How Much Water Will It Take To Fill A Swimming Pool The Size Of 18x40 And 54 Inches In Depth?

This is for U.S. Measurements:  If your pool is like a box, (rectangular, straight sides, right angles) you can calculate the volume just  like you would a box.     To find the volume of a box, multiply the length, width, and height (depth is similar to height - but from the top instead of the bottom)     convert depth to the same units of measure as the other measurements: 54-inches = 4.5-feet     18-ft X 40-ft X 4.5-ft = 3240-ft³     3240-ft^{3 x}  7.48-gal/ft³=24236.88     Since your pool has rounded corners and the bottom isn't perfectly flat, I would say it is pretty close to 24,000 gallons.  If you pay $0.01/gallon (one penny for every gallon), it will cost $240 to fill the pool.

My pool is  a 24 feet round pool and is 4 feet deep.  How much will this cost to fill?

15x42 and 40 inches

I don't think that it will take that much water. The most important thing is to be able to make it clean, isn't it? That's why you need good cleaning means and tools. Personnaly, I found them on cleanup expert website. They gave me a good piece of advice about pool cleaning.