To measure the amount of water in a shower you can take a 5 gallon bucket or gallon bucket and then use the shower head to fill the bucket and time the amount of time it takes for the bucket to fill using a timer or watch on a timer.
Then the time listed on the timer or watch timer is the amount of water that the shower is using per minute.
For example if it takes 5 minutes to fill the 5 gallon bucket with the shower then it means that the shower is using 1 gallon per minute of water.
If it takes only 2 minutes or 2 minutes and 30 seconds to fill the 5 gallon bucket then the shower is using 2.5 gallons of water per minute.
The number of gallons of water that are in a bathtub is a function of time.
The reason for the number of gallons of water in a bathtub being a function of time is because for every specific time, there will also be only one corresponding amount of water in the bathtub, given that the water in the bathtub is either being removed, added or remaining consistent.
For any given time, there's only 1 unique amount of water which can be in the bathtub.
And if you were to plot the water level in the bathtub over time, you would see a smooth and continuous curve.
And each time value would also correspond to a single water level.
For example if you were to start with an empty bathtub and then add water at a rate that is constant, then the amount of water in the bathtub at any given time will also depend solely on that time.
The amount of water that a bathtub can hold when full is between 65 gallons to 70 gallons for standard 5 foot sized bathtubs.
For larger bathtubs such as corner garden bathtubs they can old as much as 85 gallons of water to 100 gallons of water.
The amount of water that comes out of a bathtub faucet ranges from 2.5 gallons per minute for a lower flow rate faucet and for super high end bathtub faucets they can range from 10 to 21 gallons of water per minute.