There are two answers.
1. You don't provide enough information. A liter is a volume or 3-dimensional measure length width, depth). You only provide the capability to calculate a 2-dimensions area (length and width).
2. Assuming that the 18 foot dimension is a diameter, the radius is then 9 feet. Using that and the formula to calculate the area of a circle, the number of square feet contained in an 18 foot diameter circle is approximately 254.34 Square feet.
If you have an AVERAGE water depth of 3 feet in the pool, the total cubic feet of water is 763 cubic feet which converts to approximately 16, 265 liters of water.
If the average water depth is 3 1/2 feet, the number of liters jumps to 18,976.
If the average water depth is 4 feet, the number of liters jumps to 21, 687.
That should give you enough information to estimate what you need.
About 49,246.3 liters of water.
1100 gals
One cubic foot equates to approximately 28.32 liters.
141.06 gallons
13,594 US gallons of water.
Around 22,870 liters of water.
About 49,246.3 liters of water.
About 129,715 liters.
About 45,025 liters.
About 39,397 liters.
A 22-foot round pool that is 4 feet deep holds about 43,238.5 liters of water.
1 cubic foot is about 28.32 liters.
1 cubic foot of water equates to about 28.32 (28.3168466) liters.
About 28.32 liters per cubic foot.
1100 gals
right at 1809,000 liters
One cubic foot equates to approximately 28.32 liters.