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Fusion Pro Grout Calculator: How Many Gallon Pails Do You Need?

Enter your area, tile size and joint width to see how many 1 gallon (3.78 L) pails of Fusion Pro Single Component grout to buy. The calculator uses a joint-volume formula calibrated against the coverage chart in Custom Building Products TDS-242 (published 3/12/2024), 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 199 sq ft per gallon; the calculator gives about 198.

Custom Fusion Pro coverage

Source: Custom Building Products Technical Data Sheet TDS-242: Fusion Pro Single Component Grout, Published 3/12/2024, read 2026-10-16.

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 pail 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 200 sq ft floor of 12 x 12 x 3/8 in tile with 1/8 in joints and 10% extra: 220 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 1 gal (3.78 L) pail fills about 219 cubic inches of joint, so it covers 219 ÷ 1.11 = 197.7 sq ft. 220 ÷ 197.7 = 1.11, so buy 2 pails.

Custom Fusion Pro key specs

What Fusion Pro is

Fusion Pro Single Component grout from Custom Building Products is a ready-to-use sanded grout made from advanced polymers with inorganic fillers and pigments. There is no powder and no water to measure: you open the pail and spread it. Custom markets it as stain proof and color consistent, says it never needs sealing, and adds Microban antimicrobial protection. The data sheet says it meets many of the performance characteristics of ANSI A118.7 (polymer-modified cement grout) and A118.3 (epoxy grout), without claiming full compliance with either.

It is rated for walls and floors, interiors and exteriors, residential and commercial work, including countertops, tub surrounds and shower floors. Suitable tiles include ceramic, mosaic, quarry, porcelain, glass, cement-based terrazzo, polished natural stone and groutable luxury vinyl tile. Because it is sanded, Custom warns that some glass, metal, marble or stone tiles can be scratched by the silica filler and asks you to test a small area first.

Why grout is harder to estimate than mortar

With thinset or self-leveler, the bag covers a fixed area at a given trowel or thickness. Grout only fills the gaps between tiles, so the amount you need depends on how much gap there is per square foot. Small tiles mean many joints; wide joints mean each joint holds more; thick tiles mean deeper joints. That is why Custom's chart ranges from 782 sq ft per gallon for 24 x 24 inch tile with 1/16 inch joints down to 14 sq ft per gallon for 1 x 1 inch mosaic with 1/2 inch joints. Using one "coverage per pail" number for every job can be off by a factor of ten.

How the calculator works

Picture one tile plus half of the joint on every side. Together they form a rectangle (L + J) by (W + J), where L and W are the tile length and width and J is the joint width. The joint part of that rectangle has an area of (L + W) × J + J². Multiply by the tile thickness T to get the joint volume for one tile, and divide by the rectangle's area to get grout per square inch of floor. Multiply by 144 to get cubic inches of grout per square foot.

The other half of the calculation is how much joint volume one pail fills. Custom does not publish that figure, so it was worked back from every cell of the TDS-242 gallon chart. Across the 96 cells, 76 imply between 219 and 226 cubic inches per gallon. The calculator uses 219, the lowest value any cell implies, which means that for every tile size and joint width in the chart, it gives the same or slightly less coverage than Custom's own table. The site's build checks this automatically and refuses to publish if any cell would come out more generous than the data sheet. The cells where the chart is noticeably more generous than the formula are small mosaics (1 to 3 inch) with wide joints, where the calculator stays on the safe side.

Finally, the calculator adds your waste allowance to the area, divides by the coverage per pail and rounds up to whole pails. For tile sizes that are not in the chart, the same formula applies, so you are not limited to the 16 sizes Custom prints.

Entering your tile and joint

Use the actual tile size, not the nominal one, if they differ much. Many "12 x 24" porcelain tiles are a little under that, which slightly increases the number of joints per square foot. For tile thickness, measure the edge; the formula assumes the joint is filled to the full depth of the tile, which is how the TDS chart is built too. Joint widths are limited to the 1/16 to 1/2 inch range Custom lists. Custom says not to use Fusion Pro where joints or depths exceed 1/2 inch, or with irregular tile such as pebble stone.

A waste allowance of 10 percent is a reasonable default. Grout left in the pail can be kept for later touch-ups because it is a ready-to-use product, but Custom asks you to protect it from freezing and store it cool and dry.

Gallon or quart

All colors come in 1 gallon (3.78 L) pails and select colors also in 1 quart pails. Custom's quart chart is about a quarter of the gallon chart: 50 sq ft for 12 x 12 inch tile at 1/8 inch joints against 199 sq ft for a gallon. If the calculator says you need, say, 0.2 of a gallon for a small backsplash, a quart pail may be enough. Otherwise round up to gallons.

Installation notes from the data sheet

Fusion Pro is installed differently from cement or epoxy grout, and Custom says so plainly. Lightly dampen the tile surface without leaving puddles, spread with a medium to hard float for about 2 minutes, typically in 10 sq ft sections, then drag the float edge diagonally to strip excess while leaving the joints full. Cleaning starts right away, before the next section: a lightly damp sponge in a circular motion, then a rinsed sponge wiped once diagonally, then a lightly damp microfiber towel. Do not let it dry or skin over before final cleaning, and do not use a power mixer on it.

It cures by evaporation, so the setting material must be dry, the slab must not have a high moisture emission rate and the room needs ventilation. Allow 24 hours before light traffic and 7 days at 70°F (21°C) and 50 percent humidity before intermittent water exposure such as a shower. Install between 50°F and 90°F. It is not for steam rooms, pools, spas or fountains, not for movement joints or changes of plane (use a caulk or sealant there), and not for industrial areas with harsh chemicals; Custom points to CEG-IG epoxy for heavy industrial work.

Check before you buy

The numbers on this page come from TDS-242 published 3/12/2024. The printed page header on that data sheet still reads "PRISM SCG", which appears to be a template leftover; the product name, description, item codes and coverage chart in the body are all Fusion Pro. Custom revises data sheets from time to time, so check the current version linked below if you are close to a pail boundary.

FAQ

How much does a gallon of Fusion Pro cover?

It depends entirely on tile size and joint width. Custom's TDS-242 chart gives 199 sq ft per 1 gallon pail for 12 x 12 x 3/8 inch tile with 1/8 inch joints, 393 sq ft at 1/16 inch and 101 sq ft at 1/4 inch. Small mosaics use far more grout: 1 x 1 inch mosaic at 1/8 inch joints covers only 31 sq ft per gallon. Custom says the chart is for estimating and real coverage depends on installation practice.

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 pail set at the lowest value any cell of Custom's chart implies. That means 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; small mosaics with wide joints are where the chart is more generous than the formula.

Should I buy quart pails instead of a gallon?

Custom sells select colors in 1 quart pails, and its quart chart is roughly a quarter of the gallon chart (for example 50 sq ft for 12 x 12 inch tile at 1/8 inch joints). If the calculator shows you need well under a quarter of a gallon, such as a small backsplash, a quart may be enough. For floors, gallon pails are usually the better buy.

Do I need to seal Fusion Pro grout?

No. Custom describes Fusion Pro as stain proof and says sealing is not necessary. Natural stone and porous or rough tile should be pre-sealed before grouting so the grout does not stain the tile. If you do seal the tile afterward, use a water-based sealer and wipe excess off the joints; Custom says not to use solvent-based sealers or enhancers with Fusion Pro.

How long before I can use a shower grouted with Fusion Pro?

Fusion Pro cures by evaporation rather than by a chemical reaction with water. The data sheet allows light foot traffic after 24 hours, but asks for 7 days at 70°F (21°C) and 50% relative humidity before intermittent water exposure such as a shower. Cooler temperatures and higher humidity slow the cure, and covering the grout right after installation extends it.

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