Benches are set from the ceiling down
A sauna is a stratified room. The air near the ceiling is far hotter than the air near the floor, and the gradient across a seven foot room is large enough that a person sitting with their head at four feet and one sitting with their head at five feet are having two different experiences. That is why the dimension that matters is the distance from the top bench to the ceiling rather than the distance from the top bench to the floor.
Setting that gap first and working down is the whole method. Pick the headroom a seated person needs above the upper bench, subtract it from the ceiling height, and that is the bench. Everything else follows: the lower bench is one rise below, the foot step one rise below that, and if the last tier lands underground the room wants fewer tiers or a different rise rather than a compromise on the top bench. Building the top bench at a comfortable domestic sitting height and letting the headroom fall where it may is the mistake that produces saunas people describe as never quite getting hot.
Depth, and what the bench is for
Bench depth decides what the bench can be used for, and it is where small rooms make their hardest trade. A depth in the region of 16 to 18 inches seats someone with their feet on the tier below. Around 24 inches lets a person sit cross-legged, which most people end up doing. Getting past that and up toward the depth of a body is what makes lying down possible, and lying down is the arrangement that puts a whole person in the same temperature band instead of spreading them across the gradient.
Length works the same way. A run of six feet is the practical floor for lying down, and the calculator flags whether the usable run clears it. In a small room the choice is usually between a deep upper bench on one wall and a shallower L that seats more people, and there is no correct answer to that; it depends on whether the room is mostly used by one person or by four.
The wall you think you have is not the wall you get
Two things eat the bench wall before any bench is built. The door needs its swing and the space in front of it, and the heater needs its position and whatever surround the manufacturer of that specific unit requires around it. Between them they commonly take two to four feet out of a bench wall, and in a small room that is a third of the seating.
This is worth resolving on paper in the right order. Fix the heater position and its required surround from the instructions that came with it, fix the door, and only then measure what is left for benches. Doing it the other way around produces a bench layout that has to be cut back after the heater arrives, which is how people end up with a two foot stub of upper bench in a corner.
Ventilation, and why the positions matter more than the area
The calculation is unremarkable: volume times air changes per hour divided by sixty gives cubic feet per minute, and dividing that by a chosen face velocity gives the free area the opening needs. A small sauna at six air changes wants something in the region of thirty CFM, which at 200 feet per minute is a free area of around twenty square inches each way.
The part that actually determines whether the room works is where the two openings sit. Air entering low near the heater is drawn up over the stones and across the room, and air leaving on the far side below the upper bench pulls that flow through the occupied zone before it goes. Put the two openings near each other and the air short circuits between them, leaving a room that stratifies into a hot ceiling and a stale bench even though the CFM figure is correct. Every arrangement question of that kind is a question about path, not about area, and the same is true of the free area figure itself: a grille blocks a large fraction of its own opening, so the hole in the wall has to be bigger than the number above by whatever the free area ratio of that grille is.
Questions people ask
How high should a sauna bench be?
Work it out from the ceiling rather than the floor. Choose the headroom a seated person needs above the upper bench, commonly in the region of 40 to 45 inches, and subtract it from the finished ceiling height. In a 7 foot room with 42 inches of headroom that puts the upper bench at 42 inches from the floor, with a lower bench 16 inches below it at 26 and a foot step at 10. The reason for working downward is stratification: the useful heat sits high in the room, and a bench set to a comfortable domestic height leaves the sitter below it.
How deep should the upper bench be?
It depends on what you want to do on it. Sixteen to eighteen inches seats someone with their feet on the tier below and is the minimum that works. Around 24 inches allows sitting cross-legged, which is what most people settle into. Getting close to the depth of a body allows lying down, which needs a run of at least six feet as well. In a small room this is a direct trade against seating capacity, and the answer depends on whether the sauna is usually used alone or with company.
How much ventilation does a sauna need?
This page takes the air change rate as an input rather than stating one, because it depends on the heater type, whether the ventilation is mechanical or passive, and what the local requirements are. Mechanically ventilated rooms are commonly designed somewhere in the 3 to 8 changes per hour range. Whatever figure you work to, the arithmetic is the same: volume times changes divided by 60 gives CFM, and CFM divided by face velocity gives free area. A small domestic sauna lands somewhere around 20 to 40 square inches of free area each way.
Where do the intake and exhaust vents go?
The usual arrangement is intake low and close to the heater, so incoming cold air is drawn up over the stones, and exhaust on the far side of the room below the upper bench, so the air crosses the occupied zone before it leaves. What matters is that the two are far apart and that the path between them passes through where people are sitting. Openings close together short circuit, and the room ends up with a hot ceiling and a stale bench regardless of what the CFM figure says. Note that heater position itself is governed by the clearances in the manufacturer instructions, not by ventilation preference.
Why is the free area smaller than the vent I have to cut?
Because a grille is mostly not a hole. The blades, the frame and any insect screen block a substantial share of the opening, commonly a third to a half, so the free area a product delivers is well under its nominal size. Manufacturers publish a free area figure or a free area ratio for exactly this reason. Take the square inches from this calculator as the free area you need, then divide by the ratio of the grille you are buying to get the opening to cut.