Time for some math, folks. If I was trying to figure out how much liquid a person would displace in a container of a given volume, how would I determine that? Trying to plan out something for the future and I'm unable to do a "dry" run with just water.

Ask Archimedes

The volume of displaced liquid is equal to the volume of the object that's been submerged.
https://www.britannica.com/science/Archimedes-principle

Depends on the person.

You know a persons mass from a scale, you want volume, so you also need density.

Read this to come up with estimate of the persons density.
https://en.m.wikipedia.org/wiki/Body_composition

Then use Density = mass / volume solve for volume and plug in your known mass and density estimate, and thats your answer.

Yea theres Math to calculate that.. but you can use this calculator if you want to easy road

https://www.aqua-calc.com/calculate/weight-to-volume

Human's specific gravity is nearly equal to water.
If you are 70 kilograms, you are about 70 liters.

That sounds right. In US terms, water is about 8 pounds to a gallon.

A person-sized amount?