, and the coverage chart supports that: it is almost cell-for-cell the same as Custom's 25 lb chart for Polyblend Plus sanded grout. So if a project estimate says "two 25 lb bags", that is roughly two Prism boxes, not three. Each box holds two sealed 8.5 lb (3.85 kg) bags.

How the calculator works

Grout fills only the gaps between tiles, so the amount you need depends on how much joint there is per square foot of floor. The calculator treats one tile plus half of the joint on each side as a rectangle (L + J) by (W + J), where L and W are the tile length and width and J the joint width. The joint part of that rectangle is (L + W) × J + J² square inches. Multiply by the tile thickness T, since the joint is filled to full tile depth, and divide by the rectangle's area to get grout per square inch of floor; multiply by 144 for cubic inches per square foot.

Custom does not publish how much joint volume a box fills, so that number was worked back from each of the 96 cells in the TDS-128 chart. Eighty-two of the cells imply between 383 and 395 cubic inches per box. The calculator uses 383.2, the lowest value in the chart, which comes from 12 x 48 inch plank tile with 1/16 inch joints. That way, for every tile and joint the chart prints, the calculator shows the same or slightly less coverage than Custom does, never more, and the site's build refuses to publish if that stops being true. The places where the chart is noticeably more generous than the formula are 1 x 1 and 2 x 2 inch mosaics with wide joints; there the calculator stays on the safe side by up to about 20 percent.

After that, the calculator adds your waste allowance to the area, divides by the coverage per box and rounds up to whole boxes. Tile sizes not in the chart use the same formula, so you are not limited to the 16 sizes Custom prints.

Entering your tile and joint

Use the real tile dimensions rather than the nominal ones if they differ much. Many "12 x 24" porcelain tiles are slightly under size, which adds a little more joint per square foot. Measure tile thickness at the edge. Joint widths run from 1/16 to 1/2 inch, matching the chart; Custom rates Prism for joints up to 1/2 inch. Because the aggregate can scratch some ceramic, glass, metal, marble or stone surfaces, Custom asks you to test a small area first and suggests Polyblend NonSanded Grout for joints up to 1/8 inch on tile that cannot take a sanded grout.

A 10 percent waste allowance is a sensible default for floors. Prism has a pot life of 60 to 90 minutes, so mix only what you can spread and clean in that window; when using more than one box at a time, Custom asks you to blend the dry powders together before adding water so the color stays even.

Mixing and installation notes

Mix each 17 lb box with 4 to 5 pints (1.89 to 2.36 L) of cool, clean water using a trowel or a mixer under 300 rpm. Let it slake for 5 minutes, then remix. Remix occasionally as you work but do not add more water, which Custom says weakens the grout and can cause color variation, shrinkage, cracks and pinholes. Fill the joints with a rubber float held at 45 degrees, strip the excess at 90 degrees, and at 70°F do not spread more than you can clean within 30 minutes of the grout firming. Haze can come off after 3 hours with cheesecloth or a wrung-out sponge.

The setting mortar must be cured for 24 to 48 hours first unless it is a rapid-setting type. Install between 50°F and 90°F. Prism takes intermittent water after 3 days; for pools and fountains wait 14 days before filling. Exterior work needs protection from rain and snow for at least 7 days above 50°F. Prism does not need sealing, but a pH-neutral, water-based sealer can be applied after 48 hours for extra stain protection. Movement joints and changes of plane need caulk or sealant instead.

Check before you buy

The numbers on this page come from TDS-128 published 6/20/2023, downloaded from Custom's website. Custom revises data sheets from time to time, so check the current version linked below if your estimate is close to a box boundary.

FAQ

How much does a 17 lb box of Prism cover?

It depends on tile size and joint width. Custom's TDS-128 chart gives 348 sq ft per 17 lb box for 12 x 12 x 3/8 inch tile with 1/8 inch joints, 688 sq ft at 1/16 inch and 177 sq ft at 1/4 inch. Mosaics use much more: 2 x 2 inch mosaic at 1/8 inch joints covers 96 sq ft per box and 1 x 1 inch mosaic only 54 sq ft. Custom says the chart is for estimating and real coverage depends on installation practice.

Is a 17 lb box of Prism really the same as a 25 lb bag?

For coverage, yes, according to Custom. Prism uses lightweight recycled glass aggregate, so it weighs about 32 percent less than typical grout for the same joint volume. The data sheet states that a 17 lb box covers the same area as a typical 25 lb bag of conventional grout, and the coverage charts back that up: the Prism box chart is almost identical to Custom's chart for a 25 lb bag of Polyblend Plus sanded grout.

Why is the calculator's coverage a little lower than the chart?

The calculator uses one joint-volume formula for every tile size, with the volume per box set at the lowest value any cell of Custom's chart implies, about 383 cubic inches. For every tile and joint in the chart it gives the same or slightly less coverage than the data sheet, never more. Most cells are within about 1 percent; 1 x 1 and 2 x 2 inch mosaics with wide joints are where the chart is noticeably more generous.

Can I use Prism in a shower or pool?

Yes. TDS-128 lists countertops, tub surrounds, showers and submerged conditions such as swimming pools, spas, water features and fountains. Allow 3 days before intermittent water exposure and 14 days of cure before filling a pool or fountain. Custom warns that chemicals in salt-based pool filtration systems can react with blue, green and red grout colors and asks you to contact Technical Services first.

Do I need to seal Prism grout?

No sealer is required. Custom lists stain resistance and no efflorescence among Prism's features. If you want extra stain protection, the data sheet recommends a pH-neutral, water-based sealer such as Aqua Mix Sealer's Choice Gold or TileLab SurfaceGard, applied after the grout has cured for at least 48 hours.

Custom Building Products and Custom Prism are trademarks of their owner. This page is an independent estimate based on the published data sheet; check the bag label before mixing.