If you are talking a standard five foot tub,
A cast iron tub can weigh 300 pounds or more.
Most five foot jetted tubs weight less then 100 pounds.
I'm thinking of putting in a whirlpool tub to replace my original plain tub. Does my floor have to be reinfored for this
Our 4-jetted, 5 ft. bathtub was very light, I was readily able to pick it up without a problem.
Water weighs about 8#/gallon. A residential floor is designed to hold typically 40#/ft2. This includes the subfloor and floor, if present. A typical tub would not exceed those loads. Check how many gallons the tub holds, add the weight of the tub and the person sitting in it and divide by the say 15 square feet (assuming a 36x60" tub) and see what you get for a load.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013