They're measured in gallons in the US, and liters everywhere else.
This holds true no matter how big the tank is--the New England Aquarium in Boston has a tank they describe as holding one million gallons of water.
The best unit of measure to use when measuring the amount of water in a drinking glass is ounces. A liquid measuring cup can be used to measure the ounces of water.
millilitres
take a cylindrical measuring thing and measure the amount of water in it then drop the item in then measure that amount and subtract the first amount from the second
Use water displacement. Measure water before you put the rock in it, and then again afterwards. Put a measuring cup large enough to hold the rock in a pot large enough to hold the measuring cup and fill the measuring cup to the very top with water. Lower the rock into the measuring cup carefully so that the water flows over and into the pot. Carefully remove the measuring cup with the rock without spilling any additional water into the pot. When you measure the water in the pot it will equal to the volume of the rock (accuracy will depend on how careful you were). This is displacement measuring.
Measuring the amount of water is typically referred to as water metering. This process can help monitor water usage and ensure efficient management of water resources.
by measuring the amount of water it displaces
Measuring cup
You have to use a measuring cylinder and water.
You would use a measuring cup or a graduated cylinder to measure the amount of water in a fish tank. Alternatively, you can also calculate the volume of water by measuring the length, width, and height of the tank and using a formula for volume.
I woould use a small measuring spoon, as in the image
You can use a measuring cup or a measuring jug to measure the water before pouring it into the washing machine. Make sure to follow the manufacturer's guidelines for the correct amount of water required for your specific machine and load size.
It is a measure of volume usually used when measuring the water in a lake or large reservoir. An acre foot is the amount of water that it would take to cover one acre of land with one foot of water.