A cube sauna is a square or boxy outdoor cabin, and it has become the alternative to the barrel for people who find the barrel shape agricultural. The differences are not only visual: a flat-walled box uses its internal volume differently, takes glazing better, and needs a proper roof where a barrel sheds water by shape.
Usable space
A barrel's curved walls waste the corners and give you headroom only in the middle, which is why a barrel feels smaller inside than its dimensions suggest. A cube uses its full footprint: benches run wall to wall, you can stand at the edges, and a changing vestibule partitions off cleanly. For the same external size, a cube usually seats more comfortably.
Glazing and how it looks
Flat walls take large glass panels, which is why the panoramic back wall you see in modern garden saunas is almost always on a cube. Barrels can take a glass end but not a glass wall. If the view out is part of why you want it in the yard, that alone tends to decide the shape.
The roof
A barrel sheds water because it is round. A cube needs a roof: a slight pitch, a membrane or metal covering, and a drip edge. That is a real cost and a real maintenance item, and a flat-roofed cube with a cheap covering is where outdoor saunas leak. Check what the roof actually is before comparing two prices.
Heat and efficiency
A barrel's shape means less air volume for a given bench area and no cold corners, so barrels heat quickly and evenly, which is their genuine functional advantage. A cube has more volume to heat and corners where air sits still, so it wants a slightly larger heater and benefits more from a proper vent path.
Questions people ask about cube sauna
Is a cube sauna better than a barrel?
Better on usable space, glazing and how it looks in a garden. Worse on heat-up and on needing a real roof. Neither wins outright.
Do cube saunas leak?
Only if the roof is an afterthought. A proper pitch, covering and drip edge is what to check before comparing two prices.
Which heats faster?
A barrel, generally. Less air volume for the same bench area and no cold corners is its real functional advantage.