Custom Grout Calculator: Prism, Polyblend Plus and Fusion Pro Coverage

1. Pick the Custom grout and pack you plan to buy. Prism and Polyblend Plus fill in a factor and pack size you can edit; Fusion Pro and CEG-Lite switch to their per-unit coverage automatically.
2. Enter the tile length, width and thickness and the grout joint width. Use the real tile size from the box, and choose inches or millimetres first.
3. Enter the area to grout and your own waste allowance. Read how much to buy, or switch the question to the area one unit covers.

A Custom grout calculator estimates how much Custom Building Products grout a tiling job needs. It works out the volume of the grout joints, then turns that volume into the pack you actually buy, box, bag, gallon or unit, for the product you choose.

Custom sells grout in two very different ways. Prism and Polyblend Plus are dry cement grouts measured by a pounds-per-joint factor. Fusion Pro and CEG-Lite are sold ready to use by the pail or epoxy unit, so this tool counts pails and units instead of pounds.

This page is an independent tool and is not affiliated with Custom Building Products. For a brand-free estimate, use the grout calculator; for a different brand see the TEC grout calculator.

A 17 lb box of Prism covers the same area as a typical 25 lb bag of conventional grout, because Prism is a lightweight grout that Custom states is about 32 percent lighter. The joint still fills with the same volume; it just weighs less.

Custom's own data proves it. The Prism 17 lb coverage chart is identical, cell for cell, to the Polyblend Plus 25 lb chart. So one 17 lb Prism box and one 25 lb Polyblend Plus bag grout the same floor, and the Prism box is 8 lb lighter to carry.

That means a generic grout calculator over-orders Prism by roughly 1.6 times. This page uses Prism's real factor of 0.042 lb per cubic inch, so the box count is right. Never carry a conventional sanded number across to Prism.

For cement grouts the calculator finds the cubic inches of joint in one square foot of tile, then multiplies by the product factor to get pounds. For ready-to-use and epoxy grouts it divides a per-unit fill volume by the same joint volume to get coverage.

Here L and W are the tile length and width in inches, T is the tile thickness, J is the joint width, and k is the grout factor in pounds per cubic inch. The 144 converts square inches to square feet.

1. Joint length. Per square inch of tile, the joint length is (L + W) / (L x W). Multiply by joint width and thickness, then by 144, for cubic inches per square foot.
2. Cement products. Multiply joint volume by the factor k, then by area and waste, and divide by the pack weight to get boxes or bags.
3. Per-unit products. Fusion Pro and CEG-Lite coverage equals a fill volume per unit divided by the joint volume, so the tool returns gallons, quarts or epoxy units.

The cement factor is not a bulk density. It is the grout weight that Custom's coverage figures imply per cubic inch of finished joint, so use it only with this formula.

Take a 400 sq ft floor of 12 x 12 in porcelain tile, 3/8 in thick, with 1/4 in joints, grouted with Prism in 17 lb boxes. The 10 percent waste allowance is only an example.

1. Joint volume: 144 x 0.375 x 0.25 x (12 + 12) / (12 x 12) = 2.2500 in3 per sq ft.
2. Grout per sq ft: 2.2500 x 0.042 = 0.0945 lb. One 17 lb box covers 180 sq ft.
3. Whole floor: 400 x 0.0945 = 37.80 lb. Adding 10 percent gives 41.58 lb, which is 3 boxes (9.4 lb to spare).
4. Lightweight payoff: the same floor in Polyblend Plus needs 3 bags at 25 lb. That is 75 lb to carry versus 51 lb of Prism, a saving of 24 lb.

For a ready-to-use job, 220 sq ft of the same 12 x 12 in tile at 3/16 in joints needs 1.80 gallons of Fusion Pro, so you buy 2 gallon pails. One gallon covers about 135 sq ft, which matches the TDS-242 chart.

Coverage falls fast as the joint gets wider. Doubling the joint width halves the area a box covers, because the joint volume doubles.

This table shows square feet per 17 lb Prism box, computed with the cement formula. It reproduces the published Prism chart, which Custom prints identically to the Polyblend Plus 25 lb chart.

Bigger tiles cover far more per box, because they have less joint per square foot. To fit a floor to these numbers, measure the room with the tile flooring calculator first.

Fusion Pro and CEG-Lite are sold by the unit, so their coverage comes straight from Custom's charts rather than a pounds factor. The numbers below are copied from the data sheets and are the anchors this tool interpolates between.

Fusion Pro, square feet per 1 gallon pail (TDS-242)

A 1 quart pail covers about one quarter of each gallon figure. Fusion Pro is ready to use and is not for submerged or pool joints.

CEG-Lite, square feet per 9.5 lb unit (TDS-101)

CEG-Lite is a two-part epoxy; one unit is 1 Part A plus 1 Part B at 9.5 lb. The larger pack uses 2 Part A with 1 Part B and doubles every value above.

Five inputs move the answer. Four are in the calculator, and the fifth is how cleanly you spread the grout.

Joint Width

Grout quantity is directly proportional to the joint width. A 1/4 in joint takes twice the grout of a 1/8 in joint on the same tile.

Tile Thickness

Thicker tile means deeper joints. The calculator assumes the joint is filled to the full tile thickness, which is a safe basis for ordering.

Tile Size

Small tile has the most joint length per square foot. A mosaic can need many times the grout of a large-format tile.

Product Choice

Prism at 0.042 and Polyblend Plus at 0.062 differ by about a third, and the per-unit products use their own charts. Switching products changes the pack count, not just the pack type.

Waste and Technique

Spillage, mixing loss and uneven tile edges use extra grout. Set your own allowance, since no single figure fits every job. See the material waste guide.

Ordering Grout for a Floor or Shower

Enter the tile and joint details, pick the product, and read the pack count. Then add the grout to the tile order from the tile calculator.

Choosing Prism to Save Weight

If you carry grout up stairs or across a large job, Prism cuts the load by about a third for the same coverage. The verdict band shows the saving when you pick Prism.

Switching Between Dry and Ready-to-use

Compare boxes of Prism against gallons of Fusion Pro for the same floor to weigh cost against convenience.

Planning a Whole Project

The flooring and tile quantities guide shows how to measure a room before you use these numbers.

1. Using a Conventional Factor for Prism

Prism is lightweight. Applying a 0.062 sanded factor or a generic 120 lb/ft3 default over-orders it by roughly 1.6 times. Use 0.042.

2. Counting Pounds for Fusion Pro

Fusion Pro is sold ready to use by the gallon. Convert the job to pails, not pounds, or you will buy the wrong amount.

3. Using Nominal Tile Size

Some tiles measure a little less than the label. Enter the real size from the box.

4. Forgetting Tile Thickness

A 1/2 in thick tile has joints twice as deep as a 1/4 in tile, which doubles the grout volume.

5. Using a Joint Outside the Product Range

Polyblend Plus starts at 1/8 in. For a narrower joint the chart and the verdict band both flag it.

6. Rounding Packs Down

Packs are sold whole. 1.4 gallons means 2 pails. A full pack also mixes to a more even color.

The calculator reproduces Custom's published coverage within a few percent for normal floor and wall tile. Cement grouts use the same approximation as our other grout pages; the per-unit products use the exact tile geometry, which matches their charts more closely.

The cement formula uses the tile size, not the tile plus joint. For the 12 x 12 in example at 1/4 in joints, counting the joint corners exactly would lower the quantity by about 3.0 percent, and the gap is larger only for small mosaics. For exact published coverage on per-unit products, read the charts above.

It depends on tile size and joint width. For 12 x 12 in tile, 3/8 in thick, with 1/8 in joints, one 17 lb box covers about 360 sq ft. A 17 lb Prism box covers the same area as a 25 lb bag of conventional grout.

Prism is a lightweight calcium-aluminate grout that Custom states is about 32 percent lighter than typical grouts. A 17 lb box fills the same joint volume as a 25 lb conventional bag, so you carry less for the same coverage.

Prism uses 0.042 lb per cubic inch of joint and Polyblend Plus uses 0.062. Both come from Custom's coverage charts. Do not use a generic 120 lb/ft3 default for Prism, as it over-orders by about 1.6 times.

Pick Fusion Pro and the tool divides a per-gallon fill volume by your joint volume to get square feet per gallon, then rounds up to whole pails. For example, 12 x 12 in tile at 3/16 in joints covers about 134 sq ft per gallon.

CEG-Lite is sold as a two-part unit of 1 Part A plus 1 Part B at 9.5 lb. The calculator returns whole units from the published coverage chart. The larger 19 lb pack doubles the coverage.

Not with a built-in number. Classic Polyblend Sanded was not verified here, so pick it and enter the factor and pack size from its own data sheet, TDS-129, or type a coverage override.

Pounds per square foot equal 144 x T x J x (L + W) / (L x W) x k, where T is tile thickness, J is joint width, L and W are tile sides in inches, and k is the product factor.

Switch the units to millimetres and square metres. The calculator converts to inches with 1 in = 25.4 mm, then shows kilograms and square metres beside the pounds and square feet.

Prism and Fusion Pro are rated down to 1/16 in. Polyblend Plus starts at 1/8 in, so the verdict band warns you if you pick it for a narrower joint.

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• Prism Ultimate Performance Grout data sheet (TDS-128): coverage chart, 17 lb box, lightweight claim (Custom Building Products, published 2023-06-20, read 2026-10-04).
• Polyblend Plus Sanded Grout data sheet (TDS-360): 25 lb bag and 7 lb box coverage (Custom Building Products, published 2023-03-22, read 2026-10-04).
• Fusion Pro Single Component Grout data sheet (TDS-242): per-gallon and per-quart coverage (Custom Building Products, published 2024-03-12, read 2026-10-04).
• CEG-Lite 100% Solids Commercial Epoxy Grout data sheet (TDS-101): per-unit coverage (Custom Building Products, published 2023-03-22, read 2026-10-04).