In the United States, a gallon is defined as 3.78541 liters. To find out how many two-liter bottles it takes to make a gallon, the volume of a gallon is divided by the volume of the bottle.

Answer: Approximately 1.89 two-liter bottles of water are needed to equal a gallon.

This calculation is based on the conversion between liters and gallons.

The calculation is as follows:

3.78541 liters per gallon / 2 liters per bottle = 1.892705.

This result indicates that just under two 2-liter bottles are required to reach the volume of one gallon.

It is important to note that this calculation is based on the US liquid gallon. Other types of gallons, like the imperial gallon used in some Commonwealth countries, have different volumes and would therefore require a different calculation.