Underlayment calculator

Sheets of cement board, rolls of Schluter DITRA, or sheets of plywood — for the same floor, side by side. With the thin-set bed under the board that most calculators forget, and a flag for which product actually belongs under tile.

01 How the math works

Underlayment covers the floor, so the amount scales with the floor area, not the tile count. Take the area, add a waste allowance for cuts and overlap, then divide by what one unit covers — a sheet for board and plywood, a roll for membrane. Round up.

A cement backer board sheet is 3 ft × 5 ft = 15 ft² (1.39 m²). Schluter DITRA comes on rolls of 54 ft² (5 m²) and 323 ft² (30 m²); DITRA-XL on a 175 ft² roll. A plywood underlayment sheet is 4 ft × 8 ft = 32 ft² (2.97 m²). Board and plywood are sold by the sheet; membrane is sold by the roll, which is the one thing an area-only calculator quietly gets wrong when it hands you a sheet count for a product that doesn't come in sheets.

The part the sheet count doesn't show is the mortar underneath. On a floor, cement board isn't just screwed down — it's embedded in a bed of thin-set troweled onto the subfloor first, then fastened through the wet bed. DITRA is the same idea: bonded to the subfloor in a thin-set bond coat, with the tile set in thin-set on top. Either way there are two thin-set applications on the job, and area ÷ sheet size counts neither.

02 Worked example

Take a small bathroom floor: 5 ft × 8 ft (1.52 m × 2.44 m), 40 ft² (3.72 m²). With a 10% waste factor there's 44 ft² (4.09 m²) to cover.

In cement board: 44 ÷ 15 = 2.93 → 3 sheets. The thin-set bed under them is 44 ÷ 80 = 0.55 → 1 bag (50 lb), combed with a 1/4 × 1/4 in notch. Fasteners on the 8 in grid run about 34 per sheet, so 102 screws. Then the tile takes its own thin-set on top of the board — a separate order the tile calculator handles.

In Schluter DITRA the same floor is one 54 ft² roll (44 ÷ 54 = 0.81). A 323 ft² roll would also cover it and leave you with most of a roll of membrane and a question about the return policy. In DITRA-XL it's one 175 ft² roll, the smallest stocked. Plywood underlayment would take 2 sheets of 4 × 8 — but not under tile, which is the next section.

03 When this calculator is wrong

The unit counts are right; the two ways to misread them are picking the wrong product for the job and trusting a flat waste factor on a small room.

• Plywood underlayment is not a tile substrate on its own. It's a smoothing and fastening layer for resilient floors — sheet vinyl, LVT, engineered plank. For ceramic or stone tile over a wood subfloor, the tile industry calls for cement backer board or a bonded uncoupling membrane like DITRA. Some methods do bond tile over plywood, but only through a membrane — which puts you back to buying the membrane, not the plywood, as the surface the tile grabs.
• Cement board and DITRA each need a thin-set bed the sheet count doesn't include. A 40 ft² floor in cement board takes 3 sheets and roughly a full 50 lb bag of thin-set just to bed the board — before a single tile is set. Skip that line and the mortar order is about half of what the job burns through.
• A flat 10% waste factor is light for a small, cut-up room. On a job under about 10 sheets — a bathroom, a mud room, a landing — real waste runs closer to 15%, because a small floor is nearly all edges and cuts and there's little field area to absorb an off measurement. The exception is a big open rectangle, where 10% is right and sometimes generous. Bump the waste to 15% on the small stuff and round up.

04 What to do with the result

Pick the product by what's going on top first. Tile over a wood subfloor: cement board or DITRA. Tile where you also want waterproofing in one layer: DITRA, which is a membrane and a waterproofer at once. Tile you want close to flush with an adjacent hardwood floor: DITRA-XL for the extra thickness. Resilient flooring: plywood underlayment. Then take the unit count above, and for cement board or DITRA, order the thin-set bed as a separate line — it's the number most people find out about mid-job.

Buy the membrane in one dye-and-batch run where it matters and the board a sheet over, not a sheet under. An unopened sheet of backer goes back; a floor short one sheet at the doorway waits on another store trip while the thin-set bed sets without it.

05 Common questions

What underlayment goes under tile?

Over a wood subfloor, cement backer board or a bonded uncoupling membrane such as Schluter DITRA. Both give thin-set a stable surface to bond to; DITRA also waterproofs and lets the tile move independently of the subfloor. Plywood underlayment alone is not a tile substrate — it's for resilient floors.

Can you lay tile directly on plywood?

Not as a stand-alone underlayment. Plywood moves with humidity and the tile bond can fail. For tile over a wood subfloor the tile industry calls for cement board or a bonded membrane. Where a method does put tile over plywood, it's through a membrane bonded to the plywood — so you're buying the membrane regardless.

Does cement board need thin-set underneath?

On floors, yes. The board is embedded in a bed of thin-set troweled onto the subfloor, then screwed down through it, so the board sits in full contact with no voids. A 40 ft² floor takes about one 50 lb bag for the bed at a 1/4 × 1/4 in notch — before the tile's own thin-set on top.

How many screws per sheet of cement board?

Fasten every 8 in on center over the whole sheet. On an 8 in grid that's about 34 per 3 × 5 sheet — one fastener per 8 × 8 in cell. Keep them 3/8–3/4 in from edges and 2 in from corners.

Is cement board or Schluter DITRA better?

Different tools. Cement board is a rigid, cheap, proven substrate that adds a solid bonding surface. DITRA is thinner (1/8 in vs a 1/4 in board), waterproofs, and uncouples the tile from subfloor movement — worth it on floors that flex or get wet, at a higher material cost. For a dead-flat, stable subfloor in a dry room, board is hard to beat on price.