The floor should be designed for the weight to be encountered. This is not an “engineer’s estimate”, but a rough back-of-a-bar-napkin type of calculation:

Let’s say that the tub weighs 500#, which is pretty conservatively heavy unless you have an extraordinarily heavy tub; so you can adjust that weight according to what you know about your intended purchase.

The weight of the tub occupant/s, let’s also estimate at 500# (not you, of course, but let’s say that you and your husband want to use the tub together, or you have an especially heavy guest sometime).

If we estimate the tub’s dimensions as roughly 6’ x 4’ x 3’ (length, width, depth), that’s 72 cubic feet of water, or about 538 gallons of water. Let’s call it 550 gallons. Fresh water weighs about 7# per gallon, so that’s about 3,850#, which we can round up to 4000#, or two tons.

So, adding it all up: a 500# tub, 500# of occupants (and let’s say that they’re standing up, because if they were lying down, they’d displace their weight in water, which would therefore no longer be in the tub), and 4000# of water: 5000# all in. Divided by four support points, that’s 1250# per foot – and yes, that’s a lot of weight if you’re lifting it!

Residential building codes in the USA typically call for structures to be designed with floors that can support “live loads” (which this application is) of 40 pounds per square foot. That doesn’t seem to jibe with the tub weight on four points at 1250# per point. However, the “whole floor” under the tub is (assuming the dimensions of 6’ x 4’, length x width) 24 square feet, and the floor just under the tub alone should safely support 24×40# = 960#. (Again, this is especially conservative, because if you look at other “point loads” in your house, such as a loaded refrigerator, for example, you can locally easily – and safely – exceed the nominal 40psf loading.)

The high-side estimate that I gave you above represents about 5x that weight, so it’s going to be outside of “normal” for a standard just-according-to-the-code floor. However, it would not take extreme measures to stiffen that floor, especially if you’re tearing it up anyway, so that it would be perfectly adequate to support that tub.

Particular attention should be paid to the four support points, as others have already noted, so that local deformation doesn’t ruin the floor surface.