How Much Thermo-Aspen Do I Need? Sauna Coverage Math

The formula: square feet to linear feet

Linear feet = net square feet × 12 ÷ installed cover in inches × (1 + allowance). Each part of that line is one measurement you take or one number you look up.

• Walls. Add the four wall lengths and multiply by the ceiling height. A 5 by 7 foot room with a 7 foot ceiling has 2 × (5 + 7) × 7 = 168 sq ft of wall.
• Ceiling. If you panel it, multiply length by width: 5 × 7 = 35 sq ft.
• Openings. Subtract the door and any glass that replaces paneling. A 2 by 6-1/2 foot door is 13 sq ft.
• Cover.Look up the installed cover, also called covering width, in the maker’s catalog. For STS4 1×4 thermo-aspen it is 3.15 inches Thermory catalog, p. 8.
• Allowance.Add 5 to 10% for cuts, the material-loss range in Thermory’s installation guide Thermory USA guide, p. 2.

The 12 converts the cover from inches to feet. A board with 3.15 inches of cover fills 3.15 ÷ 12 = 0.26 sq ft per linear foot, so each square foot of wall takes 12 ÷ 3.15 = 3.81 linear feet.

Measure at the framing, in feet. Battens and boards move the finished surface inward, so framing dimensions overstate the paneled area slightly. That error runs toward having enough wood.

Why the installed cover is narrower than the board

Divide by the installed cover, not the size name or the full board width. On tongue-and-groove paneling, part of every board disappears into the joint with the next one.

Thermory’s June 2026 catalog lists the STS4 thermo-aspen board sold as a 1×4 at about 0.59 by 3.54 inches, with 3.15 inches of covering width Thermory catalog, p. 8. So 0.39 inch of each board, about 11% of its width, sits inside the joint. The “4” in “1×4” is a size name, not a measurement.

Dividing by the full board width would leave this room about 80 linear feet short. Dividing by the size name would leave it about 154 feet short.

Worked examples for 4 × 6, 5 × 7, and 6 × 8 ft rooms

Each room below has a 7 foot ceiling, paneled walls and ceiling, one 13 sq ft door, and a 10% allowance. Enter the same inputs in the calculator above to check any row.

The 1×5 needs about 27% fewer linear feet than the 1×4 for the same room, because its cover is wider. The wider cover also means fewer rows to fasten. A 7 foot wall is 84 inches tall. It takes 84 ÷ 4.33 = 19.4, so 20 rows of 1×5, or 84 ÷ 3.15 = 26.7, so 27 rows of 1×4.

Paneling the ceiling adds 17 to 24% to the wall area in these three rooms. Leave it out of the math only if the ceiling gets a different material.

How much extra to add for cuts

Add 5 to 10%. Thermory’s installation guide gives that range for wall paneling material loss and ties it to board length Thermory USA guide, p. 2. The examples above use 10%.

Where you land depends on how the board lengths fit your walls. A board slightly longer than a wall leaves one short offcut per row. A wall slightly longer than a board forces a joint, or a longer board, in every row.

Check each wall length against the listed board lengths before you pick a percentage. Lean toward 10% when a door or window breaks up the runs, since each opening adds short cuts.

Buy the full amount at the start, for color as well as length. Thermory’s guide says to make sure you have enough material, since shade can differ slightly between production batches, and to open several packages so boards can be matched on the wall Thermory USA guide, p. 2.

Turning linear feet into bundles

Wall paneling is sold in bundles of set board lengths. The Sauna Marketplace product pages for STS4 1×4 and STS4 1×5 list the current lengths and how many boards come in each bundle.

A bundle’s linear feet equal its board length times its board count. Divide your total by that figure and round up to whole bundles.

Choose lengths wall by wall. Long boards suit the ceiling and the longest walls. Shorter boards suit short walls, where a long board would leave a long offcut.

Thermory’s guide panels the ceiling first and the walls after Thermory USA guide, p. 3. Sort the boards by length before you start and set aside what each wall needs, so the ceiling does not use up the lengths a wall depends on.

Plan delivery a few weeks before installation. Thermory’s guide recommends storing the paneling for a few weeks in the room where it will go, flat and off the floor, in unopened packages Thermory USA guide, p. 2. That lets it adjust to the room’s temperature and humidity. More on storage is in cutting, sanding, and storing thermo-aspen.

To see the color and face before you order the full amount, order a sample of STS4 1×4 from Sauna Marketplace.

What the paneling number leaves out

The formula covers wall and ceiling paneling only. Four more items need their own count.

• Bench boards.Benches use a separate bench-board profile, and the catalog lists thermo-aspen SHP bench boards on their own page Thermory catalog, p. 16. Compare options on the profiles page.
• Trim. The catalog lists thermo-aspen mouldings Thermory catalog, p. 14. For vertical paneling, Thermory’s guide leaves a gap of at least 7/16 in at corners and covers it with a corner moulding Thermory USA guide, p. 3. Count trim in linear feet of corners and edges.
• Battens.Thermory’s guide calls for battens at least 13/16 in thick and 1-13/16 in wide, about 15-3/4 in apart and never more than 23-1/2 in Thermory USA guide, p. 2. See framing, battens, and the air gap.
• Fasteners.Thermory’s guide recommends a staple gun with stainless steel clips Thermory USA guide, p. 2. For ThermoWood® products, the handbook’s Table 10 sets Type 304 stainless (class A2) as the minimum for sauna wall and ceiling cladding ITWA handbook, p. 44. See stainless fasteners for sauna paneling.

To see the whole room with benches and heater before you buy, open a profile in the 3D sauna designer. The buyer’s guide covers the rest of a sauna package. For how the wood is made, see the process page; care and installation answers are on the questions page.