Kerdi Board Calculator: How Much Do You Need?

The Kerdi Board calculator estimates how many sheets of Kerdi Board you need to cover the walls and floor of a shower or bathroom before tiling. You enter the length, width, and height of the space (in feet or meters), and it returns the total square footage and the number of 3′ x 5′ (15 sq ft) or 4′ x 8′ (32 sq ft) boards required, rounded up to whole sheets. This article explains the math, how to account for cutouts and waste, and how to use the result when ordering.

Formula

The calculator uses the standard formula for the interior surface area of a rectangular room with a floor and four walls:

Total Area = 2 × (Length × Height) + 2 × (Width × Height) + (Length × Width)

Where:

• Length – room length in feet (or meters)
• Width – room width in feet
• Height – wall height from floor to ceiling in feet
• Board Width and Board Height – dimensions of one Kerdi Board sheet (typically 3 ft × 5 ft or 4 ft × 8 ft)

The result is then multiplied by 1.10 to add a 10% waste allowance for cuts, mistakes, and offcuts. Finally, the number of boards is rounded up to the nearest whole sheet using the ceiling function, because you cannot buy a fraction of a board.

Worked Example

Using the default values from the calculator (Length = 6 ft, Width = 5 ft, Height = 8 ft, Board = 3 ft × 5 ft):

1. Two long walls: 2 × (6 × 8) = 96 sq ft
2. Two short walls: 2 × (5 × 8) = 80 sq ft
3. Floor: 6 × 5 = 30 sq ft
4. Total area = 96 + 80 + 30 = 206 sq ft
5. Add 10% waste: 206 × 1.10 = 226.6 sq ft
6. Board area = 3 × 5 = 15 sq ft
7. Boards needed = 226.6 ÷ 15 = 15.1 → round up to 16 boards

The calculator shows 16 boards, matching this step-by-step result.

How Kerdi Board Is Used

Kerdi Board is a rigid, waterproof foam board made by Schlüter-Systems, used as a tile substrate in wet areas. Unlike cement board or drywall, Kerdi Board has a built-in waterproof membrane on both faces, so it does not need a separate vapor barrier. It is cut with a utility knife, fastened with washers and screws, and the seams are taped with KerdiBand and sealed with Kerdi-Flex or unmodified thinset. Because it is lightweight and easy to cut, it is popular for DIY shower builds and renovations.

How to Use the Calculator Result

The number of boards is the minimum you should order, but consider these factors:

• Cutouts: Subtract areas for doors, windows, and niches only if they are large. Small cutouts like pipe penetrations do not reduce the sheet count meaningfully.
• Layout: If you plan to run boards vertically (e.g., 5 ft tall on an 8 ft wall), you will need to piece the top section, which creates more waste.
• Offcuts: You can often use offcuts for small areas like the floor or curb, but plan for at least 10% waste. For complex layouts or lots of angles, increase waste to 15–20%.
• Board size: The calculator lets you choose the board dimensions. The 4×8 ft sheets cover more area but are heavier and harder to maneuver in tight spaces. The 3×5 ft sheets are easier to handle but require more seams.

Common Mistakes

One frequent error is forgetting to include the floor. Many people only measure the walls, then run short. Another is mixing units: if you measure in inches but enter feet, the result will be wildly wrong. Always convert all measurements to the same unit before calculating. Also, do not subtract small cutouts like valve openings or shower heads—they are too small to save a whole sheet. Finally, rounding down to avoid buying an extra sheet is risky; a single miscut can force a second trip to the store.

Limitations of the Formula

This calculator assumes a simple rectangular room with a flat ceiling. It does not account for:

• Sloped ceilings or skylights
• L-shaped or irregular floor plans
• Recessed niches or benches that add surface area
• Door and window openings (you can subtract them if they are large, but the calculator does not do it automatically)

For those cases, break the space into rectangles and calculate each separately, or add extra waste. The formula also ignores that boards come in fixed sizes, so you may need more than the area calculation suggests if your wall dimensions do not divide evenly by the board dimensions. For example, an 8 ft wall with 3 ft wide boards will require three boards side by side (9 ft total width), wasting 1 ft per row.

Frequently Asked Questions

Can I use Kerdi Board on the ceiling?

Yes, Kerdi Board can be installed on ceilings, but you must use the proper fasteners (washers and screws) and support the board while fastening. The calculator includes the ceiling only if you add it manually; the standard formula covers walls and floor. If you are tiling the ceiling, add its area separately.

Do I need to waterproof the seams?

Yes, all seams and corners must be covered with KerdiBand (a fleece-backed membrane) and embedded in unmodified thinset. The board itself is waterproof, but the joints are not until taped. Also apply Kerdi-Flex or Kerdi-Fix to inside and outside corners for extra security.

What thickness of Kerdi Board should I use?

Schlüter recommends 1/2-inch for walls and 5/8-inch for floors or heavy tile. The calculator does not require thickness because it does not affect area, but it affects cost and structural support. Check the manufacturer’s guidelines for your specific tile weight.

How much does Kerdi Board cost?

Prices vary by region and retailer, but a 3×5 ft sheet of 1/2-inch Kerdi Board typically costs between $30 and $50. The calculator gives you the number of sheets, so multiply by your local price to estimate the total. Remember to add the cost of KerdiBand, screws, washers, and thinset.

Can I use regular drywall instead of Kerdi Board?

You can use drywall if you cover it with a waterproof membrane like Kerdi, but Kerdi Board itself is not required. The advantage of Kerdi Board is that it is dimensionally stable and does not wick moisture, making it ideal for steam showers and high-humidity areas. Drywall requires a separate waterproofing layer and is heavier.

Is Kerdi Board suitable for floors?

Yes, but use the thicker 5/8-inch board and follow the manufacturer’s installation instructions. The calculator includes the floor area, but you may need to add extra support if the subfloor is not perfectly flat. For floors, many installers prefer a poured mortar bed, but Kerdi Board is a valid option.

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in length | Room Length | ft | 6 | 0 | 100
in width | Room Width | ft | 5 | 0 | 100
in height | Wall Height | ft | 8 | 0 | 20
in board_w | Board Width | ft | 3 | 1 | 8
in board_h | Board Height | ft | 5 | 1 | 12
out area_total | Total Surface Area | sq ft | 2 * (length * height) + 2 * (width * height) + length * width | 1
out boards | Boards Needed | boards | ceil(round((area_total * 1.10) / (board_w * board_h), 6)) | 0