Concrete slab for a hot tub
Just a gut feeling, but based on what I've seen that will hold a brick patio flat for a long time, two or three three inch lifts of packed crush under a 5&quot; slab with 10mm rebar at 12-15&quot; should be adequate.  If you want something set on dirt to stay flat and the way you set it a long time, multiple layers of tightly packed gravel give a flat, long lasting high strength base with good drainage for the brick or slab on top.  I don't know enough to be sure of slab thickness but 125 PSI seems like nothing.  It's about 0.86 MPa and Wikipedia says regular strength concrete is good for at least 10 MPa.  If you do a solid base, you can probably even get away with a 4&quot; slab as a hot tub has fairly wide weight distribution and what usually breaks 4&quot; concrete is a wheeled vehicle that concentrates its weight.
Fair warning I'm only aware of electrical codes for hot tubs and my solution might be overkill, but I try to build things to last.  Gravel is relatively cheap, digging, placing and packing are hard work, but you only have to do it once.