A semi flush uses about half the water that a full flush uses. This saves using water when it is not necessary to use more than is required. It saves money plus helps the environment.
In a modern toilet you use 13 litres per flush.
If the toilet works each time on one flush, they use roughly half the water of a standard toilet. You would then use half as much a month on the toilet. They do not always flush completely on the first flush and the toilet is only a part of the water bill.
It takes 1 bucket of water when we flush.
If you have an older style toilet (15 years or more) then you are flushing away about 18 litres of water per flush. All new toilets flush an average of 13 litres per flush and many even give the choice of a 6 or 13 litre flush with the use of a dual flush button. Many municipalities in Canada have now banned the sale of old style full flush toilets. -In US and Canada people are incredibly wasteful of water in general. We use nearly 2-3 times per capita what most other countries use.
5 gallons
If you want to use less water per flush, you put a brick in the bottom of the tank. You do not want to reduce the height of the water in the tank so much as use less water per flush.
If the holding tank is full you can flush it just once. Further flushes will have to wait until the holding tank is again full. If the tank is empty- no. If you have a way to refill the holding tank with water again, you can flush one more time for every time you refill it. When our water was going to be off for a time, I pre-filled the bathtub with water to use for this purpose.
Dual flush toilets usually use 3 and 6 liters of water depending on which way you push the flush handle - 3 liters for a little job and 6 liters for a major event. Single low-flush toilets use either 4.5 liters or 6 liters.
5 gallons
You use anywhere from 1 cup of water to 5 cups of water to flush the toilet. This is per time you flush the toilet.
It depends on what type of flush you use. Old style single flush toilets use around 11 litres of water per flush, older dual flush toilets use 4.5 or 9 litres per flush, while modern water efficient dual flush toilets use 3 or 6 litres per flush.