How many sheets of backer board do I need?

Divide your tile area by the coverage of one sheet. A standard 3x5 ft cement board sheet covers 15 sq ft, so a 100 sq ft floor needs about 7 sheets (100 ÷ 15 = 6.7, rounded up). Add roughly 10% for cuts and waste. For walls or showers, measure each surface separately and total them the same way.

What size does cement backer board come in?

The most common size in the US is 3 ft × 5 ft (15 sq ft per sheet), used for floors and countertops. Walls and showers often use 4 ft × 8 ft sheets (32 sq ft). Thickness is typically 1/4 in for floors over a solid subfloor and 1/2 in for walls and showers.

Do I need backer board under tile?

For most floors and all wet walls (showers, tub surrounds), yes — a cement or fiber-cement backer board gives tile a stable, water-tolerant surface and prevents cracking. You can tile directly over concrete or a proper uncoupling membrane instead, but you should never tile directly over drywall in a wet area or over bare plywood on a floor.

How much thinset do I need for backer board?

Backer board is set in a bed of thinset AND the tile is set on top, so you're buying mortar for two layers. Plan for one 50 lb bag of thinset to set roughly 40–60 sq ft of backer board with a 1/4 in trowel, then estimate the tile layer separately with our thinset guide.

How Much Backer Board Do I Need? (Sheets, Sizes & Screws)

Quick answer: To find how much backer board you need, divide your tile area by the coverage of one sheet. A standard 3x5 ft cement board covers 15 sq ft; a 4x8 ft sheet covers 32 sq ft. Divide your square footage by that number, round up to whole sheets, and add about 10% for cuts. A 100 sq ft floor needs roughly 7 sheets of 3x5 ft board.

Backer board (also called cement board or, by the brand names, HardieBacker or Durock) is the rigid, water-tolerant panel that goes between your subfloor or studs and the tile. It gives tile a stable base so it doesn’t crack and shrugs off moisture in wet areas. Estimating it is the same square-footage math as your tile — get your area first with the tile calculator or the square footage guide, then use the coverage numbers below.

Backer board sizes and coverage

Sheet size	Coverage	Best for
3 ft × 5 ft	15 sq ft	Floors, countertops
4 ft × 8 ft	32 sq ft	Walls, showers, tub surrounds
3 ft × 4 ft	12 sq ft	Small floors, tight spaces

Use 1/4 in thickness for floors over a solid subfloor and 1/2 infor walls and showers. Thickness doesn’t change how many sheets you need — only which product to buy.

How to estimate backer board in 4 steps

Get your total tile area in square feet (measure floors and walls separately).
Pick your sheet size and read its coverage from the chart above.
Divide your area by the coverage and round up to whole sheets.
Add about 10% for cuts around openings and fixtures.

Example — a 120 sq ft bathroom floor, 3x5 ft sheets:
120 ÷ 15 = 8 sheets exactly. Add 10% for cuts around the toilet flange and vanity: 8 × 1.10 = 8.8 → round up to 9 sheets. For a shower with 90 sq ft of wall area in 4x8 ft sheets: 90 ÷ 32 = 2.8 → 3 sheets, plus a spare if the layout is complex.

Don’t forget screws, tape, and thinset

Backer board is a system, not just the panels. For every job you’ll also need:

Backer-board screws: roughly 30–40 screws per 3x5 ft sheet, spaced about 8 in apart. Budget one 5 lb tub for a typical bathroom.
Alkali-resistant seam tape: 2 in mesh tape for every joint between sheets. Add up your seam length and buy a little extra.
Thinset — twice:you bed the board in thinset AND set tile on top, so you’re buying mortar for two layers. Size the tile layer with our thinset guide.

Where backer board is required (and where it isn’t)

Use backer board on tile floors over wood subfloors and on all wet walls — showers, tub surrounds, and splash zones. You can skip it and tile directly over concrete slabs in good condition or over a proper uncoupling membrane, which is a thinner alternative that also handles crack isolation. What you should never do is tile directly over drywall in a wet area or over bare plywood on a floor — both lead to cracked tile and failed grout down the line.

Frequently asked questions

How many sheets of backer board do I need?: Divide your tile area by the coverage of one sheet. A standard 3x5 ft cement board sheet covers 15 sq ft, so a 100 sq ft floor needs about 7 sheets (100 ÷ 15 = 6.7, rounded up). Add roughly 10% for cuts and waste. For walls or showers, measure each surface separately and total them the same way.
What size does cement backer board come in?: The most common size in the US is 3 ft × 5 ft (15 sq ft per sheet), used for floors and countertops. Walls and showers often use 4 ft × 8 ft sheets (32 sq ft). Thickness is typically 1/4 in for floors over a solid subfloor and 1/2 in for walls and showers.
Do I need backer board under tile?: For most floors and all wet walls (showers, tub surrounds), yes — a cement or fiber-cement backer board gives tile a stable, water-tolerant surface and prevents cracking. You can tile directly over concrete or a proper uncoupling membrane instead, but you should never tile directly over drywall in a wet area or over bare plywood on a floor.
How much thinset do I need for backer board?: Backer board is set in a bed of thinset AND the tile is set on top, so you're buying mortar for two layers. Plan for one 50 lb bag of thinset to set roughly 40–60 sq ft of backer board with a 1/4 in trowel, then estimate the tile layer separately with our thinset guide.

With your board count sorted, finish estimating the rest of the job: work out tiles and boxes with the free tile calculator, then size your grout and thinset. To see the full budget, read how much tile installation costs.